WO2024157266A1

WO2024157266A1 - Method of controlling soilborne plant diseases

Info

Publication number: WO2024157266A1
Application number: PCT/IL2024/050112
Authority: WO
Inventors: Mohammed ABUELHAIGA; Alexander FRENKLACH; Gali CARMI; Reuven ROZEN; Nikolay Fux
Original assignee: Bromine Compounds Ltd
Current assignee: Bromine Compounds Ltd
Priority date: 2023-01-29
Filing date: 2024-01-28
Publication date: 2024-08-02
Anticipated expiration: 2025-07-29

Abstract

The invention relates to a method of controlling soilborne plant disease, comprising applying to soil magnesium oxide, magnesium hydroxide, or both, preferably in combination with one or more compounds decomposable into ammonia in an aqueous environment, such as ammonium sulfate. Co-granules of magnesium oxide and e.g., ammonium sulfate, are also provided.

Description

Method of controlling soilborne plant diseases

Most planting media, whether it is soil or potting soil and other solid media, may contain pathogens that can be detrimental to growing produce. Soilborne pathogens include pathogens that cause plant diseases via inoculum that comes to the plant by way of the soil. The major groups of soilborne pathogens include fungi, bacteria, nematode and viruses. Soilborne pathogens usually affect belowground parts of the plant, but they sometimes cause foliar diseases.

There are different approaches to manage soilborne pathogens, chiefly cultural control, and application of disease-control chemicals. Cultural control includes solarization, replacement of growing media (a common practice with potting soil) and changing a field to one that is non-inf ected . Solarization, as the name hints, uses heat from the sun to increase the soil's temperature so that pathogens are thermally eradicated. Although this method is environmentally clean, it is fairly limited by regional climate differences. Container growers avoid pathogenic infection by discarding the media after a single use. Planting media are not expensive, so such an approach is quite reasonable. However, this option is not available to field growers.

Application of disease-control chemicals includes the use of preplant fumigants. Soil fumigation is achieved with different gases or chemicals which release gases upon exposure to soil moisture. Although current fumigation methods are somehow limited due to environmental regulations, they are still quite common and considered very efficient in terms of disinfection. The application of fungicides is also an effective tool; a variety of chemical formulations are commercially available, which consist of a single active agent or mixtures of active agents. Mixtures of several active ingredients become necessary to grow a healthy crop, as many pathogens develop resistance.

We have now found that magnesium oxide of moderate activity (as explained below) is useful as a disease-control chemical that can be applied to soil to control soilborne pathogens. Magnesium oxide (MgO; also named magnesia) possesses diverse properties and different grades of magnesium oxide are available in the marketplace for a wide range of industrial applications, e.g., it can be used as a filler additive in polymers, a pharmaceutically active ingredient (antacid or laxative) and a pharmaceutical excipient; and in producing forsterite coatings on silicon steel by manufacturers of transformers.

As to the agricultural/agrochemical sector, magnesium oxide has found use as a fertilizer, to deliver magnesium to plants. See, for example, Durrant et al. [J. Agric. Sci., Camb. (1976) , 86, 543-552] , reporting that magnesium oxide obtained from magnesium carbonate by calcination under certain conditions was a very effective source of magnesium to support the growth of plants. However, fairly little was reported on other benefits received by the application of magnesium oxide to soil. In US 5,453,277, which is directed to a method for controlling soil pests, a few magnesium compounds were mentioned, including magnesium oxide and magnesium carboxylate, indicating that they act on pests such as soil insects, to kill them. US 2014/0356461 relates to the use of magnesium oxide obtained by calcination of magnesium hydroxide at 400 to 1000 °C as a plant disease-controlling agent that can be mixed with the soil before seeding. MgO grades possessing high BET surface area (~300 m²/g) were tested and recommended in US 2014/0356461. In US 2016/0286794, it was proposed to improve the action of magnesium oxide in controlling soil-borne plant diseases with the aid of a nonionic surfactant. Experimental work conducted in support of this invention indicates that moderate activity magnesium oxide can act on a soil bacteria known to infect plants (S', scabies) to kill, and even eradicate the bacteria, achieving ~ 2.5 log reduction compared to a control, and that the action of magnesium oxide is strongly enhanced in the presence of ammonia-releasing compounds, e.g., ammonium salts such as ammonium sulfate ( (NH4)2SO4,- ammonium sulfate on its own had no effect on the bacteria tested) .

A similar indication was observed in a field study, in which combinations consisting of magnesium oxide and ammonium sulfate were tested against the notorious soilborne fungi Fusarium. A field was divided into plots, which were treated with mixtures of magnesium oxide/ammonium sulfate proportioned at different ratios; control plots received no treatment. Soil samples were collected before and two weeks after the treatment, and Fusarium levels in these soil samples were determined. The results indicate an increase in Fusarium levels in the control samples and a decrease in all treated soil samples; markedly strong effects were measured for MgO/ (NH4)2SO4 combinations, reaching >90% reduction in Fusarium levels compared to Fusarium levels measured in the control groups at the end of the test.

Additionally, pot experiments reported below indicate that magnesium oxide shows nematocidal activity when acting alone, and that its mixture with ammonium sulfate, e.g., formulated as a cogranule, exerted enhanced action against a nematode attack. Soil infested with root-knot nematodes (which cause gall formation, or "knots", on roots) was added to pots and treated with MgO, (NH4)2SO4 and co-granules of MgO/ (NH4)2SO4 prior to planting with tomato seedlings. The treatment restrained the root galling index to less than 1, in fact, down to 0, thirty days after planting (akin to the standard system for root galling rating) . Accordingly, the invention is primarily directed to a method of controlling soilborne plant disease, comprising applying to soil magnesium oxide and/or magnesium hydroxide, preferably in combination with one or more compounds decomposable into ammonia in an aqueous environment.

Magnesium oxide is prepared industrially by calcination of magnesium carbonate, or by reacting magnesium chloride with an alkaline agent such as calcium hydroxide, sodium hydroxide, ammonium hydroxide or potassium hydroxide, to form magnesium hydroxide (Mg (OH) 2) , which undergoes calcination to afford magnesium oxide. Another approach is known as the thermal decomposition process, in which magnesium chloride hexahydrate is decomposed thermally with steam to give magnesium oxide. The oxide and hydroxide forms of magnesium are related compounds that can be interconverted. Magnesium oxide in the presence of water will slowly hydrate to form the hydroxide salt while heating the hydroxide salt above 500 °C will release water to give the oxide. Both minerals have negligible solubility in water and form a suspension when added to water. The suspension is mildly basic (pH 10-11) and is stable and homogenous when specific powder properties (particle size, wettability, porosity, and other physical attributes) of such minerals are present in the as- synthesized solid. Auxiliary additives, to stabilize the suspension, may be added. For example, we have found that medium activity MgO powder with >96% assay (e.g., >97% or >98%) achieves good results, e.g., MgO powder with loss on ignition (LOT; measured at 900°C) of less than 2% by weight, e.g., less than 1% by weight (LOT is indicative of Mg (OH) 2 levels) ; specific surface area in the range from 3 to 100 m²/g, e.g., from 5 to 80 m²/g, for example, from 5 to 50 m²/g, measured by the BET method (this range of BET surface area corresponds to medium activity MgO) ; and characterized by particle size distribution, measured by wet sieving, indicating that at least 85% of the particles, e.g., at least 90%, pass through 100 mesh screen (that is, the diameter of at least 85%, e.g., of at least 90% of the particles is less than 149 pm) and at least 60% of the particles, e.g., at least 65%, pass through 325 mesh sieve (that is, the diameter of at least 60%, e.g., of at least 65% of the particles is less than 44 pm) .

Compounds decomposable to release ammonia which can be used according to the invention to enhance the effect generated by magnesium oxide alone are water-soluble ammonium salts such as ammonium sulfate, ammonium chloride, and ammonium nitrate. Watersoluble organic compounds, organic salts such as ammonium carbamate can also be used.

Ammonium decomposition to ammonia is promoted by the mildly basic pH created by magnesium oxide in an aqueous environment. Thus, when magnesium oxide is combined in an aqueous environment with a compound decomposable into ammonia, the release of ammonia gas occurs. By way of example, the decomposition of ammonium sulfate in the presence of magnesium oxide in an aqueous environment is shown below:

MgO+H₂O Mg (OH) ₂

Mg (OH) ₂ + (NH₄)₂SO₄ MgSO₄ + 2NH₃? + 2H₂O

It is seen that the method of the invention for controlling soilborne plant disease involves the use of magnesium oxide (or hydroxide) alongside ammonium salts and the formation of magnesium salts (such as magnesium sulfate) and gaseous ammonia. These compounds are known fertilizing agents with a beneficial effect on soil and plant. Magnesium is the metal center of the chlorophyll complex (green pigment) . Ammonia released by the reaction serves as a fertilizer or exerts its action as a fumigant .

There are different ways to apply to the soil magnesium oxide alone, or a mixture consisting of magnesium oxide and ammonium salt(s) before planting. The components of the mixture can be applied simultaneously or successively, e.g., through addition to the soil of an aqueous suspension of magnesia and an ammonium salt in a solid form; magnesia in a solid form with an aqueous solution of an ammonium salt; both components delivered in solid forms or the two components suspended/dissolved in (separate) aqueous carriers, respectively. To achieve a good effect, the active compounds are placed/inj ected about 10 to 30 cm below the soil surface.

For example, magnesium oxide and ammonium salt can be distributed in the soil in solid forms, e.g., as powders and granules. Magnesium oxide is commercially available in powdery or granular forms from suppliers such as ICL IP. Granular ammonium sulfate is commercially available from ICL Fertilizers and can be prepared by reacting in a pipe reactor sulfuric acid with ammonia to give the product in the form of a hot melt which is discharged into a rotary granulator, dried, cooled and screened to collect granules with suitable size, as described in https ; //feeco ♦ com/photo-of - the-week - ammo n i u -sulfate - a r a nu 1 e s .

When used in solid forms, the magnesium oxide and the ammonium salt can be added to the soil separately, that is, not as a premix, either simultaneously or successively, say, first the magnesium oxide and subsequently, before irrigation, the ammonium salt. Results reported below based on a field experiment show that the composition of the invention was highly effective when the two components (MgO/ (NH4 ) 2SO4 ) were distributed separately, at the same time, without preblending the two solids. But agrochemical premixes are often easier to apply because the individual components are mixed in advance at the desired proportion by the manufacturer. Thus, the soil fungicidal, bactericidal, nematocidal and fumigant composition of the invention, can be commercialized as a suitably proportioned dry premix (a powder blend; MgO powder + granular (NH4)2SO4) .

The ratio of ingredients (MgO and e.g., (NH4)2SO4) may depend on the mode of application. For example, when the application to the soil is in the form of individual powders, a powder premix or as granules (MgO granules + (NH4)2SO4 powder, MgO powder + (NH4)2SO4 granules or MgO granules + (NH4)2SO4 granules) , then MgO would often be the predominant component. But when applied in the form of co-granules, then the ammonium salt would generally be the major component of the co-granule.

For example, according to the first case, MgO/ (NH4)2SO4 are proportioned from 1:1 to 10:1 by weight, e.g., from 1.5:1 to 8:1, or from 2:1 to 7:1. In a field experiment described below, the molar ratio MgO/ (NH4)2SO4 was examined over the range from 8:1 to 25:1, i.e., well above the 1:1 ratio dictated by stoichiometry. Satisfactory killing rates of fungi were measured already at the lower part of the range, e.g., from 2:1 to 3:1. However, high excess of magnesia may be needed to offset the poor water solubility of magnesia, which reduces its availability to the reaction with the ammonium salt that takes place below the surface of the soil. In the presence of high excess of magnesium oxide, the likelihood of residual, unreacted ammonium salt, that did not undergo decomposition to release ammonia, is small.

Stated otherwise, when applied in the form of individual powders, a powder premix or a blend comprising granules of each compound, MgO is used in a molar excess calculated relative to the number of moles of MgO needed to liberate one mole of ammonia from the ammonia-releasing compound under consideration, e.g., MgO is applied in a molar excess of not less than 2:1, e.g., not less than 3:1 or not less than 4:1. For example, from 8:1 to 25:1, e.g., from 10:1 to 18:1. Because it is applied in excess, only part of the magnesium oxide is consumed by the chemical reaction depicted above, with the concomitant evolution of ammonia to achieve the ammonia gas fumigation effect. Unreacted magnesium oxide that is left in the soil has a twofold function: it continually acts on soilborne pathogens on its own, generating a long-term effect, and owing to its poor water leachability, there will be magnesium oxide left behind that will be available to advance ammonia gas fumigation upon periodic supply of water- soluble ammonium salts to the soil.

Accordingly, another aspect of the invention is a soil fungicidal, bactericidal, nematocidal and/or fumigant composition, comprising a mixture of magnesium oxide and ammonium salt(s) , such as ammonium sulfate.

For example, the composition MgO: (NH4)2SO4 may be proportioned as described above, e.g., at a molar ratio of not less than 2:1, e.g., not less than 3:1 or not less than 4:1, for example, from 8:1 to 25:1, or from 10:1 to 18:1, when formulated as discrete particles of each component (blend of powders, blend of granules of each type) .

Alternatively, the composition is formulated as co-granules comprising MgO and ammonium salt, e.g., (NH4)2SO4. Thus, cogranules comprising magnesium oxide and ammonium salt decomposable into ammonia in an aqueous environment, form another aspect of the invention. The co-granules can be prepared using conventional granulators, e.g., feeding MgO powder and ammonium salt powder/granules and applying pressure (by roller compactor) in the range of 5 to 15 MPa, e.g., 5 to 10 MPa. The mixing ratio by weight is from 1:9 to 1:2, e.g., 1:7 to 1:3 [MgO: (NH4)2SO4] . The granules can be sieved, for example, to collect the fractions of >2 mm, < 2mm, 1-2 mm, 0.5-1 mm, 0.25-0.5 mm, and <0.25 mm. Each of the individual fractions can be used, or the fractions may be combined. For example, granules of size < 2 mm, granules of size above 1 mm (from 1-2 mm) and granules consisting of fine particles of submillimeter size (from 0.25 to 1 mm) were shown to give good nematocidal effect in pot experiments reported below.

As pointed out above, magnesium oxide has very poor solubility in water whereas ammonium salts are very water-soluble and can be easily formulated to form a sprayable aqueous solution. That is, by adding ammonium salts to a spray tank (as a powder, water dispersible granules or tablets) , which dissolve rapidly in the tank mix to achieve the desired ammonium salt solution strength for application. As an example, the slow addition of the ammonium salt solution through the irrigation system installed in the field to a magnesia-containing soil will drive the slow release of ammonia to achieve a beneficial fumigation effect.

Magnesium oxide creates a flowable, sprayable suspension in water when added to water in an amount from 20 to 150 g/liter and the invention includes the application of such a suspension to the soil, to supply to the soil magnesium oxide either as a sole active ingredient or in combination with the solid or aqueous forms of the ammonium salt that were described above. For example, simultaneously with the delivery of a suspension of magnesium oxide to the soil, solid forms of ammonium salts are distributed as they readily dissolve by the action of the applied slightly alkaline MgO suspension. The rate of application of the co-granular product described above is from 50 to 500 kg per hectare, e.g., from 50 to 200 kg per hectare. The rates of application of the individual components, when applied separately, may be calculated according to the ratios mentioned above. The application rates are influenced by the types (s) of soilborne pathogens that exist in the soil and the severity of the infestation, and by other factors, such as the alkalinity of the soil upon exposure to the MgO or MgO/ammonium salt mixtures. Field experiments have shown that the application of MgO or MgO/ammonium salt mixtures increases the alkalinity of the soil in a tolerable manner, e.g., up to ~10, without causing damage to the crop; a normal pH is restored over time.

The method of the invention offers effective management of diseases caused by fungi, e.g., the fungal genus Fusarium, (including species such as Fusarium oxysporum, Fusarium solani and Fusarium culmorun) ; the fungal genus Rhizoctonia; soil bacteria such as Streptomyces and root-knot nematodes such as Meloidogyne incognita.

In the drawings

Figure 1 is a bar diagram showing the pre-treatment level of Fusarium in the field study (the field was divided into plots) .

Figure 2 is a bar diagram showing the results of the field study where mixtures of magnesium oxide and ammonium sulfate were tested to control Fusarium.

Figure 3 is a photograph showing an arrangement of pots used in the pot experiments where magnesium oxide, ammonium sulfate and their mixtures (formulated as co-granules) were tested to control root-knot nematodes.

Figure 4 is a photo with a comparison of plant roots - control on the left and MgO, 10% on the right side. Examples

Materials

Magnesium oxide, Technical Grade (>96%) , with the following chemical and physical properties was used (% are by weight) :

Table 1

Ammonium sulfate was obtained from ICL Fertilizers.

Examples 1 to 3

Control of soil bacteria: a laboratory study

A series of experiments was carried out to examine the antimicrobial effect of magnesium oxide alone (Examples 1A-1E) , ammonium sulfate alone (Examples 2A-2E) and mixtures thereof (Examples 3A-3E) on the microbial growth of Streptomyces scabiei 33281 (a soil bacteria) in agar plates.

100 ml samples were prepared by adding suitable amounts of MgO, (NH4)2SO4 or their mixtures as tabulated below to a flask that was previously charged with distilled water, a surfactant (Tween-80) at a concentration of 0.1 wt . % and the suspended bacteria. 1 ml of the sample was transferred to a hot (~45°C) agar under stirring. The hot liquid suspension was poured into Petri dishes and cooled to afford a solid agar gel containing the compound (s) being tested. The dishes were incubated for 48 hours (at 26°C) . The tested concentrations of the compounds and the results (microbial counts and log reduction relative to the control sample) are tabulated in Table 2.

Table 2

The results indicate that magnesium oxide has antimicrobial action on its own, achieving bacterial eradication at a concentration of 3% (Example IE) . Ammonium sulfate had no effect on the bacteria at the concentration range tested (0.2-0.6 %) . The mixture MgO (2.5%)+ (NH4)2SO4 (0.5%) exhibited a synergistic effect (Example 3D compared to Examples ID and 2D) . Example 4 Control of soil fungi : a field study

A field infected with a pathogenic fungus of the genus Fusarium oxysporum was divided into rectangular plots of equal size (about 2m x 10m each) . The plots were randomly assigned to control and treatments (two control groups each consisting of six plots and twenty-four plots equally divided between four different types of treatments) . The plots were accordingly labeled with the numbers 1 to 6; Table 3 explains the meaning of each label (i.e., control and types of treatments) .

Table 3

Before the experiment (t=0) , soil samples were collected from all thirty-six plots. Samples assigned to the same treatment group (2, 3, 4 or 5) and control groups (1 and 6) were mixed. Fusarium levels in the mixtures (total of six mixtures) were determined, expressed as CFU (colony-forming unit) per gram of soil. Quantification of Fusarium was by seeding of 1:400, 1:1600, and 1:6400 dilutions on a selective Nash-Snyder substrate and incubation for 4 days at 25 °C. Sampling was carried out at a depth of 0-30 cm, and each of the six soil mixtures was tested in the three dilutions.

The results are shown in the form of a bar diagram in Figure 1. The lines represent a standard error of the mean. The average Fusarium level in the soil in the field was 11800 units per gram of soil.

On the next day, MgO + (NH4)2SO4 mixtures were applied to the soil in the twenty-four plots according to the experimental design explained above. Note that the mixtures were applied in two ways: either as a premix prepared beforehand (treatments labeled 2, 3 and 4) or by delivery of the individual components from separate bags (treatment labeled 5) . The soil was loosened by tilling to a depth of 30 cm and well moistened (to a depth of 10 cm) by uniformly delivering to the soil 10 m³ of water. In the next fourteen days, a standard pre-planting irrigation program was applied, consisting of watering the field with 15 m³ of water once in two days using sprinklers.

After fourteen days (t=14) , soil samples were taken from each plot, for the determination of pH and the amounts of pathogen that survived each treatment.

Effect of the treatment on the pH of the soil pH was measured as follows. Soil samples were taken from each of the six plots of each treatment and mixed to form a blend assigned to the treatment labeled 2 to 5. Likewise, soil samples taken from each of the twelve plots of the control groups were mixed to form a blend assigned to the control group labeled 1+6. The soil blends were dried under the sun for three days, manually grounded, and sieved to a size of 2 mm. The sieved soil (5g) was suspended in water (25 mL) by stirring (magnet bar) for 30 min, filtered on Buchner, and the pH of the aqueous filtrate was measured by a calibrated pH meter equipped with a thermometer. pH measurements were taken on t=14 days and a month later, on t=45 days. The results are tabulated in Table 4. Table 4

The increase in soil alkalinity caused by the addition of magnesium oxide to the soil is tolerable, about two pH units compared to the control group.

Examination of pH variation with the passage of time indicates a slight increase in the control groups, apparently due to the leakage of alkaline agents from adjacent plots. As to pH variation in plots assigned to the treatments, an average decrease of 0.5 pH units was observed, indicating that normal soil pH levels are restorable with the passage of time. Planting could be scheduled accordingly, until a nearly neutral (from 7 to 8) or slightly alkaline (from 8 to 9) pH is restored. Perhaps worthy of note is the difference between the treatments labeled 3 and 5, which quantity-wise are the same, but involve different application modes. The application of MgO/ (NH4)2SO4 as a premix (treatment 3) resulted in a lower pH compared to the pH created following separate applications of MgO and (NH4)2SO4 to the soil.

Effect of the treatment on Fusarium counts in the soil

As indicated above, the average count of Fusarium across the field before the treatment was 11800 units per gram of soil.

Post-treatment Fusarium quantification was done by sampling each of the plots at a depth of 0-30 cm and measuring Fusarium in each plot separately. The method described above was applied, i.e., by seeding at dilutions of 1:400, 1:1600, and 1:6400 on a selective Nash-Snyder substrate and incubation for 4 days at 25 ° C . Each plot was tested in the three dilutions .

The results are shown in the form of a bar diagram in Figure 2 . I f a plot showed high deviation from other plots of the same treatment (by one order of magnitude ) , then it was excluded and only the counts obtained from the other five plots were taken into consideration . The first observation to be made is that the average Fusari um in the control groups ( labeled 1 and 6 ) , i . e . , the post-treatment count , was 12915 - slightly above to the initial (pre-treatment ) average count of 11800 units per gram of soil .

To assess the ef fect of the treatment , average Fusari um counts determined for each of the treatments labeled 2 to 5 at t=14 were subtracted from the average count measured at t=14 in the control groups . The results , expressed as percentage change , are tabulated in Table 5 and shown in Figure 2 .

Table 5

one plot was excluded due to high deviation

The results indicate that suitably proportioned mixtures of magnesium oxide and ammonium sul fate used in treatments 2 to 5 exhibit potent action against fusarium . Example 5 Co-granulation of magnesium oxide and ammonium sulfate

A blend consisting of MgO powder (see Table 1) and (NH4)2SO4 (from ICL Fertilizers) , proportioned 130 parts to 870 parts, respectively, was co-granulated in a roller compactor (dry granulator GL2-25 by Create, China) .

The granulator was set to operate at the following parameters: Feeder speed - 15 RPM

Roll speed - 8-10 RPM

Roll pressure - 85 kg/cm²

Granulation speed - 30 RPM

Screen size - 5mm

Current - 1.9-2.3 A

The individual components were first mixed manually, and the blend was introduced into the granulator via the feeder. The co-granules were collected, and the process was repeated until a constant bulk density was reached. The granules were sieved to collect the following fractions: >2 mm, 1-2 mm, 0.5-1 mm, 0.25-0.5 mm, and <0.25 mm. Fractions consisting of particles greater than 1 mm were combined, and fractions consisting of particles sized in the range between 0.25 and 1 mm were combined. The two fractions were tested in the pot experiments reported in the next example.

Example 6

Control of nematodes : pot experiments

The experiments were performed in pots that were filled with loess soil collected from a greenhouse which was severely infested with Meloidogyne incognita, a root-knot nematode species that attacks cucumber, tomato and other plants. The treatments are tabulated in Table 6; the control group included fifteen pots that received no treatment. The additives applied to the soil consisted of: A) MgO/ (NH4)2SO4 co-granules of Example 5, fine fraction;

B) MgO/ (NH4)2SO4 co-granules of Example 5, coarse fraction;

C) MgO powder; and

D) NH4SO4 granules.

For each of the tested additives A-D, three concentrations were studied :

1) 5 g additive per liter soil;

2) 10 g additive per liter soil; and

3) 50 g additive per liter soil.

The treatments are labeled (Al, A2, A3) , (Bl, B2, B3) , (Cl, C2, C3) and (DI, D2, D3) , with five replicates (pots) per treatment, and therefore a total of fifteen pots for each of the A, B, C and D groups. Figure 3 shows the set-up of fifteen pots assigned to one of the groups. The parameters of the experimental protocol are tabulated in Table 6.

Table 6

The soil was collected from a Meloidogyne incognita (root-knot nematode) infested greenhouse where cucumber plants were grown. The soil was thoroughly mixed in a concrete mixer, to ensure uniformity in composition and texture, before it was divided into the treatment and control groups. Two days later, the tested additive was mixed in an appropriate amount with the soil, in a 3 L volume tank for each treatment. The amount allocated for each treatment was then divided into five repetitions (pots) . The pots were irrigated with about 200 cm³ of water per pot, until a leakage from the bottom of the pot was observed. An irrigation program was initiated two days later, supplying water daily to the pots over a period of three weeks, in an amount sufficient to keep the soil wet.

During the pre-planting three weeks period, the pots were examined once weekly to determine the efficacy of the treatments in controlling sprouts. Sprouts (presumably Setaria) were observed in the pots of the control group, but were effectively restrained by the Al, A2, A3, Bl, B2, B3, DI, D2 and D3 treatments and to good extent also by the Cl, C2 and C3 treatments. Dicotyledons were observed to grow in all the pots assigned to the control group. In the case of the A2 and B2 treatments, dicotyledons were observed in just two pots. The B2 treatment was also quite effective in surpassing the development of the typical green layer caused by algae that is often seen on a surface layer of the wetted soil.

After the lapse of the three weeks period, tomato was transplanted (variety named Ikram) in the pots (two seedlings in each pot) . The day of planting is designated herein DAT=0, and days that follow are respectively marked DAT=N. The experiment was ended one month later (DAT=30) . During the post-planting one month period, phytotoxicity was evaluated on DAT= 2, 5 and 15. No phytotoxicity effects were observed, except for the D2 (to some extent) and D3 (all plants died at DAT=2) treatments.

On the last day of the experiment, the overground parts of the plants were trimmed, measured, and weighed. Root systems were thoroughly washed and examined to determine 1) the percentage of plants showing root galling and 2) level of gall formation (expressed by the 0-10 index) . The results of the pot experiments are tabulated in Table 7.

Table 7

The results indicate that almost all treatments were non- phytotoxic to the plants (except for the high dose (NH4)2SO4 D2 and D3 treatments) . Another observation worth making is about the C3 treatment, which caused changes in the texture of the soil, resulting in the formation of hard soil.

As to the nematocidal action of MgO, (NH4)2SO4 and their co- granular mixtures, the results show that they all achieved significant reduction in the frequency and severity of galls formed along the plant roots (the characteristic damage caused by root-knot nematodes) . The coarse-grained co-granular mixture of MgO / (NH4)2SO4 (group B) emerged as the most efficient soil additive .

It is worthy of note that MgO alone shows efficient activity in controlling root-knot nematodes. Another observation about the application of MgO alone relates to the development of the root system. The roots of individual plants grown in a soil that was treated with MgO powder (group C, consisting of the Cl, C2 and C3 treatments) did not intertwine with one another, such that the root systems of individual plants in each pot were separable from one another by the end of the experiment. In contrast, the roots of the individual plants in the pots of the control group and the other treatments (A, B and D) were intertwined with one another. Figure 4 shows the differences between the inseparable roots of plants of the control pots (left side of the figure) and the separated roots of plants of the C group.

The data in Table 6 suggests that MgO and (NH4)2SO4 act synergistically. A nematode control program incorporating the application to soil of a suitably proportioned co-granular mixture of MgO and (NH4)2SO4 could benefit from their nematocidal action but without the undesired effects caused by high-rate applications of these ingredients (hard soil in the case of an excessive amount of magnesia and phytotoxic effect in the case of an excessive amount of (NH4)2SO4) . The co-granular mixtures tested were proportioned ~1:7 by weight [MgO : (NH4)2SO4] , so an application rate of 10 g co-granules per liter soil corresponds to application rates of 1.25 g/liter and 8.75 g/liter of MgO and (NH4)2SO4, respectively. These amounts were lesser than the corresponding amounts of the separately applied MgO and (NH4)2SO4.

Claims

1) A method of controlling soilborne plant disease, comprising applying to soil magnesium oxide, magnesium hydroxide, or both.

2) A method according to claim 1, wherein magnesium oxide possessing BET surface area of less than 100 m²/g is applied to the soil.

3) A method according to claim 1 or 2, comprising applying magnesium oxide in combination with one or more compounds decomposable into ammonia in an aqueous environment.

4) A method according to claim 3, wherein the compound decomposable into ammonia is selected from the group consisting of water-soluble ammonium salts.

5) A method according to claim 3 or 4, wherein the magnesium oxide is added to the soil in a solid form or as an aqueous suspension, and the ammonium salt is added to the soil in a solid form or as an aqueous solution.

6) A method according to claim 5, wherein the magnesium oxide and the ammonium salt are added in the form of co-granules.

7) A method according to any one of claims 4 to 6, wherein the water-soluble ammonium salt is ammonium sulfate.

8) A method according to any one of the preceding claims, wherein the soilborne plant disease is caused by a soilborne pathogen which is fungi and/or bacteria and/or nematode. 9) A method according to claim 8, wherein the soilborne fungus is selected from the group consisting of Fusarium, Rhizoctonia and Verti cilli urn.

10) A method according to claim 8, wherein the soilborne bacteria is Streptomyces .

11) A method according to claim 8, wherein the soilborne nematode is a root-knot nematode.

12) A soil fungicidal, bactericidal, nematocidal and fumigant composition, comprising a mixture of magnesium oxide and an ammonium salt decomposable into ammonia in an aqueous environment .

13) A soil fungicidal, bactericidal, nematocidal and fumigant composition of claim 12, in the form of co-granules.

14) A soil fungicidal, bactericidal, nematocidal and fumigant composition of claim 12 or 13, comprising magnesium oxide and ammonium sulfate.

15) Co-granules comprising magnesium oxide and ammonium salt decomposable into ammonia in an aqueous environment.

16) Co-granules according to claim 15, wherein the ammonium salt is ammonium sulfate.

17) Co-granules according to claim 16, wherein the weight ratio [MgO: (NH4)2SO4] is in the range of 1:9 to 1:2.

18) Co-granules according to claim 17, wherein the weight ratio [MgO: (NH4)2SO4] is in the range of 1:7 to 1:3. 19) Co-granules according to any one of claims 15 to 18, wherein the size of the granules is <2mm.