• During cultivation

In the nursery, it is urgent to impregnate the entire substrate with an anti-oomycete fungicide solution ( e-phy Pythium ) ( e-phy Phytophthora ). It is also necessary to limit irrigations if they have been too abundant. Good wiping of the substrate will be favored after sprinkling irrigations. If the seedlings are produced under shelter, it is advisable to promote the ventilation of the latter as much as possible and subsequently to control the climate of the greenhouse in order to avoid excess humidity. Diseased seedlings and those nearby should be removed.

During cultivation and following root attacks, phytosanitary interventions are different depending on the context:
- in soil , generally few plants are affected and no treatment is to be applied;
- out of soil , fungicides can be added in the nutrient solution or locally by watering at the foot of the plants. The doses chosen must be lower in order to avoid phytotoxicity.

We should add that leaf attacks, occurring mainly in the open field, are normally well controlled by anti-mildew treatments .

Several fungicides * are commonly used in the world to combat these chromists, the active ingredient chosen will vary depending on the target organ ( e-phy Phytophthora , e-phy Pythium ).

These fungicides would prevent the formation of zoospores at low concentrations and kill these chromists at higher doses.

Note that you will benefit from alternating products belonging to different chemical families and having dissimilar modes of action. Their sexual reproduction is frequent, it is at the origin of a certain variability. This allows these chromists to adapt to their environment, and in particular to certain factors exerting a certain selection pressure (for example to fungicides). There is therefore a risk of selecting strains resistant to some of these fungicides. By way of example, strains of Pythium spp. tolerant to metalaxyl and furalaxyl have already been reported (especially in Pythium aphanidermatum ).

Maintain balanced fertilization and avoid stressing the plants.

Diseased plants and plant debris should be carefully removed during and at the end of cultivation, especially root systems.

• Next crop

In the nursery, the substrate used must be healthy or disinfected . In countries where producers make it themselves, for example from sand, reclaimed earth, various composts, there is a risk of contamination. The same goes for producers who mix their purchased substrate with the aforementioned ingredients. The clods will not be placed on the ground, especially if the latter has not been disinfected. It will be better to place them on plastic wrap or on shelves. In contaminated soils that have not been disinfected, the seedbed can be soaked preventively with an anti-Oomycetes fungicide solution ( e-phy Phytophthora , e-phy Pythium ). Hygiene measures in the nursery can be implemented. In some countries the seeds are coated with an anti-Oomycete fungicide.

In soil cultivation, crop rotations will be carried out with cereals and fodder grasses. For example, in the case of Phytophthora capsici, rotations of at least 3 years, without a susceptible host, are recommended. Heavy and wet soils will be drained. We will add organic matter to lighten them. The tomatoes will be planted on mounds in order to avoid water retention at the foot of the plants. The soil can be mulched. Fertilization should be balanced. Note that animal composts limit the actions of Phytophthora nicotianae .

In some particularly affected soils, disinfection with a fumigant will be considered. The active ingredients conventionally used (metam-sodium, dazomet, propamocarb HCl) and steam are effective against Pythium spp. and Phytophthora spp. In production areas where it is possible, solar disinfection of the soil (solarization) will be carried out. Quite spectacular results have been recorded, especially in certain Mediterranean countries: the soil to be disinfected is carefully prepared and moistened, then covered with a polyethylene film 35 to 50 µm thick, held in place for at least one month at a period of very sunny year. This economical method, with a broad spectrum of efficiency, makes it possible to control Oomycetes.

When planting, avoid planting seedlings in too wet or cold soils. The irrigations carried out at this stage of the crop should not be excessive. The plants can be led on grids, staked, in the presence of a plastic mulch, in order to avoid contact of the fruits with the soil at the end of cultivation.

Sprinkler irrigations should not be too abundant and will not keep the soil moist for too long.

In soil-less cultivation, before any cultivation, the irrigation circuit must be emptied and disinfected. It will also be in your best interest to replace the growing medium each year.

It will be necessary to be wary of the sanitary quality of the water used for the preparation of the nutrient solution and the irrigation of the plants, especially if it comes from an irrigation canal, a watercourse, a basin, etc., which may have been contaminated. If the nutrient solution is recycled, it can be disinfected. Several methods can be used to purify it: chlorination, iodination, ozonization, biofiltration, UV rays, pasteurization, TiO photocatalysis ₂ ... If the spectrum of effectiveness of these methods seems broad enough and integrates several fungi present in soilless cultures, and in particular these chromists, they do not all seem to be entirely satisfactory. Some of them reveal a very partial effectiveness and sometimes drawbacks for the production systems such as interference with mineral nutrition, in particular ferric. The most effective methods would be ultrafiltration and heat disinfection. Remember that 5 ppm bleach has sometimes been used to disinfect irrigation water.

It should be noted that monogenic resistance to Phytophthora nicotianae ( P. parasitica ) has been demonstrated in India in commercial lines. No variety sold in France seems to have this property. It should be added that sources of resistance to P. capsici have been found by a Russian team in wild species Lycopersicon and in tomato lines, in particular CRA66.

Certain antagonist microorganisms have been tested to control some species of these chromists: Trichoderma harzianum, T. virens, Gliocladium virens, Pythium oligandrum, Pseudomonas fluorescens, Bacillus subtilis, Enterobacter cloacae, Burkholderia cepiacia e…

Their use is already effective in some countries.