• During cultivation

As the spread of late blight is very rapid , it is necessary to react quickly when the very first symptoms are observed, especially if no preventive treatment has been carried out. An anti- treatment should therefore be carried out as soon as mildew fungicide possible .

Several fungicidal active ingredients * are used around the world and in France, alone or in combination, to control Phytophthora infestans ( e-phy ).

* Chemical families particularly concerned by resistance phenomena: Anilides, Strobilurines.
Note that for multisite fungicides to be relatively effective, they must be applied preventively and every week, especially in the open field. Despite their limited effectiveness over time, they still have the advantage of being quite versatile and not being affected by resistance phenomena .
This is not the case with certain unisite fungicides (such as, for example, those of the anilide family) which are rarely used alone, often combined with each other and / or with multisite fungicides in order to limit the risk of the appearance of resistance. It should be remembered that curative treatments, with relative effectiveness, further promote the appearance of strains resistant to fungicides. In addition, we expressly invite you to alternate fungicides with different modes of action . With certain products, it will not be necessary to carry out more than 2 to 3 applications per campaign, and not to intervene on declared attacks.

In the nursery, we can try to contain the first outbreak (s) by treating them with a more concentrated dose of a fungicide and eliminating the affected seedlings. Plants produced under such conditions will only be marketed if the mildew attack has been completely contained and by warning the buyer producers of the risks involved.

In addition to fungicide treatments, a number of prophylactic measures should be applied.

In the nursery, the shelters will be ventilated as much as possible in order to reduce their humidity. It will be advantageous to avoid irrigations by sprinkling or by misting . If we cannot do otherwise, they should not be done at the end of the day and in the evening, but rather in relatively warm periods so that the plants have time to dry out before dark.

In cultivation, we will adopt the same irrigation recommendations as those suggested in nurseries. In a greenhouse, it may be necessary to heat in order to reduce the ambient humidity. Everything will be done to avoid the presence of a film of water on the plants. At the end of the crop, as much plant debris as possible should be removed quickly. The residues will be buried deep in the soil to promote their rapid decomposition.

• Next crop

The following nursery will be set up in a sunny location, in no case damp and shady. If it takes place in the same shelter as the previous year, it is advisable to apply hygiene and disinfection measures .

The quality of the plants must be checked on arrival, especially if they come from nurseries located in countries or production areas in temperate climates. If you have the slightest doubt, we advise you to carry out an insurance fungicide treatment.

Of crop rotations of at least 3 years is recommended. New plantings should not be made near crops of tomato or other cultivated nightshade already affected. The tomato should not be planted in poorly drained plots where strong water retention occurs and in soils that are too rich in organic matter. The manures made must be balanced, in no case excessive. In countries or regions where this is possible, planting densities can be reduced in order to have more aerated plots, in which the humidity within the plant cover will be lower. If possible, the planting mounds and / or rows should be oriented in the direction of the prevailing winds in order to promote maximum aeration of the vegetation. Sensitive weeds will be eliminated from the plots and their environment.

The mulching and, to a lesser extent, staking contribute to reduced development of mildew. In addition, for extensive crops, inter-row maize production with tomatoes would have reduced both the incidence of late blight and early blight in El Salvador. It would be the same with sesame and soybeans in Uganda. In addition, plants fed with organic manure rather than mineral manure would be less susceptible to mildew. This would be related to the metabolism of nitrogen.

A number of other health measures , used alone but especially in combination, reduce the progression of late blight epidemics:
- the elimination at regular intervals of the attacked leaves (this measure would interfere with the good growth of the plants);
- the use of plastic covers also protecting the plants from bad weather;
- reduction in plant density .

Given the rapid progression of late blight and the risks it poses to the crop, preventive fungicide treatments will be essential in the nursery, as well as during cultivation. The products used and the rates adopted will be defined with the technician according to local cultivation practices. It should be remembered that the time between two treatments should not exceed 7 to 12 days depending on the active ingredients. Care will be taken to alternate fungicides with different modes of action in order to limit the selection of resistant strains.

It should be noted that several prediction models or decision support systems have been developed, in particular in temperate zones, in order to better position fungicide applications ( e-phy ), improve their efficiency and reduce their number: Hyre system (based on temperature and rainfall), Wallin system (based on temperature and humidity), but above all Blitecast, which integrates the two systems in a computer program. The interest of these tools is no longer to be demonstrated. Other systems are reported in the literature: Phymet in Italy, the MacHardy system, developed in New Hampshire in the United States and the Maschio and Sampaio systems developed in Brazil in particular.

Furthermore, it should be noted that experiments have taken place in the United States in order to be able to detect mildew by spectral analysis of plants, with the aim of better managing the use of fungicides. In countries where such services exist, account should be taken of agricultural warnings issued fairly regularly by the plant protection service or by other bodies.

Several genes , all from Lycopersicon pimpinellifolium but whose efficacy has been questioned by various races, have been used to control P. infestans in tomato:
- " Ph-1 " (present on chromosome 7), the resistance of which was quickly bypassed by the T-1 race;
- “ Ph-2 ” (present on chromosome 10) and resulting from accession WVa 700. It is now available in F1 hybrid varieties and confers partial resistance. Strains virulent with regard to this gene are also cited in the literature;
- " Ph-3 " (present on chromosome 10) obtained from accession L3708. This gene would control more isolates.

If we are interested in the races of P. infestans , it seems that the differentiation between them can only take place on the leaves and stems of the tomato. Indeed, the fruits would be capable of harboring all the strains. The T0 race appears mainly on tomato varieties lacking resistance genes. The T1 breed is said to be more aggressive than the T0 breed. Note that work carried out in China and using 5 tomato genotypes (Ts19, Ts33, WVa 700, LA1033, L3708), would have made it possible to distinguish 8 different races: T0, T1, T1-2, T1-2-3, T1-2-3-4, T1-2-4, T1-4 and T3.

Some varieties expressing a partial resistance to late blight are therefore available in many countries and especially in France. Although representing a real asset for tomato cultivation, this plant material should be used in conjunction with other protection methods, in particular with complementary chemical control. This will limit the risks of the emergence of the breeds described above, and therefore contribute to the sustainability of these resistances. For the same reasons, under no circumstances should mixed crops of resistant varieties and susceptible varieties be carried out and the proximity of susceptible varieties should be avoided.

A certain number of micro-organisms and plant extracts (rosemary, lavender, thyme, fennel, etc.) have been tested against P. infestans . Some of them have shown some efficacy in vitro. Unfortunately, placed in the conditions of production in the field and faced with the aggressiveness of the agent responsible for late blight, they have often proved incapable of controlling the latter's epidemics. Soils resistant to this chromist have also been described in Mexico, in the Toluca Valley. This property would be conferred by the biological activities of several microorganisms ( Pseudomonas sp., B urkholderia sp., Trichoderma sp.).


* Chemical control : As the number of pesticides available for a given use is constantly changing, we advise you to always confirm your choice by consulting the e-phy site of the Ministry of Agriculture and Fisheries which is an online catalog of plant protection products and their uses, fertilizers and growing media approved in France. This also applies to all biological products based on microorganisms or natural substances.