Biology, epidemiology

• Conservation, sources d'inoculum
  

The way in which the tomato strains of Phytophthora infestans stored are is sometimes difficult to understand, in particular in countries where winter is freezing. It seems to able to be be preserved in different forms depending on the production areas:
- through oospores resulting from sexual reproduction ; these are formed in diseased necrotic tissue and are subsequently found in the soil, along with plant debris (Figures 1 and 2). As we specified previously, for more than ten years now, new strains of the two sexual types A1 and A2 have been detected quite commonly in several countries of North America and Europe, as in France on potato and tomato. Type A2 is well represented on this last plant, more than on potatoes. Thus, P. infestans must now be able to complete its complete sexual cycle on tomato and form oospores capable of being preserved in the soil for several months or even years. Note that these preservation organs can be observed on leaflets, fruits, and occasionally on seeds. Moreover, this sexual reproduction can be at the origin of new genotypes possessing new biological properties which can call into question the effectiveness of the struggles, in particular chemical and genetic;
- in the form of mycelium inside diseased potato tubers remaining in the soil. When the contaminated tubers germinate, they give rise to infected shoots on which fruiting bodies quickly form. The importance of this inoculum is difficult to estimate. In fact, the strains of P. infestans isolated on tomato in France are quite different from those found on potatoes, confirming a parasitic specialization of the strains. The tomato strains seem more diversified and are able to also infect potatoes . We cannot therefore exclude the possibility, even limited, for the parasite to change host, especially in production areas where tomatoes and potatoes coexist;
- on various other alternative hosts , surrounding cultivated or wild plants, on which this chromist sporulates more or less: many Solanum (eggplant, red pepper, potato, pepino, black nightshade, Solanum incanum, S. indicum ) , Datura stramonium, D. metel , several species of Hypomea, Lycium hamilifolium, Nicotiana glauca, Petunia sp. and Physalis angulata , on blueberry (in Cameroon). On these hosts, P. infestans produces fruiting bodies consisting of long sporangiophores bearing sporangia. The zoospores resulting from the latter often provide the primary contaminations in tomatoes. In addition, the presence of tomato crops practically all year round in certain production areas allows the chromist to easily stay there from one year to the next. Regrowths and natural seedlings also ensure the survival of P. infestans .

• Penetration and invasion

As previously reported, once on the leaf blade, the sporangia release flagellated zoospores . The optimum temperature for their release is of the order of 13 ° C . These zoospores, once fixed, emit a germ tube which enters the limbus mainly via the stomata but also, sometimes, directly through the cuticle and epidermal cells. The sporangia can also directly give rise to a germinal filament. The infection would be realized in 3 to 4 hours . The leaf tissues are then rapidly invaded by the mycelium non-partitioned (the optimum growth temperature is 23 ° C ), the activity of which gradually disorganizes the colonized tissues.

If the weather conditions are favorable, the first spots appear between 4 and 7 days after the first contaminations.

• Sporulation and dissemination

Once installed in the host, P. infestans emits sporangiophores through the stomata (Figure 3), sometimes directly through the epidermis. These organs produce numerous sporangia citriform , several thousand per spot (24,000 / cm²) . This step requires the presence of a high humidity (relative humidity equal to or greater than 90%) and temperatures between 3 and 26 ° C . The sporangia are easily carried away by wind and rain, sometimes over long distances (several hundred meters) and reach new, still healthy plants, ensuring secondary contamination. Zoospores can also perform the same function, but more locally and essentially in the aqueous phase.

The disease is sometimes spread through infected plants. In some countries, farmers have specialized in producing large quantities of seedlings which they market in different production areas around the world. Contamination of plants sometimes goes unnoticed; they are then shipped sick to distant producers, thus contributing to the spread of the pathogen and the early development of the disease in crops. If seed contamination with oospores is confirmed in the field, they could disperse the disease over long distances.

• Conditions favorable to its development

This chromist is extremely influenced by climatic conditions. It grows more or less well at temperatures between 3 and over 25 ° C. Its sporulation is optimally between 16 and 22 ° C . It imperatively needs high relative humidity, greater than 90% . Cold nights and moderately hot days, with high humidity, favor its extension. On the other hand, a dry atmosphere and temperatures close to 30 ° C inhibit it. Rainy periods, sprinkler irrigation and dews are also very favorable to mildew epidemics. It only takes 2 hours of water on the leaves to start an infection. The production of sporangia is high at 18 ° C, it is zero at 28 ° C. The oospores are formed in quantity between 8 and 15 ° C; their production requires the presence of permanently high humidity and humidity.