Phytophthora  spp.,  Pythium  spp.

Oomycete rots

General

• Microorganisms recently reclassified in Straménopila * among which several agents of mildew are found.
• Widely distributed worldwide, reported on many vegetables in many countries on all continents.
• Rather polyphagous microorganisms; several species of Pythium and Phytophthora are listed in vegetable species, and in particular on eggplant. Distrust, some may come to colonize secondarily tissues damaged by a primary invader.
• Responsible for damage throughout the eggplant production cycle, both in nurseries on seedlings and in crops on adult plants : damping off, rots on roots and crowns. they can also act in complex with other soil pests associated with root dieback.
• Their parasitism is materialized by the presence of non-septate mycelium, oospores and sporangia in the altered tissues.
• Interact in particular with root-knot nematodes and parasitic fungi of the roots and crown.
• Observed in nurseries, in the open field, but especially under shelters.

* There are about 800 saprophytic or parasitic species of Oomycetes which have long been classified as Phycomycetes or “lower fungi” (Eumycetes). This classification was revised a few years ago because the ultrastructure of these microorganisms, their biochemistry and their molecular sequences indicated that they belonged to the Chromists , including mainly algae (green and brown), diatoms. Currently, depending on bibliographic sources, they can be associated either with the reign of the Chromista (Index fungorum), or with the reign of the Stramenopila (Tree of life).

• Organs attacked  : roots, crown, vessels, leaves, fruits
• Symptoms :
  • Damping -off (no germination, wet and brown lesions on the roots and or at the crown; wilting, collapse and death of seedlings.
  • Browning and / or rotting of taproots and lateral roots in soil and in soil ( root rot ). Browning, decomposition and disappearance of rootlets and roots of small diameter. The cortex of the main roots and the taproot also eventually rot more or less locally. The vascular tissues opposite the altered areas eventually turn brown (xylem).
  • Moist lesions initially dark green gradually encircling the neck, gradually turning brown and necrotic.
  • Yellowing and / or leaf wilting more or less rapid and marked, reversible at first.
  • Sometimes poor growth of plants and fruits of reduced size.
  • Brown and damp lesions surrounding the stems, damp rot on leaves and on eggplant fruits.

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• Signs  : white cottony, airy mycelial felting more or less covering lesions in humid conditions. Sporangia, oospores, chlamydospores can be observed with a light microscope in degraded tissue.
• Confusions possibles  : Rhizoctonia solani ,  Sclerotium rolfsii , etc.
• P ain species reported  :  several species of Pythium ( P. aphanidermatum , Pythium myriotylum ... ), Phytophthora nicotianae , Phytophthora capsici ...

Biology

• Conservation : able to live in a saprophytic state at the expense of organic matter present in the soil or substrates. Oospores (Figures 3 to 5) and chlamydospores perfectly ensure their conservation in the soil, whether in wet or dry conditions. Many hosts, cultivated or not, also ensure their multiplication and conservation.  
• Sources of inoculum : substrate, plants, irrigation water, plant debris, sludge, soil dust, etc.
• Infection : directly penetrate the epidermal tissues of young roots and fruits, but also through wounds. They rapidly invade the tissues with their non-partitioned mycelium progressing between and into the cells.
• Sporulation : more or less abundantly in and on the tissues which they have invaded (roots, crown, stems, fruits, etc.), forming in particular sporangia which can germinate directly or produce flagellated and mobile zoospores. The latter are easily disseminated in the aqueous phase of soil and in the nutrient solution of soilless crops, and are attracted by root exudates. Aerial releases are possible as a result of splashing during sprinkler irrigation or heavy rains. Certain insects, in particular flies, could be the source of contamination, via the transport of oospores.
• Favorable conditions :
  • a high density of seedlings in nurseries and roots in the breads of soilless crops;
  • excess nitrogen, high salt concentrations, etc. ;
  • the presence of water, excessive soil moisture and reduced gas exchange, heavy and / or compacted soils;
  • the temperature influences their behavior differently, with more thermophilic species such as Pythium aphanidermatum , Phytophthora capsici ;
  • the receptivity of the hosts which is not constant throughout their life;
  • the intervention of other pests leading to much more destructive interactions for vegetables.

Protection

• Carry out fairly long crop rotations in virgin land, they are no longer very effective in contaminated soil.
• Ensure good soil drainage, and provide organic matter in heavy soils.
• Ventilate the nurseries well, avoid excessive densities and excessive water supplies, monitor the quality of the seedlings when planting.
• Avoid planting too deeply, burying the crown of the plants. Plant on mounds to avoid water retention at the base of the plants.
• Put in place a plastic mulch to create a mechanical barrier between the soil and the plant organs, in particular the fruits.
• Take care of irrigation: optimal quantity, localized supply, etc. Avoid sprinkler irrigation and monitor the sanitary quality of the water.
• Maintain balanced fertilization and avoid stressing the plants.
• Eliminate healthy or diseased plant debris during and at the end of cultivation, as well as potential host weeds likely to harbor or promote the development and conservation of these organisms in the soil.

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