Botrytis cinerea Pers.

Gray mold

General  

• Mushroom, ubiquitous and very polyphagous.
• Observed on eggplant in practically all its production areas in the world. Can be found in the open, but especially in shelters.
• Biological variability little known, strains resistant to several fungicides have been reported in several countries.
• Great genetic variability within the species B. cinerea . This fungus is in fact a complex of species.
• Organs attacked  : leaves, fruits and stems.
• Symptoms :
  • Possible attacks in wet nurseries leading to damping off, cankers at the collar or on stems, etc.
  • Symptoms present on all aerial organs.
  • Rather circular and moist leaf spots , then beige to light brown in color, revealing concentric arabesques.
  • Affected tissues quickly necrotize and dry out important areas of the limbus.
  • Having invaded the petioles, B. cinerea can thus settle on the stem.
  • Wet canker lesions at first, then beige to brown in color, drying out as they develop and girdle the stems for several centimeters.
  • Branch wilting.
  • Browning of the senescent petals allowing the invasion of the flowers.
  • Soft, moist rot appearing at the stalk scar of fruits, sometimes at their tips. Lesions mostly circular to elongated, pale brown to brown; quite often the tissue eventually collapses. Rot on fruits observable both during cultivation and after harvest (during storage, transport and marketing).
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• Signs : very characteristic dense gray mold, consisting of conidiophores and conidia of the fungus. Sometimes presence of black sclerotia 2 to 5 mm in diameter, rarely visible on the lesions.

Biology  

• Storage : can be preserved on seeds, in the soil, on plant debris and organic matter thanks to its mycelium, flat sclerotia and conidia. Polyphagous, it can colonize several hundred cultivated plants or weeds which contribute to its conservation and constitute potential sources of inoculum, this is the case of the majority of market garden plants. Primary contaminations often aerial and provided by conidia transported by the wind, they germinate in a few hours in the presence of an ambient humidity of at least 95%.
• Infection : enters the tissues through the germ tube of the conidia either directly through the cuticle or from various wounds; the mycelium destroying cell walls and their contents. The infections take place approximately after about fifteen hours. B. cinerea also settles on all senescent, necrotic and / or dead tissues such as petals, necrotic sepals, old leaflets. It sometimes colonizes tissues already damaged by other pathogens or pests. It quickly spreads to tissues, which it rots in a few days thanks to the hydrolysis of peptic substances.
• Sporulation : produces a gray mold on all its hosts as well as on plant debris, it consists of mycelium and many long and branched conidiophores. At their ends emerge ovoid to spherical conidia which ensure the dissemination of B. cinerea . Sporulation can start 3 days after the first infections.
• Dissemination : wind and air currents which disperse conidia, to a lesser extent rain and splashing water. It is also ensured by the workers during cultivation operations. The mycelium is at the origin of contamination by contact of diseased tissues with healthy tissues.
• Conditions favorable to its development : particularly likes humid environments, a relative humidity of around 95% and temperatures between 17 and 23 ° C (conditions found especially in shelters, but also in the open field during rainy periods or after sprinkler irrigation). Plants that are etiolate or overgrowth are particularly vulnerable. The quality of the plastic films more or less influence the development of B. cinerea , as well as  the agrotextiles sometimes used to protect plants from insects which cause an increase in humidity, aggravating the damage. Under favorable conditions, the duration of a cycle is quite short, of the order of 4 days.  

Protection

• Create a crawl space under shelters, replace the plastic covering the floor if necessary.
• Disinfect the structures and walls of the shelters before setting up the culture in order to destroy the spores B. cinerea present.
• Ventilate the shelters as much as possible in order to reduce the ambient humidity and, in particular, to avoid the presence of free water on the plants. In a period of cloudy and humid weather, ventilating and heating the shelters makes it possible to better manage the humidity.
• Remember that organic substrates make it possible to obtain plants that are more vegetative and therefore more sensitive to gray mold.
• Replacing nitrates with chlorides made it possible to very significantly reduce the damage of B. cinerea less in soil- cultivation under shelters. In addition, the calcium enrichment of eggplant tissues would greatly lower its sensitivity to this fungus.
• In the open field, a localized drip irrigation system that is preferable to sprinkling. Prepare and drain well the soil of future plots in the open field in order to avoid the formation of puddles conducive to attacks by B. cinerea on the leaves in contact with the soil. Orient the rows of planting in the direction of the prevailing winds so that the plant cover is well ventilated. Avoid planting densities that are too high and injure the plants.
• Crop rotations are disappointing for controlling B. cinerea .
• Irrigate under cover only when the plants are able to evaporate in order to avoid obtaining plants that are more vegetative, with waterlogged and succulent tissues, and therefore more susceptible to B. cinerea . In the open field, the sprinkler irrigations carried out rather in the morning - and never in the evening - will allow the plants to dry as quickly as possible.
• Avoid any stress on the plants leading to spurts of growth. Nitrogenous manure must be controlled. It will be neither too strong (at the origin of very receptive succulent tissues), nor too weak (sources of chlorotic leaves constituting ideal nutritive bases for B. cinerea ).
• Remember that leaf stripping from the lower part of plants promotes fruit ripening, simplifies picking, and helps improve the climate of the plant cover by improving air circulation in the crop. Do not prune during cloudy and humid periods.
• Very quickly remove plant debris during cultivation, in particular moribund plants with one to several cankers on the stem and rotten fruits on which B. cinerea sporulates. Do the same at the end of the crop, so that they are subsequently buried in the ground and that the fungus can be preserved there. Deep plowing will facilitate the decomposition of the few remaining waste
• Preventive fungicide treatments are often essential in greenhouses at certain times of the year to control this fungus.
• Take care as soon as the vegetation becomes heavy, especially in cloudy weather.
• It should be noted that several families of fungicides are affected by resistance phenomena, and therefore alternate the chemical families of fungicides. Treat following leaf stripping, harvesting, etc. in order to protect the wounds produced. A thick fungicidal spray can be applied to the young cankers after they have been removed, also covering the lesions on the stem well.
• Control other diseases and predators because they are the source of wounds and tissue necrosis conducive to the establishment of B. cinerea .
• A number of alternative methods have been or are being tested in several countries (compost extracts, antioxidants, lemon essential oils, extract Reynoutria sachalinensis , baking soda, etc. have also been tested).
• Bacteria and several antagonist fungi have been evaluated to control this fungus ( Streptomyces spp., Bacillus spp., Several yeasts, Aureobasidium pullulans, Microdochium dimerium, Gliocladium virens, G. catenulatum, Trichoderma harzianum, Ulocladium atrum, Chaetonostium globachosum, Cladium atrum, Chaetonostium globachosum, rosae, Rhodosporidium diobovatum, etc. Several organic preparations are currently on the market.