Biology, epidemiology

• Conservation, sources d'inoculum 

Athelia rolfsii can be stored for several years in the soil on plant debris , in the form of aggregated mycelium ( figure 1 ), but especially sclerotia ( figure 2 ) which constitute the primary inoculum. These structures are found free in the soil or associated with plant debris. Their persistence is greater in the first few centimeters than in depth. This fungus is also capable of maintaining itself on different organic substrates in its mycelial form in the saprophytic state.

It is a highly polyphagous pathogen that can attack and persist on some 500 plants, cultivated or not, belonging to around a hundred botanical families. Among the vegetables, let us point out in particular: tomato, pepper, eggplant, various salads, melon, cucumber, watermelon, beans, artichoke, beetroot, carrot, cauliflower, celery, garlic, onion, radish, turnip, sweet potato . Many ornamental plants are also likely to harbor it: narcissus, lily, zinnia, chrysanthemum. Groundnuts and soybeans are important potential hosts in areas where they are grown. It is also found on apple trees.

• Penetration and invasion

The first contaminations take place via the mycelium already present in the soil (figure 3) or from the sclerotia. They are often localized on the parts of plants located at the level of the surface of the soil, near or in contact with the latter. A. rolfsii , when in proximity to a potential host, produces lytic enzymes such as polygalacturonases as well as oxalic acid , a poison which causes cell death . Thus, it can penetrate directly into the tissues and invade them after having decomposed them thanks to the combined action of the preceding compounds. Its inter and intracellular evolution is very fast, especially if the climatic conditions are favorable to it. This parasitic process , which lasts from 2 to 10 days depending on the climatic conditions, is at the origin of the wet deteriorations and rots observed on the various organs.
 

• Sporulation and dissemination

On damaged tissue, it quickly forms mycelium and sclerotia. These can germinate quickly because they do not need a dormant phase. They are, however, able to keep for several years in the soil without germinating.

The transmission of this fungus is ensured by soil contaminated by sclerotia , tillage, soiled tools and implements, water and plants produced in infested nurseries.

The perfect form of A. rolfsii would rather rarely produce basidiospores on the mycelium at the periphery of the lesions. These spores could be at the origin of its aerial spread over long distances. Their epidemiological role, poorly understood, should be quite limited given the low frequency of this form of reproduction and the absence of contamination on the upper parts of plants.

• Conditions favorable to its development

A. rolfsii appreciates hot climates and thrives after humid periods and / or irrigation . It is able to survive in a wide range of surrounding conditions.

It likes acidic soils , its mycelial growth being optimal between pH 3 and 5; germination of the sclerotia takes place between pH 2 and 5. The latter seems to be inhibited above pH 7.

Periods of hot and humid weather, and asphyxiating soils favor its extension. It grows well between 25 and 35 ° C , its growth is blocked below 10 ° C and above 40 ° C. If its mycelium degenerates below 0 ° C, its sclerotia withstand temperatures of the order of - 10 ° C. A. rolfsii grows well and produces a large number of sclerotia between 27 and 30 ° C. Humidity influences their germination. Contrary to what one might think, this would be inhibited in soils saturated with water, some authors reporting that it would be optimal at relative humidity between 25 and 35% . The disease incidence may be higher in well-drained, sandy soils. The alternation of wet and dry periods would stimulate the germination of sclerotia. The presence of organic substrates, such as senescent leaves, could increase the severity of the disease.