Controlling sclerotinia (White mold)

by Matthieu Morel

Chef Produit Biofongicides

co-authors

Caroline de Rauglaudre

Sclerotinia, also known as “white mold” is a serious disease that affects a great number of crops (most broadleaf crops are susceptible to it).

It is mainly the sclerotia (the survival organs of the fungi) of the species of Sclerotinia sclerotiorum and Sclerotinia minor, which are at the origin of the attacks on crops. These sclerotia reactivate each time a favorable crop is planted (e.g., lettuce, rapeseed, sunflower, beans, carrots, etc.) allowing the disease to develop. These sclerotia are able to survive for at least ten years in the soil. The plot history is therefore a key element in assessing the level of contamination.

Sclerotinia is, without a doubt, a disease of great economic importance, extremely damaging in the case of a strong attack. Some figures for example: on soybeans, losses can amount to 20% in the event of an attack, i.e., several hundred million dollars in the USA alone (Guyon et al., 2014), on rapeseed, they can exceed 15 q/ha or even 32% of the endive harvest (Benigni and Bompeix, 2010).

1 - Biology of the disease

Sclerotia are small black nodules consisting of the highly condensed mycelium of Sclerotinia sclerotiorum or of Sclerotinia minor.
These sclerotia form on the affected organs and/or in the plant debris left on the plots and represent a very resistant form of conservation for these pathogenic fungi, which can live in and on the surface of the soil for 5 to 10 years.

Sclerotinia minor

Contamination by Sclerotinia minor is mainly through the intermediary of the mycelium from the sclerotia found near the roots and leaves/stems touching the soil.

Sclerotinia sclerotiorum

Contamination by Sclerotinia sclerotiorum can be produced in the same way by Sclerotinia minor.

However, this fungus also creates formations on its sclerotia called apothecia. These are organs ensuring its sexual reproduction, which generate numerous asci (reproductive cells) containing the ascospores (the spores from the asci).

Thus, during a period of 2 to 3 weeks, millions of ascospores can be released into the air and be the cause of aerial contamination.

2 - Sclerotinia in arable crops: rapeseed, sunflower, soybean

Symptoms and damage

In the spring, leaves in contact with the soil can be infected. A white mycelial mass at the collar of certain plants is the sign of an attack at root level.

On rapeseed leaves, sclerotinia can be recognized by gray rot. On the stalk, gray-white spots encircle and develop in the leaf axils; necroses and sclerotia then appear in and on the stems, preventing sap flow. The upper part of the plant is scalded (more or less complete cessation of grain filling or ripening). The siliques can also be affected: they turn white and dry out, with the possible formation of sclerotia. In the case of an attack on the main stems, yield losses can reach or even exceed 15 q/ha.

Photo of sclerotinia damage on oilseed rape – Source *La France Agricole*

On soybean, the fungus first develops on the floral node and spreads up the stem and then the top of the plant dries up.

Photo of sclerotinia damage on soybeans – Source Terres Inovia

On sunflowers, sclerotinia attacks start on the young apical leaves, forming a rosette which will then contaminate the bud. Contamination occurs at the 6-8 leaf stage. If mild, humid weather sets in (in May-June), the infection quickly progresses to the heart of the bud and the rot destroys it completely.

Photo of sclerotinia damage on sunflower – Source Ephytia

Prophylactic methods

To anticipate the risks of sclerotinia, there are several points of interest on which to act preventively:

Rotation

Avoiding susceptible crop succession is one of the keys. This fungus is present on many cultivated species.
The level of risk is at plot level.

Watering regime

On sunflowers, avoid irrigations during flowering.

On soybean, the percentage of attack varies according to the ambient humidity. A first start of irrigation as late as possible and a spacing of ten to fifteen days between each round of water

reduces the attack by sclerotinia. The dry phase between two waterings is unfavorable to its development.

Varietal choice

For rapeseed, all the classic varieties are susceptible to sclerotinia apart from certain new varieties developed for their greater tolerance to this disease, such as BRV703. Their choice may prove to be judicious in addition to classical control measures.

In soybean cultivation, according to the earliness group, the risk of sclerotinia is more or less elevated. The variety should also be chosen based on its susceptibility to the lodging. In general, susceptibility to sclerotinia and the lodging are linked.
The lodging is often correlated with canopy closure, with a dense population creating a humid environment favorable to the germination of the fungus.

For sunflower cultivation, avoid choosing susceptible varieties.

Density

Overcrowded planting increases the risk of sclerotinia. The denser the cover, the more the humidity maintained is favorable to the fungus developing.

When sowing, it is therefore necessary to control density.

Weed management

Good weed management is essential, on the one hand, to remove the broadleaf weeds, host-plants for sclerotinia, and on the other to avoid maintaining a humid climate in the vegetation favorable to the pathogen.

Fertilization management

Avoid excessive vigor in the plants.