Mold in the net: Sclerotinia sclerotiorum

White mold, caused by the fungus Sclerotinia sclerotiorum (Lib.) de Bary, is one of the oldest plant diseases in the world, affecting more than 400 species, including soybeans, beans, cotton, sunflower, canola, tomatoes, peas, among other crops of economic importance. Damage occurs with greater frequency and intensity in regions at altitudes above 800m, with rainy weather, mild temperatures and high relative humidity.

From the 2006/07 harvest onwards, a significant increase was observed in the incidence and levels of damage to soybean crops in Brazil, both in the highest areas of the Cerrado and in the more traditional cultivation areas of the South and Southeast, reducing productivity by up to 70% in certain crops.

It is estimated that approximately 23% of the Brazilian soybean production area is infested by the pathogen, comprising approximately 7,7 million hectares that require the adoption of integrated disease management measures. The states most affected by white mold are Goiás (with more than two million hectares infested), Bahia, Mato Grosso and Paraná (one to two million hectares infested), Minas Gerais (with 0,5 to one million hectares infested) and Mato Grosso do Sul, São Paulo, Santa Catarina and Rio Grande do Sul (with less than 0,5 million hectares infested).

A striking characteristic of the pathogen is the production of sclerotia, which are survival structures formed by clusters of hyphae, commonly rounded or elongated, black in color and firm in consistency. The sclerotium can germinate in a carpogenic or myceliogenic way, triggering new cycles of the disease. Mycelogenic germination occurs through the formation of white mycelium from sclerotia, while carpogenic germination occurs through the formation of apothecia in sclerotia. The production of apothecia is the main source of infection in soybean plants. In the apothecia, spores of the fungus, called ascospores, are formed, which are ejected under pressure, reaching the soybean flowers, where they find ideal conditions for germination and initiation of infection.

Maintaining soil moisture is essential for the occurrence of the disease, since sclerotia germination depends on high humidity (frequent rains), temperatures between 15°C and 25°C, and low incidence of sunlight (soil shading by plants). Due to these conditions, the occurrence of white mold in soybeans varies in intensity between harvests, being more frequent in regions with altitudes above 800 m.

The fungus can infect any part of the soybean plant, but infections most frequently begin in the inflorescences, axils, petioles and lateral branches. The fungus can attack the entire aerial part of the plant, affecting leaves, stems and pods. The infected soybean plant initially presents watery lesions, on which hyphae grow, forming abundant white mycelium, which characterizes the name of the disease.

The attacked tissues become necrotic as a result of the action of various toxins and oxalic acid, produced by S. sclerotiorum. At this stage, rotting of branches, pods and leaves or even of the main stem can be observed, with death of the entire plant. Sclerotia are formed both on the surface and inside the infected stems and pods, and can detach naturally or be thrown to the ground during harvest, increasing the inoculum in the area.

The objectives of disease management are to reduce the inoculum (sclerotia in the soil) and reduce the incidence and rate of progression.

The reduction of inoculum is achieved by making sclerotia in the soil unviable and by reducing the production of sclerotia in diseased plants, through measures such as the formation of mulch for uniform soil coverage, preferably from grasses; rotation and/or succession with non-host crops; use of biological control through soil infestation with antagonistic agents; use of good quality seeds treated with fungicides; use of chemical control, through foliar spraying of fungicides during the period of greatest vulnerability of the plant (R1 to R4).

To reduce the incidence of white mold and its rate of progression, it is important to choose cultivars with plant architecture that favors good aeration between plants (few branches and small leaves) and with a shorter flowering period, as well as the use of plant population and row spacing appropriate to the cultivars.

Another measure that contributes to reducing the spread of the fungus S. sclerotiorum is the cleaning of machines and equipment after use in an infested area, to prevent the spread of sclerotia to new areas. Effective control of white mold in soybeans is only achieved with the integration of these measures, and does not produce satisfactory results in isolation.

Since 2008/09, network trials have been conducted by public and private institutions with the aim of comparing the efficiency of fungicides for controlling white mold in soybeans. Based on the results of these cooperative trials, accumulated over nine years of implementation, average control efficiency levels of some fungicides were found to vary between 62% and 72%.

In the 2016/17 harvest, the cooperative chemical control trial of white mold was carried out in 14 locations, distributed in the states of Goiás, Paraná, Minas Gerais, Mato Grosso do Sul, Mato Grosso, Bahia and the Federal District.

The applications were carried out with CO2-pressurized backpack sprayers and a spray volume of 150 L/ha. Nine fungicide treatments were compared in two applications, the first at the beginning of flowering or at the closing of the soybean rows and the second application ten days later.

Only the treatment composed of thiophanate methyl (T2) was applied four times at ten-day intervals. A treatment without fungicide application (T1) was maintained as a control.

The incidence of white mold and soybean productivity were evaluated, and the mass of sclerotia obtained from the trail of the plants in each plot was also quantified. The results were analyzed together, in the locations where there was homogeneity of data variance.

The best levels of chemical control ranged from 71% to 79% and were observed in treatments with two applications of the fungicides procymidone (T3), fluopyram (T5), dimoxystrobin + boscalid (T6) and with the combination of carbendazim and procymidone (T7).

The average sclerotia production (sclerotia mass) of S. sclerotiorum, collected from plants in the uncontrolled treatment (T1), was 2.708 g/ha. The treatments procymidone (T3 and T9), fluopyram (T5), dimoxystrobin + boscalid (T6), combination of carbendazim and procymidone (T7) and fluazinam + thiophanate methyl (T8) showed a significant reduction in sclerotia production, compared to the uncontrolled control (T1), ranging from 57% to 79%.

The treatments procymidone (T3 and T9), fluazinam (T4), fluopyram (T5), dimoxystrobin + boscalid (T6), combination of carbendazim and procymidone (T7) and fluazinam + thiophanate methyl (T8) showed the highest average soybean productivity. A 19% reduction in productivity was observed in the treatment without white mold control (T1) in relation to the most productive treatment (T6).

However, considering that sclerotia production still occurs, even if reduced due to chemical control, other management measures must be adopted in order to make these sclerotia unviable during the off-season, promoting integrated disease management.

* By Mauricio C. Meyer and Claudia V. Godoy (Embrapa Soybean); Hercules D. Campos (UniRV); Carlos M. Utiamada (Tagro); Edson P. Borges (Chapadão Foundation); José Nunes Junior (CTPA); Fernando C. Juliatti (UFU); David S. Jaccoud Filho (UEPG); Luciana C. Carneiro (UFG); Luís HCP da Silva (Agro Carregal); Marcio Goussain (Assist); Monica C. Martins (Green Circle); Monica P. Debortoli (Phytus Institute); Wilson S. Venancio (CWR)