How to manage white mold on vegetables

Biological control of diseases has currently become an indispensable tool, applied with the aim of controlling a population of harmful organisms, through beneficial organisms. Different biocontrol agents are studied, among which species of the fungal genus stand out. Trichoderma which is an antagonist of several phytopathogenic fungi. Among the mechanisms of action used by this agent are the production of metabolites and enzymes with antifungal properties, hyperparasitism and competition for nutrients. Furthermore, it is considered a growth promoter in plants. Species of bacteria have also been widely used in

biocontrol of phytopathogens, with registration of products formulated from Bacillus subtilis being applied to peanuts since 1983, in the USA. Trichoderma spp. It is also currently used in several crops, mainly to control soil pathogens, and can be found on the market in different formulations such as wettable powders (PM), dispersible granules, concentrated suspensions (SC), emulsifiable oils, colonized grains and dry spores. Sclerotinia sclerotiorum (Lib.) de Bary is a soil pathogen that has more than 408 host plant species, with approximately 106 species found in the Asteraceae family. The control of S. sclerotiorum It is very difficult, due to the fungus' ability to form resistance structures known as sclerotia. The disease caused by the fungus S. sclerotiorum It is popularly called white mold or sclerotinia wilt. In it, necrosis occurs on the stem and the leaves turn light brown, with a moist appearance, then wither. External signs are the growth of white cottony mycelium on the surface of infected tissues and the presence of numerous rounded, dark-colored sclerotia. Under favorable conditions and in the presence of a susceptible host, the sclerotia germinates and can form mycelium, which infects the neck and roots of plants, and apothecia, which emerge on the soil surface and release sexual spores called ascospores. In conditions of high relative humidity, above 70%, and temperatures close to 20ºC, the apothecia release thousands of ascospores over several weeks, which are responsible for infecting the aerial part of plants.

In vegetable production, S. sclerotiorum It is a serious problem, especially in tomatoes, potatoes, peas, eggplant, brassicas and lettuce, especially when grown in contaminated soils under conditions of mild temperature and high humidity, such as irrigated soils. The fungus is commonly found in commercial vegetable crops in the South and Southeast regions of the country, causing losses of up to 100%. The pathogen causes significant losses in productivity in several crops. In lettuce, in Colombia there are reported losses of between 20% and 70%, while in California (USA), losses of around 60% were estimated. Around 80% of parsley, 80% of coriander and 70% of carrot plants, inoculated with S. sclerotiorum, died within ten days after inoculation with the pathogen. In addition to vegetables, it can cause significant damage to sunflower and soybeans, and also infect invasive plants, in which it finds refuge throughout the year. The incidence of white mold is also favored by high planting density and prolonged periods of precipitation.

Controlling the disease is very difficult due to the formation of sclerotia, which remain in the soil for several years. These structures ensure the presence of the pathogen in the soil for periods of at least six to eight years, making control difficult through crop rotation. It is important to highlight that for the vast majority of crops there is no supply of resistant cultivars and chemical control is not always efficient, due to their rapid transformation and degradation in the soil. Thus, control measures are based on the purchase of certified seeds, crop rotation, adequate spacing and the use of biological control.

The Federal University of Santa Maria (UFSM) has been developing work focusing on the biological control of soil pathogens such as Fusarium spp. And Sclerotinia sclerotiorum in different cultures. In the tests, isolates of biocontrol agents from the laboratory's library and commercial products are used. In general, Trichoderma spp. And Bacillus subtilis have demonstrated increases in shoot and root mass, reduction in incidence and severity of white mold in crops such as lettuce and beans. Furthermore, these microorganisms have the ability to remain in the soil, generating benefits in controlling other fungi that will possibly infect the successor crop in the area.

The research group also carries out work involving the association of forms of control, since biological control must be combined with other methods. In recent work involving the use of Trichoderma spp. associated with soil solarization (physical control) in field beds, a beneficial interaction was observed between the two forms of control, which after 48 days rendered 100% of the sclerotia unviable. S. sclerotiorum.

The bibliography contains numerous works involving the use of biocontrol agents. More specifically, in controlling Sclerotinia sclerotiorum, isolated from Trichoderma koningii were aggressive against the pathogen, colonizing 100% of the sclerotia in seven days (vitro) and within 60 days in infested soil (in vivo). Furthermore, around 50% of lettuce seedlings used in the control treatment containing only the pathogen survived 21 days after sowing, compared to 82% when using the treatment with Trichoderma harzianum, obtaining healthier and more vigorous seedlings. Similar results were obtained using T. harzianum native, in tomato seedlings, proving its effectiveness of more than 80% in controlling S. sclerotiorum. Recent works indicate that Bacillus subtilis presents a good antagonistic effect at all stages of the cycle S. sclerotiorum. The use of B. subtilis as a biocontrol agent it is conditioned by factors such as cultivar, in the case of lettuce, and concentration of bacterial cells used in the suspension.

Biological control therefore presents a series of advantages to the agroecosystem. However, technicians and producers must pay attention to the quality, expiration date and technical recommendations of the product to optimize results on the property.

This article was published in issue 85 of Cultivar Hortaliças e Frutas magazine. Click here to read the issue.