Fungus "Sclerotinia sclerotiorum" on sunflower

The sunflower (Helianthus annuus) is an annual plant, belonging to the Asteraceae family, and cultivated for the purpose of producing oil and edible fruits. Characteristics such as greater tolerance to drought, adaptability to different latitude, longitude and photoperiod conditions, lower incidence of diseases and pests and nutrient cycling, especially potassium, are factors that favor its expansion, especially in the Central-West region after harvest. of soybean cultivation.

Sunflower is an option in crop rotation and succession in grain producing regions. Although the crop is rustic, a fungal disease is worrying, gaining prominence due to the significant damage to the crop's productivity, known as white rot, caused by the fungus. Sclerotinia sclerotiorum. It is considered the most important pathogen in sunflower cultivation in the world, due to its ability to cause damage, its permanence in the area and because it is widespread in all producing regions, whether temperate, tropical or subtropical. Due to the climatic conditions in the South and Central-West regions of Brazil, the possibility of epidemics is alarming and deserves attention.

The disease has a parasitic and saprophytic phase. The parasitic phase occurs only on host plants, being favored by high soil humidity and temperatures around 25°C to 35°C. In the saprophytic phase, the fungus survives in crop residues and organic matter present in the soil until it finds a new host plant. It is a polyphagous fungus, which means that it can survive on countless plants, reaching up to 408 species.

The fungus exhibits sexual and asexual reproduction. In the asexual form, myceliogenic germination occurs, which gives rise to mycelium and sclerotia. The hyphae form an abundant, cottony and loose mycelium with a cotton appearance. As a survival strategy, structures called sclerotia are formed, which end the pathogen's life cycle. At the beginning of their development, they are small in size and white in color. During their formation process, they acquire a black color and a spherical or irregular shape. This mechanism is an obstacle for the producer, as it allows the fungus to survive in the area under unfavorable conditions for up to ten years. The mycelium is unable to produce spores, so dissemination occurs through sclerotia, contaminated plants and seeds. Hyphae penetrate the plant through intact surfaces, wounds and natural openings such as stomata and hydathodes.

In sexual reproduction, carpogenic germination occurs, resulting in the production of apothecia. The sclerotia, when located in the superficial part of the soil and encounter favorable conditions, will germinate and give rise to apothecia. Its emergence is favored by moist soil conditions over a long period and sunlight. On its surface, ascospores are produced, which, when temperatures range from around 3°C to 22°C, are spread over long distances by wind. Spread is passive, as it occurs through seedlings, seeds, soil (plowing and harrowing) and water (runoff or splashes). Upon finding the host, infection begins, causing rot of the stem and capitulum. As they present high genetic variability, they end up generating new breeds, which are increasingly persistent in farming. The ascospores need to reach the flowers, where penetration occurs through the petals, so the critical period of infection is between the emission of the first flowers and their absence.

Infected tissues are colonized through the action of chemical substances produced by the pathogen such as organic acids, toxins and enzymes. Plant cells that are close to the point of fungus penetration are killed by the fungal structures. As the tissue is decomposed, the mycelium grows on its surface, colonizing the hyphae inter and intracellularly, where, under conditions of high humidity and temperature, it will favor the reproduction of the pathogen, forming reproductive structures and later sclerotia, which will give rise to pathogen signals on the plant.

In sunflower crops, white mold can cause three symptoms that differ according to the infected plant organ, called basal rot, rot in the middle part of the stem and capitulum rot. Basal rot can occur from the seedling stage to maturation. When the infection occurs in seedlings, it will cause tipping over, reducing the plant stand. At maturity, sudden wilting occurs, without presenting lesions, as the disease progresses, a light brown, soft and soggy lesion appears, which is located on the stem. If moisture is present, white mycelium tends to develop and subsequent formation of sclerotia (hyphae curling). Rot in the middle part of the stem occurs at all stages of the crop, presenting symptoms on leaves, petioles and the upper half of the stem.

Differing from the other symptoms, capitulum rot occurs at the end of flowering or later. Initially, symptoms are characterized by dark, water-soaked lesions located on the dorsal side of the capitulum, with the formation of a white mycelium that covers the tissue. With colonization, sclerotia form inside the plant organ, where the fungus is capable of destroying the interior of the capitulum, resulting in its total disintegration and subsequent fall.

Controlling the disease is a major challenge for producers, due to the ability of its resistance structures to survive in the soil and its wide range of hosts. Sclerotia remain viable for years and can be carried along with harvested grains, soil particles and infected parts of the plant. Therefore, the joint adoption of cultural, chemical and biological control is essential to reduce the incidence of the disease.

Chemical control is one of the main measures for integrated disease management, however its efficiency directly depends on climatic conditions. Preferably, fungicides should be applied preventively, that is, before the onset of infection during the period in which the host is susceptible, or better yet, from the beginning of flowering until maturation. It is necessary to alternate the active ingredients used in the area, so that selection pressure on the fungus can be reduced, delaying the emergence of resistant strains. Chemical seed treatment, both industrial and on farm, can help protect the seedling, preventing fungal structures from attaching to the surface of the seeds.

Solarization, which is nothing more than the use of transparent polyethylene on the surface of the soil, is a good alternative to reduce the population of the fungus that causes the disease and other pathogens that affect productivity. This practice is unfeasible in sunflower cultivation, due to the size of the cultivated areas. However, in crops that are implemented on a smaller scale, it is a great option, especially in tomato cultivation.

Cultural control is the best option for the farmer due to its economic viability. The use of certified seeds is the main means of preventing the disease from entering the area, as it is the most important vehicle for spreading white mold. During harvest, sclerotia are found next to the seeds, or even the mycelium colonizing their internal tissues, surviving through their slow metabolism. Another option that proves to be efficient in controlling it is the elimination of cultural remains in the area, however this practice does not meet the requirements of soil conservation practices.

Choosing cultivars that are more upright and sowing with greater row spacing make it difficult for the pathogen to develop in the host. If the crop implantation area is located in a dry climate, the use of irrigation or even the use of smaller spacing will provide the formation of a microclimate favorable to the development of the disease. It is noteworthy that more upright cultivars promote better aeration and make it difficult for the pathogen to establish itself. The manifestation of the pathogen is also influenced by the planting time, due to S. sclerotiorum preferring milder climates with high humidity, therefore, planting should be avoided during the rainy season. As a control methodology, options include crop rotation with non-host plants, the use of soil cover with grass straw, soil disturbance, equipment cleaning, chemical control and biological control, which should be carried out preventively.

It is important to highlight the ability of this fungus to persist in the soil for a long period, both when subjected to favorable and unfavorable conditions, through the formation of resistance structures, becoming part of the soil microbiota. Monitoring the area is essential, due to the intensity of the pathogen's occurrence in crops, also presenting a risk for several crops, such as soybeans, canola and beans. It is worth mentioning that the most efficient option for the farmer is the joint adoption of preventive measures, adhering to cultural, chemical and biological controls, to avoid the entry of the fungus into the area and the conditions for its proliferation.

By Marcos Lenz e Jayne Deboni da Veiga, Federal University of Fronteira Sul - UFFS

Article published in issue 283 of Cultivar Grandes Culturas magazine