hidden obstacle

Rhizoctoniosis or black crust, caused by the fungus Rhizoctonia solani (Teleomorph Thanatephorus cucumeris), is a disease of widespread occurrence in potato crops, mainly in intensively cultivated areas. Associated with the soil, it can affect all underground parts of the plant and occurs at different phenological stages of the crop (emergence; tuberization and maturation of tubers). On stems and stolons, the typical symptoms of the disease are characterized by reddish-brown lesions (cankers), elongated, ringed that may or may not be associated with strangulation.

In tubers, the disease is called black scab and is characterized by the massive presence of sclerotia attached to their surface. They are dark brown to black in color, have variable sizes and shapes, and are formed by the coiling of the fungal mycelium. The disease can also present secondary symptoms such as: slow germination, reduced stand, loss of vigor, plant stunting, yellowing, leaf curling, reduction in the number and size of tubers, emission of aerial tubers, and misshapen tubers.

Rhizoctonia solani It has septate hyphae, brown to ochre mycelium, with the presence of lateral branching at right angles. The fungus can be divided into different subgroups depending on the fusion capacity of its hyphae (Anastomosis Groups - GA).

Potato rhizoctonia is mainly caused by anastomosis groups 3 and 4 (GA-3 and GA-4). Group GA-3 is the most severe and specific to potatoes, and can cause serious losses, while group GA-4, although pathogenic, rarely causes significant losses. In Brazil, in addition to potatoes, group GA-3 is reported in tobacco and group GA-4 in soybeans, beans, peanuts, tomatoes, melons, watermelons, spinach, peppers and broccoli.

The disease is favored by clayey, cold (5ºC to 25°C), humid, poorly drained soils, poorly decomposed organic matter, deep plantings and environments with temperatures between 18ºC and 22°C. The fungus can survive in the soil for long periods, remaining in the form of sclerotia or mycelium colonizing crop remains. In the germination/emergence phase, the sclerotia present on the seeds or in the soil germinate and infect the shoots and young stems. The formation of sclerotia in tubers occurs at any stage of tuberization and/or filling, however, it is more significant when the tubers are close to maturity.

In addition to the cultures mentioned above, the anastomosis groups of R.solani associated with potato cultivation can also affect invasive plants such as: joá-de-capote (Nicandra physaloides L.), purslane (Portulaca oleracea L.), caruru (Amaranthus deflexus L.) and Maria Pretinha (Solanum americanum L.).

Several products have been developed specifically to control rhizoctonia. In general, these promote increases in germination, plant vigor, yield and tuber quality. The use of fungicides should follow all the manufacturer's recommendations regarding dose, volume, time of application, safety interval, use of personal protective equipment (PPE), storage and disposal of packaging, etc. To avoid the occurrence of resistance to fungicides, it is recommended that specific (systemic) fungicides be used alternately or formulated with nonspecific (contact) products; that the repetitive use of products with the same mechanism of action be avoided; and that curative applications not be made in situations of high disease pressure.

Different control measures to limit disease severity

• Planting healthy seed potatoes (free of sclerotia).

• Correct soil preparation to eliminate “grid feet” to facilitate aeration and moisture accumulation in the surface layers.

• Plant shallowly, between 5cm and 7cm, to encourage rapid emergence of the plants.

• Avoid planting in cold soils (less than 10°C) or excessively moist soils.

• Seed treatment with registered fungicides.

• Planting cultivars with some level of tolerance such as: Bailla, Chipie, Colorado, Gredine, Opaline, Soleia and Innovator.

• Avoid excessive use of nitrogen, as stems become more tender and more susceptible to disease. Adequate levels of calcium can reduce disease.

• Elimination of alternative hosts and volunteer plants.

• Proper irrigation management to avoid moisture accumulation in the soil.

• Crop rotation with grasses for two to three years to reduce the inoculum potential in the area.

• Avoid injuring the stems during hilling. Carry out the operation when they are more rigid.

• Biological control of the disease can be carried out with formulations of Trichoderma sp. The microorganism should be applied before planting and should be used in combination with other control strategies to achieve good levels of control.

• Studies have shown that incorporation followed by decomposition of cruciferous and mustard crop residues can reduce disease thanks to soil fumigation caused by the release of cyanides.

* By Jesus G. Töfoli, Josiane T. Ferrari e Ricardo J. Domingues (APTA - Biological Institute)

Article published in issue 87 of Revista Cultivar Hortaliças e Frutas

To find out which pesticides are registered for the control of "Rhizoctonia solani", click here.