How to manage rhizoctoniosis in potato plants

Potatoes are affected by several diseases caused by different phytopathogens, with those caused by soil pathogens being the most important in the crop and the most difficult to manage. These soil diseases can be limiting and cause serious damage to the production chain, significantly affecting the germination, emergence and vegetative development of the crop, consequently reducing the productivity and quality of the harvested tubers. Potato diseases are responsible for high production losses and can be caused by fungi, oomycetes, bacteria, viruses and nematodes. Among the diseases, rhizoctonia canker stands out due to the damage it causes to production.

Rhizoctonia leaf spot is caused by the fungus Rhizoctonia solani and occurs frequently in the main potato-producing regions. Its incidence and severity may be favored in regions that present the main conditions for the development of the disease, such as the presence of moist and cold soil, poorly drained soils, with temperatures that can vary from 18°C ​​to 25°C, heavily cultivated soils where crop rotation with non-host species of the pathogen is not practiced, accumulation of undecomposed organic matter and soils with intensive cultivation of potatoes or other hosts of the pathogen.

Causal agent of the disease

The disease is caused by R. solani, which in its perfect form corresponds to the basidiomycete Thanatephorus cucumeris (Frank) Donk. This perfect phase occurs at the end of the potato cycle, forming a network of grayish-white mycelium on the stems close to the soil. This fungus can be classified and characterized in different subgroups according to 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), and can cause seed rot, pre- and post-emergence damping off and, in some situations, root rot in crops such as beans (Phaseolus vulgaris), peanuts (Arachis hypogaea), tomatoes (Solanum lycopersicum), melons (Cucumis melo), watermelons (Citrullus lanatus) and peppers (Capsicum annuum Group).

The fungus Rhizoctonia solani has the ability to survive for several years in the soil in the form of sclerotia, or in the form of mycelium, in crop residues that are present in the area. Its spread over long distances occurs mainly through seed potatoes infected by the pathogen and agricultural machinery with soil residues. Spread over short distances is caused by water used for irrigation, tools and agricultural implements. Infection by R. solani commonly occurs from the soil, mainly affecting sprouts and tubers. The phase of greatest susceptibility to the disease occurs from the planting stage to the hilling stage, equivalent to the period of 25 days to 40 days after sowing. The attack of this pathogen can be facilitated by the emergence time of new shoots, since the increase in this time delays the exposure of tissues susceptible to attack by this pathogen.

Symptoms

In many situations where R. solani is present in the field, laboratory confirmation is required, where, with the aid of microscopy, the structures of the pathogen can be visualized. The most common symptoms are delayed emergence, presence of reddish-brown lesions on young stems, leaf curling, purple pigmentation on the leaves, emission of aerial tubers, deformed and small tubers. These symptoms can be confused with those presented by other diseases, making diagnosis difficult. The fungus R. solani can attack any organ of the potato plant, including the aerial part, and occurs mainly in conditions of high air humidity. It is worth noting that the infection is more severe in organs located close to the soil and underground organs, such as sprouts and tubers. When infection occurs in the sprouts, it can cause delayed emergence due to the death of the sprouts and even the death of the plants, leading to a smaller stand, irregular development of the plants and resulting in reduced production.

Infected shoots can also emerge and later present cankers that will compromise their development and may lead to the death of these shoots. The consecutive emission of new shoots to balance the dead ones stimulates the depletion of tuber reserves, leading to their destruction and compromising the density of potato plants in the field. The fungus can also attack the plant when it is already fully developed, causing cankers on the runners and at the base of the branches.

Intensified attacks on the tuber result in the presence of a superficial black crust adhered to the skin, formed by sclerotia of R. solani, which are dark in color and vary in shape from 1 mm to 5 mm in diameter. They are not normally eliminated, even after washing the tubers.

Infected tubers may also present symptoms of cracking, malformation and necrosis.

There are also reflex symptoms of the disease, such as leaf curl and chlorosis, which are more severe in the apical part of the plant, and are commonly confused with the symptoms of the leaf curl virus (PLRV). In some cases, purple pigmentation may occur on the leaves due to the accumulation of anthocyanin, general stunting of the plant and production of aerial tubers.

Integrated management

Preventive management is recommended to control this disease, since R. solani is a soil pathogen and there are no potato varieties resistant to its attack. It is recommended to avoid planting in areas with a recent history of the disease. Considering that the fungus survives in the soil after the harvest and has the ability to attack other crops, soil preparation should be carried out in advance so that organic matter can decompose, leading to faster degradation of crop residues that may contain the fungus. Due to the ability to spread the fungus through diseased tubers, it is recommended to use healthy, treated seed potatoes, preventing the spread of the pathogen to new areas. Another very important practice for control is crop rotation with soybeans, to avoid the accumulation of biotypes that are pathogenic to potatoes.

Planting should be avoided in clayey or excessively humid soils, and seed potatoes should be planted shallowly, between 5cm and 7cm, to promote rapid germination and emergence of the plants. The incorporation of green manures can be an alternative, as the increase in organic matter stimulates the development of beneficial microflora which, by competing for food, can reduce the population of the pathogen present in the soil.

Another widely used control method is biological control, in which treatment can be carried out by applying formulations of the fungus Trichoderma sp. to seed potatoes or in the planting furrow. This management should be applied with other control practices to obtain better results. It is recommended to eliminate and destroy crop residues, tubers that show symptoms or signs of the pathogen, weeds, tubers remaining from the last harvest and possible alternative hosts.

Sanitization and disinfestation of all implements (tractors, tools, boots, scissors, hoes, etc.) must be carried out before carrying out cultural treatments in the area and avoid using these implements that were used in infested areas. Carry out adequate irrigation management in the area, to avoid the accumulation of moisture in the soil and consequently promote an environment conducive to the fungus. It is recommended to carry out hilling when the stem is more rigid, taking care to avoid causing injuries to the stems during hilling, as well as during cultural treatments.

Chemical control is carried out using fungicides. According to the Ministry of Agriculture, Livestock and Supply (Mapa), there are 25 fungicides registered to control the disease. The product must be applied according to the manufacturer's recommendations regarding dose, volume, time of application, interval, use of personal protective equipment (PPE), storage and disposal of packaging. However, chemical control of this disease is not always technically and economically viable, due to the pathogens' structures being protected within the soil. The use of products to control the pathogen will depend on the recommendations of a trained professional. 

* By Francisca Hortencia Couras Dias e Luciana (Federal University of Paraíba)