The potato plant is affected by many diseases and requires special care so that its production, in quantity and quality, is guaranteed. Those caused by soil pathogens stand out, which are more difficult to control. Rhizoctoniosis, also known as rhizoctonia canker, asphalt, asphalt stain, is caused by the soil-dwelling fungus Rhizoctonia solani, which stands out in this group for causing frequent and significant losses to potato farmers. It occurs in all regions where potatoes are planted in the world, but its presence is more common in intensively cultivated soils and where crop rotation with non-host species is not practiced.
In the 2013 harvest, potato crops with a high incidence and severity of rhizoctoniosis were observed in some of the main producing regions, such as the municipalities of Serra do Salitre (Minas Gerais), Água Fria, Catalão, Cristalina (Goiás) and Mucugê (Bahia) . Some of these epidemics occurred in seed potato production fields, which could make the tuber unviable for this purpose, causing high losses to producers.
disease symptoms
Normally, the pathogen attack on plants occurs in reboleiras, causing the formation of patches of diseased plants and healthy plants in the field. The pathogen can attack all organs of the plant, including its aerial part, when in conditions of high air humidity. However, the attack is more common in the plant's underground organs or those close to the ground. When the fungus attacks the tuber shoots, it has the power to cause delayed emergence and/or death of the shoots, resulting in smaller stands, irregular plant development and a consequent reduction in production. Attacked shoots tend to emerge and present cankers, which can grow, leading to their death. The fungus can also attack the already developed plant, causing cankers on the roots, stolons (compromising the formation of tubers) and at the base of the branches, strangling them and leading to their death.
The attack on the tuber is characterized by the appearance of a black crust on its surface, not removed during washing, also called asphalt stain, formed by sclerotia of the pathogen. Infected tubers may also present symptoms of small cracks and “scab”. Attacked plants have stolon growth inhibited, with the formation of “bunch” tubers, which results in their deformation.
As reflex symptoms of the disease, chlorosis and curling of the leaves are observed, normally more severe in the apical part of the plant, a symptom that can be confused with that of the leaf curling virus (PLRV). Other reflex symptoms are the formation of aerial tubers, general stunting of the plant and leaf necrosis.
Disease-causing agent
the fungus Rhizoctonia solani (perfect form is the basidiomycete Thanatephorus cucumeris) is a polyphagous pathogen, capable of attacking many plant species from different botanical families. In Brazil, at least 37 hosts of this fungus have been reported, in different botanical families, including ornamental and vegetable species. The attack generally occurs from the soil, causing root rot, root and stem cankers and seedling tipping over. Furthermore, it can cause fruit rot in tomato plants and also cause diseases in flood-irrigated crops, such as rice, as well as attacking the aerial part of some plants such as beans and tobacco.
The fungus, in its asexual form (R.solani), does not form spores and presents thick, dark brown, multinucleated hyphae, which in culture medium branch at angles close to 90 degrees.
Although attacking several hosts, there is a certain specificity in R.solani, which is divided into several anastomosis groups (AG) and these differ in terms of the host cycle and the type of disease they cause, among other characteristics. Potato pathogenic isolates have been grouped mainly in the AG-3 group.
Epidemiology of the disease
On potatoes, the fungus R.solani It is transmitted by seed potatoes and can survive, in the form of sclerotia, in tubers, crop residues and soil. Sclerotia are resistant structures capable of surviving in the soil for several years. Since the pathogen is polyphagous, it can multiply and survive in other hosts, mainly dicotyledons, and in their cultural remains.
Favorable conditions for the development of rhizoctoniosis are soil temperatures around 18oC and high humidity. Poorly drained soils favor the development of the disease and the formation of sclerotia on the developing tubers.
It is spread over long distances through infected seed potatoes. In the field, pathogen propagules are spread through machinery, agricultural implements and surface water. When favorable conditions occur, sclerotia germinate and attack the tuber, stem and shoots of the plant, penetrating mainly through wounds.
Disease management
As it is a disease caused by a soil-borne pathogen and there are no resistant potato varieties, its control must be carried out preventively. In this sense, it is important to plant in areas free from the pathogen, with no history of occurrence or that have undergone crop rotation with non-host species. Prevent machines and vehicles that have traveled through contaminated areas from carrying pathogen structures to new areas. Shallower planting should be done in crops grown during colder periods of the year to encourage rapid emergence of shoots. As the pathogen is transmitted by seed potatoes, it is essential to use healthy seeds or treat them with fungicides; however, the use of healthy or treated seed will not be efficient if planting is done in infested soil. The elimination of contaminated crop residues avoids the presence of volunteer plants (ratoons) that can maintain the pathogen in the soil. In the Agrofit System of the Ministry of Agriculture, Livestock and Supply (Mapa) there are 15 commercial fungicides registered to control rhizoctoniosis (Table 1). However, chemical control is not always technically and/or economically viable in case of soil contamination and conditions favorable to the disease.
Embrapa Vegetables
Table 1 - Active ingredients registered against rhizoctonia solani
| metiram (alkylenobis(dithiocarbamate)) + pyraclostrobin (strobilurin) | |
fluazinam (phenylpyridinylamine)
fluazinam (phenylpyridinylamine)
Metam-sodium (methyl isothiocyanate (precursor of))
metiram (alkylenobis(dithiocarbamate)) + pyraclostrobin (strobilurin)
fluazinam (phenylpyridinylamine)
fluazinam (phenylpyridinylamine)
fluazinam (phenylpyridinylamine)
fluazinam (phenylpyridinylamine)
Source: Agrofit
Check out the article in issue 83 of Revista Cultivar Hortaliças e Frutas.
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