Sucha zgnilizna ziemniaka (ang. Dry rot of potato)

The disease occurs in all potato cultivation regions, with its severity depending largely on weather conditions during the growing season. Losses caused by the development of dry rot can reach up to 50% of the yield. In the course of dry rot, three forms can be distinguished: rotting of tubers, rotting of sprouts, and decay of the base of stems. Each of these forms occurs at a different time and on a different plant organ. They may represent consecutive stages in the development of the disease, but they can also occur independently of each other.

Rotting of Tubers

On the surface of the tuber, black, granular scabs with diameters ranging from a few to several millimeters and thicknesses of 3-5 mm are formed. These are sclerotia, which serve as the survival form of the pathogen and, at the same time, a source of infectious material for the next season. These sclerotia can be easily scraped off the skin surface. Potatoes covered with sclerotia are largely unsuitable for planting but may be used for animal feed and industrial processing.

Rotting of Sprouts

This is the most dangerous form of the disease, and losses caused by its development can reach up to 25% of the plant stand in the field. Brownish necrotic spots appear on the surface of the sprout, which, as they develop, lead to withering and rotting of the sprout. In response to the damage to the initially strongest sprouts, the plant produces new ones, which, however, are weaker and thinner and can also be affected. Sprout damage significantly delays emergence, and on the field, empty spaces and weaker plants with fewer shoots compared to healthy ones can be observed.

Decay of the Base of Stems

This form of the disease appears later and is a decay process in which decomposing brown tissues are dry and resemble decaying wood, hence the name of this form. This process occurs only on the underground parts of stems covered with soil. The destruction of underground stem parts leads to disruptions in the transport of water and assimilates. Due to the resulting disturbances, secondary symptoms of stem base decay can be observed on the above-ground part of the potato. Disruption in the transport of water and mineral salts causes the wrinkling and rolling of leaves in the top part of the plant. These symptoms may resemble the primary symptoms of leaf roll, but in the case of dry rot, the leaves are soft, not leathery and rigid. Another symptom resulting from the destruction of conducting bundles is the formation of air tubers in the leaf axils, which occur due to difficulties in the transport of assimilates. After the rows close, when abundant rainfall occurs, brownish-white stem base decay appears just above the soil surface. This is the sexual form of the pathogen.

Deformations of Tubers

The disease, developing on the underground parts of the plant, damages not only the stems but also the stolons, leading to the dwarfing and deformation of tubers. These symptoms may sometimes resemble a flat form of common scab – the photo shows a tuber on the right with symptoms of common scab. In recent years, another symptom of tuber damage has become more common. This is corkiness (dry core). On the skin, there are round, slightly concave, sharply defined brown spots with a diameter of 3-6 mm. The skin inside the spot is often torn and covers the hole with a corky plug. This may resemble the symptoms of wireworm feeding, but unlike dry rot, the holes are not covered with skin and often extend deeper.

The development of the disease is favored by the following factors:

• Cool, moist spring.
• Too deep planting, especially in unheated soil.
• Use of infected seed potatoes.

Prevention

• Use of healthy, certified seed potatoes.
• Planting on the same field after a 4-5 year break to naturally reduce the amount of infectious material in the soil.
• Chitting and stimulating seed potatoes, allowing for faster plant growth and planting them earlier, as well as shortening the emergence period.
• Harvesting potatoes after the skin matures. Do not prolong the time between desiccation and harvesting, as it will promote the settling of the fungus sclerotia on the tubers.
• Treating potatoes before planting if more than 20% of the seed potatoes in the lot show symptoms of rot. The planting method should be adapted to the technical capabilities of the farm and local soil and climatic conditions. Treatment using professional equipment, e.g., a seeder-mounted treater, Mantis and Mafex devices, or ULV technique. Treating agents: azoxystrobin, flutolanil, pencycuron.

Compiled by: Dr. Eng. Jerzy Osowski