RHIZOCTONIA

RHIZOCTONIA CANKER OF POTATO

Rhizoctonia stem and stolon canker, commonly called black scurf because of its tuber phase, is an occasionally serious disease of potato found in all production areas of the world. This disease affects potato development from emergence to harvest. Under cold, moist soil conditions following planting, Rhizoctonia can affect emergence and the developing plant. The development of resting structures (sclerotia) causing the black scurf or "dirt that won't wash off" phase of this disease occurs after vine desiccation. Most reports of this disease claim it will limit total yield of a potato crop only under extreme conditions. Rhizoctonia can seriously limit marketable yield, however, by causing infected plants to produce an abnormally high percentage of tubers that are misshapen, knobby and cracked while also adversely affecting size distribution. Rhizoctonia will invariably decrease the amount of bruise-free potatoes during harvest due to the increased percentages of these misshapen and knobby tubers.

SYMPTOMS

Perhaps the most readily apparent phase of Rhizoctonia disease is the black scurf or sclerotia present on tuber surfaces. These sclerotia can vary in size from very small, flat, superficial black specks to large, raised, irregularly shaped masses that can cover a majoor portion of the tuber. While black scurf can, under extreme conditions, affect the marketability of tablestock/fresh pack potatoes, it is an extremely important seed-borne phase of the pathogen that deserves attention by all potato producers

Lesions characteristic of Rhizoctonia on stems and stolons are brown to black and sunken. These cankers can continue to expand and are capable of girdling stems and stolons of young developing plants. Rhizoctonia infection of older plants very seldom lead to girdled stems that die.  However, the health of these plants can be severely compromised and they can frequently become more susceptible to other diseases, particularly early blight.

Potato plants are most severely affected in the spring when underground sprouts can be killed prior to emergence. The secondary sprouts that develop are generally less vigorous and emerge much later causing irregular, uneven stands. Sprouts of severely affected plants that do not die are frequently stunted and remain so for the rest of the growing season. Infections of stolons that occur early in the growing season frequently result in pruning to tuber formation or abortion of tubers early in their development.

Mid-season Rhizoctonia infections of potato plants will result in deep, sunken cankers on the main stem. The above ground portion of the plant will appear yellow with some purpling and upward curling of the foliage. These symptoms are frequently confused with leaf roll or purple top. A key in differentiating Rhizoctonia from these two diseases is the frequently formation of aerial tubers in the lower leaf axils and the brown, sunken lesions just below the soil surface.

THE PATHOGEN AND THE DISEASE

Rhizoctonia canker is caused by the fungus Rhizoctonia solani. There are a number of strains or biotypes of this fungus, called anastomosis groups (AG) that affect a wide range of crops. Several are reported to attack potato with AG-3 being the most common. While it has been generally believed that AG-3 attacks only potato, it has been recently reported as occasionally attacking other crop plants, such as sugar beet.

R. solani can be either a soil-borne or seed-borne pathogen. The fungus survives in soil as mycelium in decomposing plant tissues. It also survives as sclerotia on tuber surfaces (seed-borne) or in the soil for extended periods. Populations of R. solani decline in the absence of a susceptible host although the rate of decline is affected by soil type, rotational crops and possibly the amount of organic matter present in the soil.

Disease development on emerging sprouts is favored by cold, wet soil conditions. Although these conditions may not increase disease incidence, disease severity is generally much greater. These conditions slow sprout development but favor germination of sclerotia and infection causing cankers to develop on young, under-developed tissues. Tuber-borne inoculum is very important in this phase of the disease while soil-borne inoculum is believed to generally more important in stem and stolon infections. However, research performed at North Dakota State University indicated that tuber-borne inoculum is also important in the pruning of the first stolons developed by the plant. When stolons become pruned, the secondary stolons are generally weaker and much shorter in length. Tubers produced on these shorter stolons are generally the ones that become misshapen because they tend to grow around the plant stem. Cool, moist conditions, with moisture being the most critical factor, also favor disease development on stems and stolons. Disease development is optimum at around 65F and decreases as soil temperatures increase.

DISEASE MANAGEMENT

Management of Rhizoctonia canker must be integrated since no single tactic is totally effective. First and foremost is crop rotation. In this regard it is important to consider both the number of years between potato crops and the rotational crops used in the system. Most research has shown that one potato crop in three or four is adequate to reduce soil inoculum to economic levels. However, soil type can potentially impact the rate of pathogen decline. Although not straight forward, observations suggest that the rate of Rhizoctonia decline in sandy soils exceed that of loamy soils with more organic matter. Rotational crops such as sugar beet and dry beans, which are affected primarily by different AG's of R. solani tend to raise the general activity of Rhizoctonia in the soil thereby increasing the inoculum potential of soil-borne inoculum. Cereal crops such as oat and barley are generally regarded as being good rotations with potato for the reduction of soil-borne inoculum.

Since young developing potato crops are more susceptible to injury by Rhizoctonia, practices that encourage rapid emergence and plant development will reduce disease severity. Potatoes planted into soils with excessive amounts of residue from a previous crop, such as corn, tend to have serious Rhizoctonia problems. Soils with large amounts of residue remain cold for extended periods and more moist, thereby favoring disease development. Therefore, high amounts of crop residue should be dealt with prior to planting.

Cultural practices such as pre-cutting seed, shallow hilling or hill drag-off and delaying planting until soil temperatures are >60F encourage emergence and will also reduce sprout pruning. It is also important to avoid high soil moisture during this period. Early irrigation, prior to emergence, should be kept to a minimum. Potato hills with steep sides and sharp peaks generally warm more rapidly than large, flat beds which can also reduce Rhizoctonia disease severity.

Seed selection is also important. Seed tubers free of obvious sclerotia is more desirable than seed with a significant amount of black scurf. Early research performed at our institution and elsewhere has shown that sclerotia coverage as little as 5% can significantly increase disease incidence and severity. In reality, however, much less black scurf can significantly increase disease development. One black sclerotium per square inch, which is far less than 1% coverage, can significantly increase disease pressure under environmental conditions that favor disease development. For this reason, disease-free seed is the best option. Harvesting seed tubers within 2-3 weeks of vine desiccation will reduce the amount of sclerotia formation. Tubers left in the soil for >3 weeks after vine desiccation will generally have significantly higher levels of black scurf which can lead to deep discounts on the fresh market.

A number of fungicide seed piece treatments have been tested for their effectiveness in controlling the seed-borne of Rhizoctonia. Of these, Maxim® has consistently provided the best control of tuber-borne Rhizoctonia. This product has been recently registered in the U.S. and will be available for the 1998 growing season. It is not registered for use on seed potatoes, however, due to concerns in the development of resistance in targeted pathogens.

SUMMARY

Rhizoctonia canker can cause serious losses in marketable yield of a potato crop if not managed properly. Cultural practices that minimize soil inoculum and favor rapid emergence and plant development are recommended in an integrated approach for disease management. Disease-free seed is also important although some seed piece treatment fungicides are active on this source of inoculum