| | Early blight | Fusarium dry rot | Late blight | Leak | Pink rot | Powdery scab | Rhizoctonia | Seed piece decay | Silver scurf | Verticillium wilt
Return to the Guide to Commercial Potato Production on the Canadian Prairies.
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Early Blight
Alternaria solani
Early blight leaf lesion
Courtesy of C. Schaupmeyer
Each season, early blight causes significant yield losses on the Prairies. The pathogen causes dark brown to black concentric lesions on leaves and elongated brown or black lesions on stems and petioles. Leaf lesions become angular if a large vein retards them. The lesions enlarge, join together and may cover the entire leaf, which will eventually die. The pathogen may occasionally cause sunken circular to irregularly shaped lesions with a raised purple to brown border on the tuber, however, these symptoms have never been reported in Western Canada. The fungus can survive over winter in soil or on plant debris and initiate infection in the crop. Lesions on infected potato plants produce spores that spread to healthy plants and cause infection. The pathogen attacks weaker tissues; young tissue with high nitrogen content is somewhat resistant to the disease. Alternating wet and dry conditions and temperature between 64 and 77°F (18 and 25°C) are very favorable for disease development during the growing season.
Control strategies:
- Plant disease-free seed
- Maintain good soil fertility and crop vigor
- Harvest when skin is mature to avoid bruising and in turn infection of tubers
- Avoid continuous potato rotations (i.e. planting potatoes in subsequent years in the same field)
- Apply protectant fungicides to the foliage, and follow a fungicide program throughout the growing season.
- Consult the provincial Guide to Crop Protection for registered fungicides
Fusarium dry rot, seed piece decay, and wilt
Fusarium spp.
Fusarium dry rot
Courtesy of P. Bains, Alberta Agriculture, Food and Rural Development
Potatoes can be infected by different Fusarium spp. throughout the season. The various Fusarium pathogens cause seed-piece decay, wilt, and dry rot. The disease may initiate from infected seed or from inoculum present in the soil. Wounding is necessary for the development of dry rot and seed piece decay. Wounds to seed pieces at planting are ports of entry for the pathogen to cause seed rot. Wounding at harvest leads to dry rot development in storage. Vascular wilt develops as a result of soil-borne Fusarium infection of the roots. In the field, the symptoms of Fusarium wilt resemble those of Verticillium wilt, and a lab test is required to distinguish these diseases. Tuber yield and quality may be reduced due to wilt, but this disease is not as common as Fusarium dry rot.
Control strategies:
- Avoid wounding tubers at any stage of cropping cycle
- Provide conditions that encourage proper wound healing
- Use registered fungicide seed treatments
- Avoid planting in extremes of cold or hot and dry or soggy soil
- Harvest in dry and cool weather
- Promote wound healing after harvest: 50-55°F (10-13°C) relative humidity at 95% with plenty of air for 10-14 days
- Apply post-harvest fungicide (resistance of Fusarium spp. to thiabendazole may reduce effectiveness)
Late blight
Phytophthora infestans
Late blight is one of the most devastating diseases of potatoes. The pathogen can infect all parts of the plant. Depending upon the environmental conditions and age of the tissue, appearance of the lesions may vary. The disease starts as small necrotic spots, which may or may not be surrounded by a pale green border.
Courtesy of R. Howard, Alberta Agriculture, Food and Rural Development
Lesions may also start as small water soaked areas at the tips of the leaf and enlarge inward. Older lesions generally have a necrotic centre and a pale green border. Dark green to black water soaked lesions develop on stems and petioles.
Courtesy of R. Howard, Alberta Agriculture, Food and Rural Development
Stem and petiole infections destroy soft tissue and leave only structural parts of the stem. As a result, stems remain standing in heavily infested, defoliated fields. Under humid conditions, a white fluffy growth appears at the lesion edges on the under side of infected leaves.
Courtesy of B Geisel, Gaia Consulting Ltd.
Tubers near the soil surface can be infected if they are exposed or spores are washed into the soil. An irregular and shallow (1/4-1/2 inch, 4-13 mm) coppery brown dry rot spreads through the outer tissue of the tuber. In storage, infected tubers are susceptible to secondary rots caused by other fungi and bacteria; this can result in extensive damage. The late blight pathogen can survive only in living host tissue. It is known to over-winter in seed tubers, cull piles, and volunteer potatoes that over-winter in the field. High humidity and temperatures of 64-71°F (18-22°C) are ideal for development of this disease.
Control strategies:
- Use certified disease-free seed
- Destroy cull piles by freezing or deep burying
- Destroy volunteer potato plants in nearby fields
- Throughout the season, destroy (desiccate, disc or flail and desiccate) infected plants to avoid spread
- Reduce periods of leaf wetness and high humidity within the crop canopy by appropriately timing irrigation
- Follow a recommended fungicide spray program. The program should start prior to the arrival of the pathogen.
- Consult your provincial Guide to Crop Protection for registered fungicides.
- Consult your local late blight forecast for disease risk information, if available
- Desiccate vines prior to harvest (refer to section Vine Killing).
Leak
Pythium ultimum
Courtesy of Manitoba Agriculture, Food and Rural Initiatives
Leak is a tuber disease that has the potential to cause significant storage losses. Diseased tissue of tubers infected with leak is soft, watery, granular and dark grey or black in colour. Severely infected tubers may drip or leak, however, in less advanced stages, infected tissue may only be seen when tubers are cut. There is usually a distinct line between healthy and diseased tissue. Tubers may develop soft rot, and then become slimy and foul smelling. The fungus is present in many soils where it over-winters in plant debris, particularly in wet soils. Tubers become infected through wounds in the periderm. The incidence of leak increases with warm temperatures, as 77-89°F (25-30°C) is ideal for disease development.
Control strategies:
- Grow potatoes on well-drained soils
- Harvest when tubers are mature
- Harvest below a tuber pulp temperature of 65°F (18°C), especially if the soil is moist
- Grade infected tubers prior to storage to reduce spread to healthy tubers
- Store tubers promptly after digging at recommended temperatures and humidity
- Cool tubers harvested in hot sunny weather to below 50°F (10°C) and market immediately
- Consult your provincial Guide to Crop Protection for registered fungicides
Pink Rot
Phytophthora erythroseptica
Courtesy of Manitoba Agriculture, Food and Rural Initiatives
The symptoms of pink rot are typically observed in tubers, but severe infections may develop foliar symptoms such as wilting, yellowing, and aerial tubers. Infected tubers exude a clear, watery liquid. The tissue remains intact, but has a rubbery texture. When infected tubers are cut open, a pink colour develops within 30 minutes, and later turns black. The tuber decay may proceed into the tuber from the stolon end, and a line may be visible between healthy and infected tissue. The pink rot pathogen lives in soil and can infect any below ground part of the plant. The pathogen thrives in wet, poorly drained soils and disease progresses quickly during warm temperatures. The disease can spread to healthy tubers during harvest and storage.
Control strategies:
- Plant in well drained soils
- Avoid wounding during harvest and handling
- Grade infected tubers prior to storage to reduce spread to healthy tubers
- If pink rot infected areas of fields are identified or suspected, market directly after harvest. If this is not possible, store separately from healthy tubers
- Use a fungicide registered for pink rot control. Consult your provincial Guide to Crop Protection for recommended products.
Powdery scab
Courtesy of J. Thompson, University of Saskatchewan
This disease has been a serious problem for potato growers in New Zealand and parts of Europe for many years, and has recently been recognized as a problem in North America. Powdery scab is caused by the fungus Spongospora subterranea. It produces a resting spore stage (spore balls) that can survive in soil for at least six years. Initial infection occurs at early tuber set, and is favored by high soil moisture and soil temperatures of 57-64°F (14°C-18°C). The pathogen causes scabs that are initially rounded and discrete, with torn edges of potato skin surrounding the scabs. Lesions frequently form in a band around the tuber or are clustered at one end. Later lesions may coalesce. By harvest time a dry, powdery mass of spore balls may be present in the scab lesions. The presence of these ovoid spore balls, visible under a dissecting microscope, is necessary for a positive identification of the pathogen. It is possible to confuse powdery scab lesions with those produced by common scab. Powdery scab also infects root tissue, forming galls 1/8-3/8" (1-10 mm) that produce spore balls that remain in the soil at harvest. Powdery scab infections increase grade-out, and the scabs may allow entry of secondary rot organisms. The powdery scab fungus is also the only known vector of potato mop-top virus (Refer to the section on mop-top virus).
Control strategies:
- Plant disease-free seed into non-infested soil
- Reduce soil moisture levels at tuber set
- Plant late to increase temperature at tuber set, harvest early
- Increase time between potato crops to at least 6 years
- Plant less susceptible cultivars such as Russet Burbank
- No registered fungicides are available (as of December 2001)
**Differentiation between common and powdery scab is important to allow correct management decisions to be made.
Rhizoctonia stem canker and black scurf
Rhizoctonia solani
Rhizoctonia stem canker
Courtesy of P. Baines, Alberta Agriculture, Food and Rural Development
The symptoms of the disease include development of rusty brown lesions on underground stems and stolons and black sclerotia (fungal bodies) on progeny tubers. The leaves may turn pale green or purple and become curled and upright. Development of aerial tubers in leaf axils may also be observed. Under humid conditions, a white cottony growth develops on the lower stem. The disease initiates from black fungal bodies present on infected seed or from the pathogen present in the soil on plant debris. Cooler conditions at planting favor the development of Rhizoctonia. Delayed emergence increases the likelihood of infection.
Control strategies:
- Use only certified and black scurf-free seed
- Use a four year rotation, preferably with cereals
- Plant in warm (60-68°F or 16-20°C), well-drained soil
- Treat seed tubers with a registered fungicide seed treatment
- Harvest the tubers as soon as they are mature, within 4 weeks of vine kill
Seed-Piece Decay
Several species of soil and seed-borne fungi and bacteria cause seed-piece decay. Seed-pieces may develop dry rot or soft rot depending upon the microorganisms involved. Consult individual diseases for their seed piece decay phase.
Control strategies:
- Avoid bruising the seed
- Preferably plant whole seed
- If using cut seed, follow proper cutting, fungicide seed treatment and storage procedures (see Seed Cutting)
- Prevent condensation on the surface of the seed by warming the seed near to soil temperature prior to planting, and plant in a warm, moist-but-not-wet soil to promote wound healing and rapid sprout growth. If soil temperature is too cold (below 50°F or 10°C) cut seed will not suberize, if it is too warm (above 64°F or 18°C) it is very favorable for
- multiplication of pathogens.
- Avoid exposure of cut seed to hot sun or drying winds
Silver scurf
Helminthosporium solani
Courtesy R. Howard, Alberta Agriculture, Food and Rural Development
This is a disease of the potato skin. The disease has markedly increased since the early 1990s due to the development of resistance in isolates of the pathogen to thiabendazole (Mertect), the only fungicide that was recommended for control of this disease. The skin spots, which are small and pale brown at harvest, enlarge during storage. Older lesions appear silvery, especially when wet. The disease, though more pronounced on white or red-skinned tubers is also found on russet-skinned cultivars. Although the pathogen can survive on plant debris in soil, the diseased seed tubers are considered the most important source of silver scurf initiation in the field. Under warm and humid conditions, the pathogen multiplies and infects daughter tubers. Delayed harvest, especially after vine kill, increases the disease severity. New tubers can be infected during harvest by intermingling with diseased tubers and during the first 2-3 weeks of storage. The pathogen can survive on structural material.
Control strategies:
- Plant certified silver scurf-free seed
- Treat seed tubers with a fungicide seed treatment registered to control silver scurf.
- Avoid planting potatoes in a field that had silver scurf the previous season
- Apply post-harvest fungicide treatments to newly
- harvested tubers (resistance in pathogen populations to thiabendazole may reduce effectiveness of this fungicide)
- Thoroughly disinfect storages before filling
- Harvest as soon as possible after vine kill or maturity
- Reduce the amount of soil and plant debris going into the storage
- Use air to dry wet tubers
- Remove field heat from tubers as soon as possible, and avoid conditions that promote condensation in storage.
Verticillium wilt
Verticillium spp.
Verticillium wilt of potato can be caused by V. dahliae or V. albo atrum. The disease can have a significant impact on the crop by reducing both tuber yield and quality. Plants infected with wilt start to show symptoms in the middle of the growing season. Individual leaves first turn pale green or yellow, leaves on affected stems then wilt, and finally the entire plant dies prematurely. Initial symptoms often develop on one side of the plant. The lower stems of diseased plants and tubers have brown discoloration in the vascular tissue when cut open. Verticillium spp. are soil-borne fungi and once established, can live for long periods in the soil even if a potato crop has not been planted for many years. The pathogen can become established in a field through the use of infected seed or by movement of infested soil.
Control strategies:
- Maintain fertility at levels optimum for high yield
- Irrigate to reduce moisture stress after flowering and during tuber bulking, but do not over-water
- Disease severity may be reduced by incorporating green manure crops in the season prior to potato production
- Practice at least a three year rotation, and preferably a four year rotation
- Do not contaminate clean fields with soil from diseased fields, diseased tubers or plant refuse.
Written by Tracy Shinners-Carnelley, Piara Bains, Debbie McLaren, Jill Thomson |
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