Potatoes

Tuber � Bacterial

SOFT ROT, Erwinia carotovora

Symptoms. Infections appear as tan to dark brown, circular water-soaked spots on tuber surfaces. Spots rapidly enlarge and underlying tissue will decompose under moist conditions. Under dry conditions, lesions become sunken, dry and hard. Internal soft rot appears wet and mushy, cream to tan-colored with a black border between diseased and healthy tissues. Decayed tissue is very soft with slightly granular consistency. Decay may consume most of the tuber, and skin above will collapse. Early stages are generally odorless; as secondary decay sets in, a foul odor and a ropy or tacky consistency will develop. If conditions are dry, decayed tissue will dry to a chalky-white mass.
Survival and Spread. Bacteria do not persist for long periods in soil without plant debris; however, they occur widely in surface water, and may infect if favorable environmental conditions exist. Moisture is critical for development; disease can develop under a wide range of temperatures if moist. Bacteria spread into soil when the tuber or stem decays. The bacteria move into soil water and from there into lenticels of tubers of adjacent plants. Bacteria spread during cutting operation and planting. Spread rapidly in field by rain splash or overhead irrigation.
Control. Use disease-free, certified clean seed pieces. Harvest only mature tubers, when skin is developed and soil temperatures are below 68°F. Minimize bruising and other damage during harvesting and handling. Store under optimal conditions after a minimum of three to five days under high humidity, high oxygen levels and 55-60°F to promote healing and suberization of tissue. Under less than optimal conditions, allow 14 days for healing process. Then cool to 50°F or cooler and provide good ventilation. Avoid free moisture at all times.

RING ROT, Clavibacter sepedonicum

Symptoms. Development favored by soil temperatures of 64-72°F. Some stems may wilt and look stunted while others appear normal. Initially, wilting of foliage on lower portions of infected plants may occur during hot periods of day, with recovery at night. Later, areas between leaf veins become pale yellow, with continued yellowing and drying of these areas and upward rolling at leaf margins. These symptoms are accompanied by progressive wilting, which eventually kills the plant. Some cultivars show atypical symptoms of dwarfing and rosetting (Russet Burbank). Some cultivars do not exhibit strong symptoms. Plants with advanced stages of disease can be cut across the base of the stem and squeezed; a creamy exudate may be expelled. Some strains cause mild to no symptoms. Infection of tubers begins at stem end. Vascular ring may show a cheesy rot, creamy yellow to light brown. In advanced cases, when tuber is squeezed, a creamy exudate comes from vascular end and there is a definite separation of tissue at vascular ring. Tubers may be cracked on outer surface.
Survival and Spread. Bacteria live in infected tubers and as bacterial slime on equipment. Later in the season, bacteria move to and infect new tubers. Wounds are necessary for infection. Bacteria do not overwinter in soil, but they can survive at least three years in dried stem or tuber tissue or on contaminated equipment. Survival is longest under cool, dry conditions and retarded by warm, moist conditions.
Control. Plant disease-free, certified clean seed pieces. Thoroughly disinfect all containers, tools, and equipment before handling new seed lots. It is essential to do this each year. Clean out storage and disinfect before placing new crop in storage. Disinfectant must be in place 10 minutes to kill bacteria.
Remarks. Recent research shows that sugarbeet can act as a symptomless host for ring rot bacterium. Further research is being done. Laboratory tests are necessary to confirm ring rot. Brown rot, which does not occur in North Dakota, and freezing injury can be confused with ring rot. Wilted plants may result from other disease or drought conditions.

Tuber � Fungal

FUSARIUM DRY ROT, Fusarium sambucinum (= F. sulphureum)

Symptoms. Easily identified by dry, crumbly decay and presence of the fungus growing in the rotting tuber. May be reddish, white, yellow or dirty tan. May cause vascular discoloration similar to Verticillium wilt. Secondary bacterial soft rot also may be present. May be difficult to see in seed lots, as decay is often internal. If infections are left unchecked, tubers will slowly rot during storage.
Survival and Spread. Seed and soil-borne. Gains entrance into tubers through wounds or injuries. Primary sources of infection are internal decay, spores on seed tuber surfaces, or in soil clinging to tubers. Also soil-borne and persists in soil for many years. Fusarium produces large numbers of spores which act as continual inoculum. Wounds to tubers during harvest act as entry sites for infection. The fresh cut surface of cut seed also acts as a site for dry rot decay.
Control. Harvest tubers after vines are dead to assure skin maturity. Avoid injuring or bruising tubers at harvest. Maintain tubers at 55° to 65°F and 90 to 95% relative humidity (RH) in storage for one to two weeks to promote wound healing. Thiabendazole (TBZ) (Mertect) may be applied post-harvest; however, resistance of Fusarium to TBZ has recently been found. Dithane ST may be applied going into storage; for seed only. Warm seed tubers before planting or cutting. Treat with fungicidal dust. Do not pile more than 6 feet deep. Plant in well-aerated moist soil at 50°F.
Remarks. Most varieties do not have resistance to dry rot.

SILVER SCURF, Helminthosporium solani

Symptoms. Light brown spots develop on tubers; later these become silvery in appearance. Heavily infected tubers may shrivel in storage. Red skinned varieties may lose color.
Survival and Spread. Infested seed tubers are the primary source of infection. Favored by warm moist soils. Leaving tubers in soil after maturity favors disease development. Disease development continues in storage, especially if humidity and temperature are high.
Control. No resistant varieties. Use disease-free seed. Harvest tubers as soon as they are mature. Store tubers at lowest possible temperature. Use seed treatment, low humidity during first month of storage, separate storage for early generation, fungicide at seed shipping time. Resistance to TBZ (applied to tubers going into storage) is common.

Stem and Tuber � Bacterial

BLACKLEG, Erwinia carotovora

Symptoms. Stems exhibit inky-black decay, which may extend for less than an inch or the entire length of the stem. May develop during any stage of plant development. Leaves roll upward at margins and turn yellow. Growth becomes stunted. Plant will eventually wilt and die as stem becomes girdled. May kill seed-piece sprouts before emergence (pre-emergence blackleg or blanking). Tubers may display vascular discoloration or soft rot of the central portions. Entire tuber may decay due to soft rot/blackleg.
Survival and Spread. Originates from infected seed pieces. Aerial blackleg is spread from other blackleg-infected plants in the area or from cull piles through wounds caused by hail, wind or cultivation. Progresses rapidly in wet weather. Bacteria do not persist in soil for long without plant debris; however, they occur widely in surface water and may infect if favorable environmental conditions exist.
Other Crops Affected. Blackleg generally affects only potato, but the disease has been reported on sunflower in North Dakota and Mexico.
Control. Use disease-free, certified clean seed. Plant in well-drained soil. Avoid excessive irrigation after planting. Plant when soil temperature is 50 to 56°F. Thoroughly clean and disinfect seed cutting, handling and planting equipment between seed lots. Remove cull piles and infected plants. Treat seed pieces with a recommended seed-piece treatment before planting.

Stem and Tuber � Fungal

SCURF & BLACK CANKER, Rhizoctonia solani

Symptoms. Emerging sprouts develop red-brown lesions or girdling cankers, become stunted or die. Some hills may have no plants emerge. Surviving plants may be purple or chlorotic, have upward leaf roll and produce aerial tubers. Tubers become crusted with hard black bodies called sclerotia which appear as "dirty spots that won't wash off."
Survival and Spread. Survives as sclerotia on tubers. Sclerotia infect emerging shoots. Disease development favored by poorly drained, warm and moist soils.
Control. Use seed that is disease free or has low levels of infestation from sclerotia. Plant into moist (not wet) soils with a temperature of 50 to 56°F at planting depth. Use seed treatment to suppress stem lesions.

Leaf and Stem � Fungal

EARLY BLIGHT, Alternaria solani

Symptoms. Symptoms develop in July or August on lower foliage. Lesions begin as dark brown to black spots that are about 1/16 inch in diameter and initially roughly circular. They enlarge to 1/4 inch in diameter and develop a somewhat irregular shape and usually develop concentric rings that give a target pattern to individual spots. Tissues surrounding spots turn yellow; entire leaflets may turn yellow, and the entire lower portion of the plant becomes yellow. Lesions increase in size and coalesce and may occur on upper leaves. When infection is severe, brown to black target spots may develop on the stems.
Tubers may develop dark brown to purple, circular to irregularly shaped lesions. Lesions tend to become sunken with a dark purplish, raised border. A yellowish discoloration develops next to the dark border. Lesions do not penetrate deeply into the tuber. Soft rot may become a secondary infection.
Survival and Spread. Survives on tubers and potato crop residue, as well as on and in soil. Infections begin on lower foliage early in the season and are the source of fungus buildup. Lesions are most numerous on lower foliage. Favored by warm weather, especially alternating wet and dry. Disease development is favored on older foliage, stressed foliage, and plants with poor mineral nutrition. It is more serious on early maturing varieties. When disease is severe, spores are washed from foliage down into soil and onto surface tubers. Immature tubers are easily infected, but mature tubers are infected only through wounds. Tuber lesions are blue-gray in color and sunken.
Control. Crop rotation helps reduce levels of overwintering early blight fungus. To prevent tuber infection, allow tubers to mature prior to harvest. Use harvest practices and equipment that minimize wounding and bruising.
Chemical Control. Effective fungicides include mancozeb (Dithane, Manzate, Penncozeb), maneb (Maneb 80, Maneb 75, Manex), chlorothalonil (Bravo, Ensign, Echo, Terranil) and triphenyltin hydroxide (Super Tin Ag Pak). Copper fungicides are less effective. Fungicide applications should begin when first lesions are visible on lower leaves of early varieties or when indicated by a disease forecasting system; information on this disease forecasting is available through a telephone service called the Potato Blight Hot Line. Fungicides will not kill existing leaf spots but will protect against new infections. Complete coverage with fungicide applications is necessary for good control.

LATE BLIGHT, Phytophthora infestans

Symptoms. Late blight can be a serious problem in areas where cool, humid weather is common. Outbreaks in warmer, drier areas are sporadic, occurring only when weather conditions are favorable. Infects leaves, stems and tubers. Initial symptoms on leaves are dark brown to purplish-black water-soaked spots, often starting at the edges. Lesions are not restricted by the veins and may spread until the entire leaflet, leaf, or plant is killed. Borders of the spots are light green to greenish-yellow and water-soaked. In humid weather, a fuzzy white growth develops along lesion borders on the lower leaf surface. This growth (sporulation) does not form in dry weather. A characteristic odor is noticeable in any field that has a significant amount of blighted foliage.
Tuber infections initially appear as a shallow rot with a distinct border. The infected area is a metallic dull brown, reddish or purplish brown or black. The tuber rot is firm, dry and penetrates the tissues irregularly. No yellow dis-coloration is found along the border; this is a way to tell late blight from early blight. Soft rot may set in as a secondary infection.
Survival and Spread. Overwinters on infected tubers. Primary infection comes from infected sprouts of diseased seed pieces or tubers. Spore production occurs on infected sprouts when humidity exceeds 90% and is favored by temperatures around 70°F. The spores are carried by wind and rain for at least 50 miles. After they land on susceptible potato plants, they germinate in one of two ways: they may produce a germ tube that infects the leaf (direct germination) or they may produce tiny swimming spores (zoospores) which are disseminated in water and later form germ tubes that infect the leaf (indirect germination). The optimum temperature for direct germination is 75°F and the optimum temperature for indirect germination is 54°F. Disease buildup is most rapid when nights are cool and humid (around 54°F) and the days are cloudy and around 70°F.
Tubers not covered by soil are infected by zoospores that are washed from infected foliage. Tubers close to the soil surface also may be infected by swimming zoospores.
Control. Use blight-free seed and destroy cull piles and volunteer plants. Use crop rotation to avoid or reduce carryover of the fungus. Follow the Blight Hot Line to determine when conditions favor late blight. Hill soil up around plants to prevent spores from reaching tubers. Kill vines at least two weeks before harvest so tuber skin is mature and any infected tubers will have disintegrated before digging.
Apply protectant fungicide before the rows close. Follow recommendations on the Blight Hot Line. Since the A2 mating type of the fungus first appeared in 1995, the disease is more difficult to control and timely applications are critical for late blight management.

VERTICILLIUM WILT, Verticillium albo-atrum and V. dahliae

Symptoms. Wilt is the first symptom to appear, with plants that wilt during the day and recover at night. Later, leaves turn pale green or yellow. Plants die early. Vascular ring of stems is discolored. Vascular ring of tubers is brownish, especially toward the stem end. Eyes may show a pink discoloration.
Survival and Spread. Short crop rotations favor disease buildup. Highly susceptible varieties such as Kennebec favor rapid disease buildup. Tiny black fungus bodies that resemble finely ground pepper and are called microsclerotia form in stems and survive several years in soil. Irrigation early in season favors early germination of microsclerotia and early infection.
Control. Crop rotation with cereals, grasses or legumes. Remove severely infected potato vines after harvest. Some varieties have moderate to good resistance and limit buildup of pathogen in soil. Use seed treatment of seed pieces. Soil fumigation can be used but is expensive.

Leaf and Stem � Mycoplasmal

PURPLE TOP, Aster yellows mycoplasma

Symptoms. Upper leaves are curled and slightly reddened or purpled. Plants may be stunted and die early. Stems may be thickened and aerial tubers may form in axils of leaves. Infected tubers may produce thin sprouts called "hair sprouts."
Survival and Spread. Transmitted by aster leafhopper, Macsosteles fascifrons. Mycoplasma multiplies in the leafhopper. Mycoplasma and leafhoppers survive in southern climates and are carried north in the spring by wind. Potato is not a preferred host of aster leafhoppers, which tend to move around, infecting many plants.
Control. No practical control at present.
Remarks. Aster yellows infected tubers produce abnormally dark potato chips.

Sugarbeet

Leaf Diseases � Bacterial

BACTERIAL LEAFSPOT, Pseudomonas syringae

Symptoms. Dark gray to black leaf spots up to ¼ or ½ inch in diameter. Leaves tear or become tattered.
Survival and Spread. Favored by rain and storms, spread by splashing water. Infects through wounds.
Control. None developed. Usually none needed.
Remarks. Usually appears in late June or early July, slightly before Cercospora leafspot. Severe bacterial leafspot may confound early season diagnosis of Cercospora. Cercospora leaf spots have tiny black dots in them; bacterial leaf spots do not.

Leaf Diseases � Fungal

CERCOSPORA LEAFSPOT, Cercospora beticola

Symptoms. Circular spots about 1/8 inch (occasionally 3/16 inch) in diameter with ash gray centers and dark brown to reddish purple brown borders. During warm, rainy, humid weather, spots may coalesce to kill whole leaves. Gray centers of leaf spots usually will have tiny black dots or a fuzzy blue-gray appearance (spore production). Severely diseased leaves wither and die, resulting in severe defoliation. Disease begins on older leaves and progresses to younger leaves.
Survival and Spread. Most common source of Cersospora fungus is infected beet debris in field. Spores are produced most readily at temperatures of 68 to 79°F and relative humidities (RH) of 90 to100%. Spores do not form at less than 50°F. Spores germinate and greatest infection occurs when temperature is 75 to 77°F, the RH is 100% for at least 8 ½ hours and there is free water on the leaf. Day temperatures of 80 to 90°F and night temperatures above 60°F favor disease development. Symptoms may occur in as few as five days after infection, with more spores produced in another five days.
Control. Crop rotation is important; a three-year or longer rotation is minimal for reducing carryover of the fungus. Beets should be planted more than 100 yards from a field that was in beets last year. Burying beet refuse by tillage helps reduce inoculum survival and dispersal. Fall tillage is most effective but may increase soil erosion. Some varieties are more resistant than others to this fungal infection.
Chemical Control. Protectant fungicides and systemic fun-gicides are currently registered (1996). Protectant fungicides must be applied before the rainy or humid weather occurs, with enough time to allow spray droplets to dry. Careful scouting and use of the Cercospora prediction model can provide proper timing of treatments. Check for current recommendations for fungicides. Tolerance to triphenyltin hydroxide (Super Tin) is common. Use of tank mixes or alternating with other classes of fungicide is recommended. Systemic fungicides currently registered are of the benzimidazole class and should be used only once in a season and only in a tank mix with another class of fungicide, such as mancozeb, in Minnesota or North Dakota because of the high prevalence of resistant strains of Cercospora.

SUGARBEET POWDERY MILDEW, Erysiphe betae

Symptoms. Wispy growth of white to light gray threadlike filaments, often radiating from a central point appears on lower leaves first. Early symptoms are not easily detected. Disease may progress rapidly in favorable weather. A powdery white or gray-white growth may cover a leaf within a week, and mildew may begin to appear on middle leaves. When disease is severe, mildew may appear on upper, not completely expanded, leaves. Characteristic odor similar to a musty basement may be noted. Severely mildewed leaves may turn yellow within a month of initial symptoms. Mildewed leaves are killed by light freezes that do not kill healthy leaves. Powdery mildew is less severe where nitrogen fertility is high. Look in areas of the field showing signs of nitrogen depletion; if not found there, it is unlikely to be found in other areas of field.
Survival and Spread. Favorable conditions are: long periods of drought, warm days, cool nights, and wide fluctuation in day-night temperatures. The most favorable temperatures for production of fungus spores (conidia) are day/night temperatures of 81°F/54°F. Other favorable temperatures are 86°/50°F and 90°/45°F. Periods of major infection appear to occur when temperatures are highly favorable for several days to a week at a time. Once infection occurs, the incubation period can be as little as five days if temperatures are around 77°F. Overwintering of powdery mildew fungus appears to be sporadic.
Control. Sulfur is a relatively inexpensive and effective control. Application at first sign of mildew is essential; delay may result in little yield improvement. Bayleton is also registered for control of powdery mildew on sugarbeets but is more expensive. Check the label for recommended rates.
Remarks. Yield losses result from losses in tonnage, in lowered percent sucrose, and increased impurities.

Root � Fungal

RHIZOCTONIA ROOT AND CROWN ROT, Rhizoctonia solani

Symptoms. May attack and kill six- to eight-leaf seedlings. Later in the season plants wilt and die. Leaves of dead plants lie flat on ground, with black petioles. Roots develop a sunken and cracked brown rot. Entire beet may rot, leaving "holes in the ground" at harvest.
Survival and Spread. Favored by warm moist soils. Survives several years in soil and on crop refuse. Infection may occur through crown of plant.
Other Crops Affected. Dry beans, soybeans, flax, canola, lentils.
Control. Minimum of three year rotation. Avoid other susceptible crops in rotation. Deep plowing helps bury and dilute inoculum. Some specialty varieties show some degree of resistance. Avoid throwing soil on crown of plant during tillage operations.

APHANOMYCES ROOT ROT, Aphonomyces cochlioides

Symptoms. Seedlings may be killed; surviving seedlings are stunted and unthrifty. Later in season plants develop a light green color and wilt at mid-day. Root tips rot; tap roots are short with many fibrous roots on the base.
Survival and Spread. Favored by warm and wet soils. Most serious if infection occurs in seedling stage. Survives 20 years in soil. Infects some common weed hosts, which may help maintain population between sugarbeet crops.
Control. Crop rotation is of no value. Promoting good field drainage may reduce infection levels. Some specialty varieties have tolerance to Aphanomyces. Seed pelleting with hymexazol (Tachigaren) gives early season protection (three to four weeks) but does not provide season-long control; most effective if used with a tolerant variety.

Root � Viral

RHIZOMANIA, Beet Necrotic Yellow Vein Virus (BNYVV)

Symptoms. Patches of plants show poor growth. Plants turn light green or yellow green and may be stunted. Leaves become somewhat narrow, petioles sometimes long and erect. Taproot may have "wine-glass" appearance, with constriction in middle, or may be shortened. Proliferation of secondary roots, giving a "whiskered" appearance. When cut open, vascular tissues of taproot may be discolored.
Survival and Spread. Caused by beet necrotic yellow vein virus (BNYVV). Virus transmitted by a soil fungus, Polymyxa betae. Fungus produces swimming spores called zoospores which carry BNYVV; virus is carried into root hairs when fungus infects them. Fungus grows in roots, and forms resting spores called cystosori, which are released into soil when root decays. Cystosori survive 20 years in soil; virus survives inside the cystosori. Infection favored by warm, wet soils. Early infection can cause severe losses.
Control. Once soil is infested, crop rotation is impractical. Minimize spread to other fields: avoid moving soil between fields. Harvest diseased fields last, power wash and steam clean equipment after harvesting. Promote good drainage to avoid waterlogged soils. Resistant varieties should be available by year 2000.
Remarks. Identified in a number of fields in southern Minnesota in 1996. Complete distribution unknown at present (1996) but may be more widespread. Positive identification can be done only by sophisticated laboratory tests using serological techniques.