Diseases Primarily of Solanaceous Crops

Anthracnose on peppers (Colletotrichum piperatum)

On peppers, infected fruit develop circular depressions. Red-ripe fruit are most susceptible. Fungal inoculum overwinters on debris including seeds from infected fruit. Transplants also serve as a source of inoculum. Disease incidence increases as humidities and temperatures rise. Use of disease-free seed and crop rotations will lower field incidence. Rapid field heat removal and storing fruit at 45 to 50 degrees F will help slow the development of sunken areas in fruit harvested from infected plants.

Anthracnose on tomatoes (Colletotrichum spp.)

This fungal disease is not a problem on commercial fresh market tomatoes because fruit are harvested before ripening. Anthracnose is a problem on processing tomatoes and in home gardens, because ripe fruit are left in the field for extended periods. Initial symptoms are small, circular, slightly sunken, water-soaked lesions which become darker and more depressed and spread to form a semi-soft internal decay. Rotation to another crop for two years will help control anthracnose.

Bacterial canker (Clavibacter michiganense)

Bacterial canker can attack any part of the plant at any stage of development, and can survive up to five years in the soil, and up to two years in compost. Canker is promoted by conditions favoring succulent plant growth. Symptoms are accentuated in sandy soils. Soaking seeds in a 20 percent bleach solution for 40 minutes, or 133 degrees F water for 25 minutes destroys bacteria on the seed surfaces but not inside seeds. Transplants are a potential source of infection and can be treated with streptomycin or copper (effective only if applied early). Cultural controls include sterilizing pruning tools frequently (after every row), avoiding irrigation and drainage water from canker-contaminated fields, staying out of fields when they are wet and disking plant debris after the final harvest.

Bacterial leaf spot (Xanthomonas campestris)

Bacterial leaf spot produces spots on leaves, fruits, and to a lesser extent, stems. On the undersides of young leaves, the spots are small, yellowish green and slightly raised. On older leaves, initially the spots are dark, water soaked and not noticeably raised. When spots are few, they may enlarge to 1/8 to 1/4 inch in diameter. Severely spotted leaves turn yellow and drop off. Seedlings infected in the plant bed often lose all their leaves except those at the top of the plant. In the field, plants may lose many of their leaves.

On fruits, the small blister-like spots are somewhat circular and may be 1/4 inch in diameter. Spots turn brown and appear cracked, roughened, and warty. In damp weather, various secondary decay-producing organisms enter through these spots and cause the fruit to rot.

Contaminated seed is a common source of first infection so seed should be disinfected with bleach. Bacterial spot also overwinters on residue of infected plants and is spread in the field on machinery. Bacteria can overwinter on debris buried more than 12 inches below the surface but are destroyed on the soil surface or if brought to within 12 inches of the surface. Plants infected in the seedbed are the primary source of the disease in the field. Dry weather will usually slow the disease. Prolonged periods of warm, rainy weather, especially when driving rain and wind have broken leaves or stems, are often followed by severe outbreaks. Florida research showed that damage from wind and wind-blown sand, and from splashing during rain storms, increa sed the number of bacterial spot lesions. Windbreaks, such as sugarcane or other tall grasses, planted approximately every 12 rows usually reduce wind damage.

Bacterial spot (Xanthomonas campestris) and speck (Pseudomonas syringae pv. tomato)

These bacterial diseases can cause defoliation and blossom blight in tomato. Lesions on immature fruit appear in warm, moist weather. 'Spot' fruit lesions are larger and less numerous than 'speck'. Small water-soaked spots also appear on leaflets, followed by a yellowish halo. These small spots may coalesce into large, irregular dead spots on the leaves.

Control measures include: 3-year rotations, use of disease-free seed and regular applications of transplants, and combinations of fungicides and fixed copper (starting soon after transplanting). Mixing copper-maneb solutions the night before application increases their effectiveness in Florida where the natural pH of water is high. In California, it is not usually economical to treat for speck if infection first occurs after the 5-leaf stage because there is little injury to the plant after that time.

Bacterial wilt (Pseudomonas solanacearum)

The initial symptoms of bacterial wilt of tomato are wilting of the younger leaves and yellowing of the older leaves (lower foliage), followed by browning of the vascular system of the roots and lower stems. Eventually the entire plant wilts and dies. This disease is favored by warm weather (85 to 95 degrees F).

Control of bacterial wilt is extremely difficult because the pathogen can remain viable in the soil for many years. This bacteria overwinters in weed hosts and plant debris and can be spread by infected seeds and machinery. Nematode infections increase the severity of wilt. The wide host range of bacterial wilt includes all solanaceous crops and peanuts which severely limits crops that can be used in rotations. Fumigation is of limited value. Tolerant cultivars are listed at the end of the Tomato chapter.

Experiments at two sites in Florida tested the effect of composted organic amendments on bacterial wilt of tomato. At one site, yard waste, mushroom compost, and yard waste plus poultry litter suppressed bacterial wilt compared to sewage sludge, yard waste plus cow manure or no organic amendments. On the other site, only mushroom compost reduced disease incidence compared to controls. Site differences were attributed to possible differences in pathogen strains at the two sites.

Blackleg (Erwinia carotovora)

Blackleg, or soft rot of potato, is a bacterial disease usually affecting young plants. Beginning below the soil line, a wet, black stem lesion spreads and eventually girdles the shoot. Stands are poor, plants appear sickly, and leaves wilt and die. This fungus also causes a stem rot in tomatoes.

Cool, wet soil conditions retard the healing and skin hardening process on cut surfaces of seed pieces and retard early growth and development of the vines, factors which increase disease severity. Seed cutting knives and equipment not cleaned after cutting infected tubers can contaminate healthy tubers. Bacteria may be spread in the field by seed corn maggots or seed potato maggots which have fed on infected plants.

Planting only disease-free certified seed, sterilizing seed cutting knives and equipment, and controlling maggots reduces the incidence of blackleg. Planting on well-drained soils and using a three-year rotation out of susceptible crops such as beans, carrots, cucumbers, turnips and beets also helps. Cereal crops are excellent rotational crop choices.

Early blight (Alternaria solani)

Early blight affects both tomatoes and potatoes. Symptoms include collar rot of seedlings, and leaf spots or blight which can defoliate the plants. Distinctive symptomatic features are the 'bulls-eye' appearance of leaf spots, and defoliation starting at the base of the plant. Early blight can lead to complete defoliation during humid weather at temperatures near 75 degrees F.

In tomatoes, early blight development can be restricted by maintaining plants in a vigorous state, minimizing injury to the plants and limiting the time leaves are wet. Staking or trellising tomatoes facilitates better airflow around plants and improves fungicide coverage, thereby slowing the onset and rate of development of early blight. Where early blight is a problem, increased yields of plants grown on trellises compared to those grown on the ground often compensate for trellising costs. Indeterminate cultivars produce foliage all season and are therefore less susceptible than determinates to yield loss related to early blight defoliation. Regular sprayings with fungicides at 7- to 10-day intervals are now necessary in areas such as the Appalachians where disease pressure is high, but disease-forecasting programs are being developed to predict the rate of early blight development based on weather patterns. Such programs should enable growers to better time their fungicide applications.

In potatoes, initial early blight symptoms are spots with yellow halos on the lower leaves. Major leaf veins often limit expansion of the early lesions, but later lesions may merge with adjacent lesions to cover the entire leaf. Under conditions of heavy dews, frequent rains, and warm temperatures, fungal spores develop rapidly and are spread by wind and some insects. Spores and dormant mycelia overwinter in the soil in potato debris, including tubers. Infected tubers used for seed are a primary source of infection. In potatoes, early blight develops slowly throughout the season and affects only the foliage so, despite its name, it is rarely a problem on early potatoes. Early blight should be monitored by observing lower leaves, and fungicides should be applied when 5 percent of leaves are infected.

Fusarium root rots and seed piece decay in potatoes

Fusarium infection just after planting can weaken plants and infect potato tubers internally. Planting cut, unhealed seed pieces in wet, cold soils increases their susceptibility to Fusarium and related root rots.(49) Rotation out of solanaceous crops, planting of disease-free seed, improving drainage, and increasing water holding capacity of soils by organic matter additions, will lower root rot incidence.

In Maine experiments, chisel plowing was more effective than moldboard plowing in reducing the incidence and severity of Fusarium stem lesions on potato. This was attributed to more rapid decomposition of infected residue after chisel plowing.

Rotational crops also appeared to be important where residues of these crops were chisel plowed under. Fusarium root rot incidence was lower in potatoes following broccoli than in potatoes following oats, buckwheat, lupine, or peas. Oats increased Fusarium severity in subsequent potato crops when residues were turned under with a moldboard plow.

Gray mold (Botrytis cinerea)

Gray mold develops on tomato under cool, wet conditions, especially when air circulation is poor. Fungicides will not suppress an established infection, but offer some protection if used before infection occurs.

Late blight (Phytophthora infestans)

Initial symptoms of late blight on tomatoes or potatoes are blackish-purple water-soaked lesions on the tips and edges of lower leaves. If the weather turns warm or humidity drops below 90 percent, these lesions dry up and disease spread is restricted. When favorable blight conditions exist, however, a white spore-producing mildew appears on lower leaf surfaces at the outer edge of the lesion. Under ideal conditions for disease development (55 to 65 degree F night temperatures accompanied by rain, lingering dew, fog or very high relative humidity), the crop can be destroyed within a week.

Infection sources include infected seed stock, blighted tubers in cull piles, volunteer plants growing from infected tubers, and wind-blown spores. Spores develop on diseased plant parts and are splashed to other parts of the plant or other plants. Spore germination is favored by warm daytime temperatures.

In potatoes, current control practices require spraying every 5 to 10 days after the vines reach 6 to 12 inches if disease spread is indicated by predictive programs such as BLITECAST (described below). Other control measures include: using disease-free seed; eliminating cull piles; planting resistant cultivars; and killing potato foliage 10 to 14 days before harvest. Killing foliage prevents migration of disease from foliage to tubers.

BLITECAST is a computerized system for forecasting potato late-blight epidemics based on maximum and minimum temperatures, rainfall and relative humidity. Depending on conditions, growers are advised to postpone spraying, to be alert to potential blight favorable conditions, or to spray every 5 or 7 days. BLITECAST was first available in 1983. By 1987 it was being used on at least 28,000 acres of potatoes in the United States.

In tomatoes, late blight can be introduced on transplants or can migrate from potato or other tomato fields as airborne spores. Available resistant cultivars are listed in the Tomato chapter. Controls include isolation of tomatoes from potato fields, using disease-free transplants, and destroying potato cull piles.

Phomopsis rot (Phomopsis vexans)

Phomopsis rot is a fungal disease of eggplant favored by warm weather. Symptoms include: large, circular, sunken tan or black areas on the fruit; canker-like lesions on the stem; and gray-brown spots on the leaves. Controls include cleaning up plant debris where the pathogen can overwinter, a 3-year crop rotation, and using clean seed. Resistant cultivars are listed at the end of the Eggplant chapter.

Potato early dying

Premature vine senescence and declining yields characterize production areas where potatoes have been grown in the same fields for a number of years. Although it appears gradually, and growers may view it as normal, it is considered by the potato industry to be one of the most important constraints to potato production in North America. The optimal conditions for early dying development are 70 to 80 degree F temperatures and wet soils. In most cases, early dying is associated with invasion of root- lesion nematodes into plant roots, followed by Verticillium infection.

There is considerable disagreement on whether crop rotations are effective in reducing injury from Verticillium. Rotation problems include identifying crops that are non-hosts for both Verticillium and root-lesion nematodes, and keeping susceptible crops out of the field for long enough to reduce populations of viable microsclerotia, the long-lived reproductive Verticillium propagules. For example, cereals, corn, grasses, onions, carrots, beans, peas and asparagus are not susceptible to Verticillium, but corn, soybeans, snap beans and wheat are susceptible to root-lesion nema-todes. Alfalfa is a good rotational choice. Microsclerotia of Verticillium can survive in the soil for at least eight years without a host, although the population of microsclerotia, as a whole, declines each year because of soil microbial activity. It takes a four-year rotation out of potatoes to decrease microsclerotia populations sufficiently to reduce crop loss.

Potatoes are less susceptible to early dying if irrigation is reduced once tubers are initiated. Studies in Oregon and Wisconsin showed that early dying severity was greater and yields lower with excessive irrigation (25 percent in excess of the water used by the plants) compared to deficit irrigation (only 75 percent of the water used by the plants was replaced).

Another early dying management strategy is the use of green manure crops such as corn, oats, peas, rape, rye and sudangrass for 2 to 3 years. The disease suppressive properties of these crops are apparently related to changes in the soil microflora.

Scab of potato (Streptomyces scabies)

Potato scab, caused by the bacteria Streptomyces scabies, infects tubers, roots, stems and rhizomes. Damage to the skin of young potatoes is the most serious economical consequence. Three types of symptoms occur: 1) russet scab with superficial, corky lesions; 2) raised scab with erupting lesions; and 3) deep-pitted scab with sunken dark brown lesions. Scab tends to be more severe in fields newly planted to potatoes and in fields with high undecomposed organic matter content. Scab is spread by infected tubers and contaminated soil and manure.

Adequate soil moisture, especially during tuber initiation and early growth, lowers scab incidence. Scab is generally more severe under dry, warm conditions. Maintaining a soil pH of 5.0 to 5.3 also slows disease development. Rotating with grains, soybeans or alfalfa (but not root crops such as carrots or radishes) lowers disease incidence. Although there are resistant cultivars (see the Potato chapter), no chemical controls exist.

References