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Garden Diseases

SOME COMMON VEGETABLE DISEASES
An Aid to Identification and Control


Fursarium Wilt of Tomatoes


Fursarium Wilt of Tomatoes is caused by the fungus Fursarium oxysporium lycopersici.


The main symptoms are brown discolorations of the vascular tissue in the stem and gradual yellowing and wilting of the entire plant. This fungus survives in the soil for many years. At least two races of the fungus are present in the United States.


Many of the newer tomato varieties show good resistance to Race 1. However, not all existing tomato varieties have this resistance. Many other plants are affected by forms of Fursarium sp., but the form that attacks tomato is specific to tomato.

 

Prevention & Treatment: Control can be obtained by growing plants in pathogen-free soil, using disease-free transplants and growing only cultivars at least resistant to races 1 and 2 of Fusarium wilt (indicated by FF following the tomato cultivar name). Some newer cultivars are resistant to races 1, 2 and 3, and can be found listed in Table 4. Raising the soil pH to 6.5-7.0 and using nitrate nitrogen (such as in calcium nitrate) rather than ammoniac nitrogen (as in 5-10-10, 10-10-10, or 34-0-0) will retard disease development. No chemical control is available.

 

Southern Blight of Tomatoes

 Southern Blight of Tomatoes is caused by the fungus sclerotium rolfsil.


Distinguishing sing is white mold, often with “seeding like” sclerotia, present on infected stem near soil line. This fungus also causes rot of tomato and other plant parts in contact with soil. The fungus lives in the soil for many years. It causes wilting and dying of many different kinds of plants.


Southern Blight is more common in the South in hot summer months. Tomato varieties have poor resistance to this disease.

 

Prevention & Treatment: Crop rotation with non-susceptible grass crops and removal of plant debris immediately after harvest will help to control the disease. Do not plant tomatoes after beans, pepper or eggplant. Calcium nitrate may be applied at transplanting.


Bacterial Wilt of Tomatoes

Bacterial Wilt of Tomatoes is caused by the bacteria Pseudomonas solanacearum.


Rapid, sudden wilting of the entire plant denotes this disease. Pith of the main stem near soil line is usually brown at first, but soon develops a slimy rot. Bacteria attack numerous other plants, especially in the Solanaceae(night shade) and legume families, and may live in soil for many years.


Wilt is more apt to be a problem on newly cleared land or fields that have been in weeds for several years. Tomato varieties show poor resistance.


Prevention & Treatment: Control of bacterial wilt of plants grown in infested soil is difficult. Rotation with non-susceptible plants, such as corn, beans and cabbage, for at least three years provides some control. Do not use pepper, eggplant, potato, sunflower or cosmos in this rotation. Remove and destroy all infected plant material. Plant only certified disease-free plants. The cultivar Kewalo is partially resistant to bacterial wilt, but is an uncommon cultivar. Chemical control is not available for this disease.

 

Early Blight of Tomatoes


Early Blight of Tomatoes is caused by the fungus Alternaria solani.


This disease usually begins on lower leaves and gradually spreads upward. Concentric rings (target like) that develop in leaf and stem spots are characteristic. The fungus may live in soils for 1 to 2 years. In addition, it is spread on transplants and by windblown spores.


It may appear on fruit, mainly as firm, dark-colored decay at stem end. Some of the newer tomato varieties have a moderate degree of resistance.

 

Prevention & Treatment: Use resistant or tolerant tomato cultivars. Use pathogen-free seed and do not set diseased plants in the field. Use crop rotation, eradicate weeds and volunteer tomato plants, space plants to not touch, mulch plants, fertilize properly, don’t wet tomato plants with irrigation water, and keep the plants growing vigorously. Trim off and dispose of infected lower branches and leaves.If disease is severe enough to warrant chemical control, select one of the following fungicides: maneb, mancozeb, chlorothalonil, or copper fungicides. Follow the directions on the label.


Late Blight


Late blight is a potentially serious disease of potato and tomato, caused by the fungus Phytophthora infestans. Late blight is especially damaging during cool, wet weather. The fungus can affect all plant parts. Young leaf lesions are small and appear as dark, water-soaked spots. These leaf spots will quickly enlarge and a white mold will appear at the margins of the affected area on the lower surface of leaves. Complete defoliation (browning and shriveling of leaves and stems) can occur within 14 days from the first symptoms. Infected tomato fruits develop shiny, dark or olive-colored lesions, which may cover large areas. Fungal spores are spread between plants and gardens by rain and wind. A combination of daytime temperatures in the upper 70s °F with high humidity is ideal for infection.


Prevention & Treatment: The following guidelines should be followed to minimize late blight problems:


Keep foliage dry. Locate your garden where it will receive morning sun. Allow extra room between the plants, and avoid overhead watering, especially late in the day. Purchase certified disease-free seeds and plants. There are no late blight-resistant tomato cultivars. Destroy volunteer tomato and potato plants and nightshade family weeds, which may harbor the fungus. Do not compost rotten, store-bought potatoes. Pull out and destroy diseased plants.


If disease is severe enough to warrant chemical control, select one of the following fungicides: chlorothalonil, copper fungicide, maneb or mancozeb. Plant resistant cultivars.

 

Septoria Leaf Spot


This destructive disease of tomato foliage, petioles and stems (fruit is not infected) is caused by the fungus Septoria lycopersici. Infection usually occurs on the lower leaves near the ground, after plants begin to set fruit. Numerous small, circular spots with dark borders surrounding a beige-colored center appear on the older leaves. Tiny black specks, which are spore-producing bodies, can be seen in the center of the spots. Severely spotted leaves turn yellow, die and fall off the plant. The fungus is most active when temperatures range from 68 to 77° F, the humidity is high, and rainfall or over-head irrigation wets the plants. Defoliation weakens the plant, reduces the size and quality of the fruit, and exposes the fruit to sunscald (see below). The fungus is not soil-borne, but can overwinter on crop residue from previous crops, decaying vegetation and some wild hosts related to tomato.

 

Prevention & Treatment: Currently grown tomato cultivars are susceptible to Septoria leaf spot. Crop rotation of 3 years and sanitation (removal of crop debris) will reduce the amount of inoculum. Do not over-head irrigate. Repeated fungicide applications with chlorothalonil, copper fungicide, maneb or mancozeb will keep the disease in check.


Bacterial Spot


This disease is caused by the bacterium Xanthomonas vesicatoria, which attacks green but not red tomatoes. Peppers, bean pods, peaches and pumpkins are also attacked. The disease is more prevalent during wet seasons. Damage to the plants includes leaf and fruit spots, which result in reduced yields, defoliation and sun- scalded fruit. The symptoms consist of numerous small, angular to irregular, water-soaked spots on the leaves and slightly raised to scabby spots on the fruits. The leaf spots may have a yellow halo. The centers dry out and frequently tear.


The bacteria survive the winter on volunteer tomato plants and on infected plant debris. Moist weather is conducive to disease development. Most outbreaks of the disease can be traced back to heavy rainstorms that occurred in the area. Infection of leaves occurs through natural openings. Infection of fruits must occur through insect punctures or other mechanical injury.


Bacterial spot is difficult to control once it appears in the field. Any water movement from one leaf or plant to another, such as splashing rain drops, overhead irrigation, and touching or handling wet plants, may spread the bacteria from diseased to healthy plants.


Prevention & Treatment: Only use certified disease-free seed and plants. Avoid areas that were planted with peppers or tomatoes during the previous year. Avoid overhead watering by using drip or furrow irrigation. Remove all diseased plant material. Prune plants to promote air circulation. Spraying with a copper fungicide will control the bacterial disease.

 

Gray Leaf Spot of Tomatoes


Gray Leaf Spot of Tomatoes is caused by the fungus Stemphylium solani.


Characteristics symptom is numerous, rounded leaf spots, brown with gray centers, which are smaller than those of early blight. These spots often grow together to kill the entire leaf. The disease does not occur on the fruit.


The fungus may live for 1 to 2 years in the soil. It can be spread on transplants and by windblown spores. Good resistance is found in some of the newer varieties.

 

Prevention and Treatment


Plant only high quality transplants, discarding any that show any sign of disease or stress.


Practice crop rotation on a three-year cycle with non-hosts. Remove crop debris, volunteer tomato plants, and weeds that can host the disease. Avoid wetting the leaves when possible by watering the base of the plant. Plant resistant varieties (many are available).Fungicides are very effective at controlling the disease


Corn Smut


Corn Smut is caused by the fungus Ustilango maydis.


Smut is a common disease of both sweet and field corn, but usually causes only minor crop loss. It may affect ears, stalk, leaf, and tassel, but it invades only young tissues. The fungus can live in the soil for 1 to 2 years and is spread by wind and water. Some varieties show good resistance.

 

Prevention & Treatment: Pick off and destroy infected ears and galls while they are immature and have not yet released spores. Remove galls carefully, since spores can readily blow to nearby plants, causing more disease. Corn smut overwinters on plant debris in the soil, so do not put infected crop residues back into the soil. The most susceptible plants are those grown in soils high in nitrogen. No chemical controls are available.

 

Seedling Disease (Damping-off)


The fungi Pythium and Rhizoctonia cause damping-off of tomato seedlings. Seedlings fail to emerge in the greenhouse or small seedlings wilt and die soon after emergence or transplanting. Surviving plants have water-soaked areas on the stem close to the soil line.

 

Prevention & Treatment: Damping-off is often a problem in plants that are planted too early in the spring. The fungi are more active in cool, wet, rich soils. To prevent damping-off, take these precautions:


Start seeds indoors in sterilized potting mix. Do not start seeds in soil that has a high nitrogen level. Add nitrogen fertilizer after the seedlings have produced their first true leaves. Allow the surface of the soil to dry between watering.

 

Powdery Mildew


Powdery Mildew is caused by the fungus Erysph sp. Many different plants are attacked by the powdery mildews, but in general each powdery mildew fungus is restricted to a few closely related host species. White, moldy growth mostly on upper leaf surface is characteristic. It does not usually develop on the fruit.


The fungus is spread long distances by wind. Varieties of many crops are resistant.

 

Cultural Controls


As with all diseases, optimum plant health is the first line of defense. This begins with selection of healthy plants that are planted properly and in the proper location, giving attention to requirements for light, soil, and moisture. Space them so they are allowed to grow without being crowded and water thoroughly during establishment, and later during dry periods. Avoid overhead irrigation which raises the level of relative humidity within the plant canopy.


If powdery mildew is noticed on a few leaves, simply removing them will help with control. At the end of the growing season, prune out infected stems and remove fallen leaves which can serve as a source of further infection. Suckers are common on crape myrtle, dogwood and other plants. These should be pruned as they develop because they are especially susceptible and the disease will spread from them upwards to other plant parts. Fertilize to optimize plant health, but avoid over fertilization with nitrogen as it stimulates young, succulent growth which is more susceptible to infection. Plants with a severe infection should be monitored closely the following spring so that if infections reoccur, they can be treated early.

 

Chemical Control


For fungicides to be effective, they must be applied as soon as symptoms are noticed. Product labels will provide information on how often to spray. When ranges are given, use the shorter interval during cool, damp weather. Be sure to cover both the upper and lower surfaces of the leaves. Plants Myclobutanil, propiconazole, thiophanate-methyl, and triadimefon have systemic properties and can be sprayed less often than sulfur or copper-based fungicides. When powdery mildew persists and sprays are repeated, it is recommended to rotate (alternate) fungicides to decrease the chance of fungi developing resistance. When deciduous plants are infected, consider the season. Generally, foliar diseases occurring in late summer do little damage. The leaves have already produced food for the plant and are going to fall off soon anyway. Just be sure to rake and dispose of them as they fall. As with any pesticide, read the label and heed all precautions. Sulfur, for example, can damage plants if applied when temperature and humidity are high.


Nematode Root knot


Nematode Root knot is caused by the nematode Meloidogyne sp.


Distinguishing symptom is galls on roots. Affected plant becomes unthrifty, off-colored, stunted, and may eventually die. Greatest damage is likely to occur during dry periods.


Root knot is common in both field and garden soils and attacks a wide variety of plants. Resistance is found in some crops. Control is mainly by soil fumigation and by cultural methods.

 

Prevention & Treatment: When nematodes are not yet present, move the garden location every year, purchase disease-free plants, pull up and dispose of roots immediately after harvest, and use resistant cultivars (indicated by N following tomato cultivar name). When root-knot nematodes are present, relocate the garden to a nematode-free area. Use nematode resistant tomato cultivars. Establish a rotation system using marigold cultivars Tangerine, Petite Gold or Petite Harmony, which reduce root-knot nematode populations in soils.

 

Stem and Root Rot on Bean


Stem and Root Rot on Bean is caused by the fungus Rhizoctonia solani.


Characteristics symptom is reddish-brown lesions or rots on stem near or below soil line. The entire root system may be rotted. Affected young bean plants appear stunted and off-colored, and may slowly die.

 

Prevention & Treatment:


Avoid chipped, cracked, or discolored seed. Test soils before planting and maintain proper balanced fertility and pH based on a soil test. Do not plant beans in low, poorly drained areas. Plant on raised beds. Plant after the soil has warmed to 69° F at a 4 inch depth. Reduce disease buildup in the soil by rotating locations in the garden where you plant bean or pea with other vegetables. Try to avoid injury to the root system, which often occurs during planting, through cultivation or due to a large population of nematodes in the soil. Remove crop debris immediately after harvest. Plant seeds previously treated with captan.

 

Apply chemicals according to directions on the label. Certain herbicides have been shown to have both direct and indirect effects on this fungus. Some herbicides may inhibit soil microorganisms which normally compete with Rhizoctonia, allowing a rapid buildup of the fungus. Others may cause growth stresses on young plants, making them more susceptible to injury by Rhizoctonia. Excessive herbicide rates, improper application, or poor incorporation may also affect the level of Rhizoctonia. Fertility may also have an important role in disease losses. Damage is usually greatest where phosphorous or potash is deficient or where pH levels are unfavorable. Rhizoctonia can also invade tissues at wound sites. Stem infections, hail injury, or mechanical damage from equipment are common and can result in plant death especially when warm, wet conditions prevail. Treat seed with a protectant fungicide which will control Rhizoctonia.

 

Downy Mildew

Downy Mildew is caused by several different genera and species of fungi.


Distinguishing sign is downy fungal growth on the underside of leaf spots in wet, humid weather, particularly in the early morning. Yellow spots first develop on upper side of leaf. The fungus that attacks cucurbits do not overwinter in South Carolina but moves northward in late spring and early summer as windblown spores.

 

Prevention and Treatment


Rotate fungicide products to reduce the risk of resistance to the active ingredients. In most parts of the U.S. downy mildew is resistant to Ridomil, all strobilurin fungicides (Group 11—Cabrio, Quadris, Flint, Pristine, and Reason), and Revus. These fungicides are not recommended against cucurbit downy mildew. Tank mixing fungicides specific for downy mildew with protectors also helps prevent fungicide resistance. Once the first spray is applied, continue spraying on a
7- day schedule. Apply fungicides before a predicted rain rather than after it rains. Fungicides must be present on the leaves before spores arrive or germinate. Cucurbits have lots of leaves that form a very dense canopy. High pressure (minimum 100 psi) and high volume (minimum 100 gallons of water per acre) are needed once vines touch each other.


Fungicides


Two different fungicide programs are recommended. The first program is for prevention. It should bused before downy mildew is found in a field. The fungicide choices are Gavel, Tanos, and Previcur Flex.Tanos and Previcur Flex must be tank-mixed with chlorothalonil or mancozeb. Once downy mildew has been found in a field, different fungicides should be used to get the best control. Spray Ranman (FMC) alternated with Presidio (Valent); each fungicide should be mixed with chlorothalonil or mancozeb.


Downy Mildew on Watermelon


Watermelon is at risk from downy mildew in the fall. It spreads very quickly after infection on unsprayed crops. Chlorothalonil and mancozeb provide some protection from initial infection, but they are not enough to stop downy mildew once it starts in a field. Apply fungicides specific for downy mildew as soon as it is found

 

Squash Fruit Rot


Squash Fruit Rot is caused by the fungus Choanephora cucurbitorum.


Dense, black fuzzy mold on the surface of rotted fruit or flowers denotes this rot. It does not affect leaves and stems. This disease is a problem primarily during periods of high humidity and on crowed vigorous plants.



Prevention and Treatment


Test the soil and apply the recommended amount of lime before planting. Mulch with 2 to 3 inches of materials such as grass clippings, pine straw and leaves. Mulching prevents rapid soil drying and allows roots to take up available calcium efficiently. Do not over fertilize plants with nitrogen or potash. Excessive amounts of these nutrients depress the uptake of calcium. Water plants during extended dry periods. Add organic matter to the soil. This will help "loosen" clay soils and will improve the water holding capacity of sandy soils. In either soil, organic matter will increase plant uptake of water and calcium. Grow squash in raised beds to improve drainage. (Do not grow squash in raised beds in the sandy Central region.)

 

Squash have separate male and female flowers on the same plant, and pollen must be transferred from the male flowers to the female flowers by bees. Poor pollination can result in improperly shaped fruit. Observe plants closely when blooming begins to determine if bees are present. Use insecticides late in the evening to prevent killing bees.

 

Disorders

Blossom End Rot: Blossom end rot is a physiological disorder of tomato. Symptoms are water-soaked spots on the blossom end of the fruit. These spots enlarge and become black. Secondary infection by decay-causing organisms usually follows.


The cause of this disorder is a calcium deficiency in the developing fruit. Extreme fluctuations in moisture, insufficient soil calcium, root pruning from nearby cultivation, and excessive ammoniacal (NH4 +) nitrogen, potassium, or magnesium fertilization can also increase the chances of blossom end rot, especially early in the season.


Prevention & Treatment: Late spring planting of tomatoes should be at the recommended date for your area. The soil should be limed according to recommendations of a soil analysis report to bring the soil pH to 6.5, and to provide adequate calcium levels in the soil. Limestone is best applied 3 to 6 months in advance and tilled into the garden soil. Follow the soil report for recommendations for pre-plant nutrient (fertilizer) applications. If calcium levels are not sufficient but the soil pH is correct, then gypsum (calcium sulfate) is best tilled into the soil before planting at 1 to 2 pounds per 100 square feet.


Avoid excessive potassium or magnesium fertilization as these nutrients will compete with calcium for uptake by the plants. Epsom salts is an example of a magnesium source, so do not apply to garden soil unless a recent soil report indicates a magnesium deficiency.


Blossom end rot symptoms on tomato:


Avoid ammoniacal nitrogen fertilizers for side dress applications (beside or around the plants), as ammoniacal nitrogen also will compete with calcium for uptake. Examples of fertilizers with ammoniacal nitrogen are ammonium sulfate, ammonium nitrate and most complete fertilizers, such as 10-10-10. A calcium nitrate (15.5-0-0) side dress fertilizer is usually the best choice, and is applied monthly at 2 pounds per 100 feet of row. Maintain a uniform supply of moisture through irrigation and adequate soil mulches. Mulches will not only keep the soil cooler and more evenly moist, but will suppress weeds, thus reducing the need for nearby cultivation that may damage tomato roots. Remove fruit with blossom end rot symptoms from the plants. However, if the soil was not tested lime or gypsum was not applied pre-plant, and blossom end rot occurs, then applying gypsum at 1 to 2 pounds per 100 square feet as a side dress supplement has proven beneficial.

 

Growth Cracks: Tomatoes crack when environmental conditions (drought followed by heavy rain or watering) encourage rapid growth during ripening. Some cracks may be deep, allowing decay organisms to enter the fruit and cause fruit rot.


Prevention: Maintain even soil moisture with regular watering. Some tomato cultivars are crack-tolerant.

 

Sunscald: Sunscald occurs when tomatoes are exposed to the direct rays of the sun during hot weather. It is most common on green fruit. Decay causing fungi frequently invade the damaged tissue.


Prevention: Cover exposed fruits. Control leaf diseases.

 

Poor Fruit Set: Poor fruit set occurs for several reasons:


Extreme temperatures: The blossoms drop off without setting fruit when temperatures are below 55 °F or above 90 °F for extended periods. Try Arkansas Traveler, Talladaga Hybrid, Homestead 24, Bella Rosa Hybrid, Top Gun Hybrid, Solar Fire Hybrid, Florida 91 Hybrid, Sioux or Costoluto Genovese for heat-tolerance.
Dry soil: Blossoms dry and fall when the plants do not receive enough water.


Shading: Few blossoms are produced when the plants receive less than six hours of sun a day.


Excessive nitrogen. High nitrogen levels in the soil promote leaf growth at the expense of blossom and fruit formation. Correct the nitrogen imbalance with superphosphate or 0-20-20 fertilizer.

 

Cat facing: This is a disorder caused by cold temperatures during fruit set. The fruit is extremely malformed and scarred, usually at the blossom end. Fruits that develop later in the season will not be affected. The cultivar Homestead 24 is resistant to cat facing.

 

Leaf Roll: Leaf roll of tomatoes may be caused by high temperatures, prolonged periods of wet soil conditions, and drought. It may also occur when tomatoes are pruned severely. The symptom is mostly on older leaves, with an upward curling of the leaflets, but may progress to affect up to 75 percent of the foliage. The rolled leaves may feel leathery and stiff. Often the condition of leaf roll occurs once the plants are under the stress of a heavy fruit set. Some cultivars are more prone to leaf roll than others.


Prevention & Treatment: The symptom of leaf roll does not significantly damage the crop. To help prevent this disorder, tomatoes should be planted on well-drained soil and be irrigated during periods of drought.