Sclerotinia Drop (fungus - Sclerotinia sclerotiorum): The causal organism is a soilborne fungus that infects the stem of lettuce at or near ground level. Lower leaves wilt and collapse, the entire plant wilts and dies. Later, a soft, watery, brown rot destroys the plant. White, cottony growth of the fungus may be found on the surface of leaves and stem. Hard, black resting bodies called sclerotia, can be found in the cottony growth. The fungus is soilborne and can remain in the field for several years. The fungus first infects dead leaves at the base of the plant and later moves to live tissue. Under moist conditions, infection occurs when plants are reaching maturity. Avoiding infested fields is recommended.
Mosaic (virus): Young plants first show a slight leaf rolling, followed by light green to yellow mottling. Foliage has a general unhealthy yellowish color and plants are small or dwarfed. Symptoms vary according to type or variety of lettuce. The virus is seed-borne and overwinters in wild hosts and weeds. Infected seed is the most common source of infection. Once infected plants are present in the field, aphids further spread the virus. The virus can also be mechanically transmitted. There is no cure for mosaic once a plant is infected. Disease-free seed offer an effective means of control.
Big Vein (virus): Infection may appear at any stage of growth. Very young seedlings are killed. Surviving plants are smaller in size, quality of heads is affected and maturity is delayed. Infected leaves have an upright habit of growth and the veins turn yellow and begin to clear. Areas around the veins turn white causing the veins to appear enlarged. The virus is soilborne and may persist in the soil for a long period of time. The virus can be transmitted by Olpidium brassicae, a soilborne fungus found in the rootlets of many growing plants. Avoiding excessive soil moisture and rotation are means of reducing disease losses.
Tip Burn (physiological): Margins of leaves die and turn brown. The first symptoms produced are small, dark brown spots about a quarter of an inch from the edge of the leaf. Tissue around these spots dies and turns brown along the margins. Tip burn usually becomes infected with secondary bacteria causing soft rot. The disease is believed to be related to fluctuations in soil moisture, drastic changes in temperature, and available calcium. No satisfactory control for this disease has been developed.
Damping-Off (fungi - Rhizoctonia sp., Pythium sp.): Small plants wilt and die soon after emergence. Excessive wet and prolonged cool, wet periods of weather are conducive to damping-off. Use only high quality seed which have been treated with a fungicide. Plantings should be on raised beds.
Soft Rot (bacteria - Erwinia carotovora): The bacterium enters through mechanical wounds or following damage caused by other diseases, insects or frost. Once the disease is established, it spreads rapidly in the field if warm weather and other conditions are favorable. There is no practical control of the disease once it becomes established and losses will increase during transit and marketing. Infected or damaged lettuce should not be packed; lettuce waiting to be shipped should be kept refrigerated.
Market Disease (various causes): Several disorders can occur on lettuce after harvest and during transport, which could easily be confused with disease caused by pathogenic organisms. Lettuce can be injured by excessive concentration of carbon dioxide (CO2), low oxygen levels, exposure to ethylene gas, and fluctuations in temperature. Brown stains on leaves, spotting of midribs, rib discolorations and russet spotting are some of the symptoms usually associated with these disorders.
Nematodes: (See section on Nematodes.)
Root Knot Nematode (nematode - Meloidogyne sp.): Okra is highly susceptble. Root becomes enlarged and distorted. See the section on root knot nematodes. No resistant varieties are available.
Leaf Spot (fungi - Alternaria sp., Ascochyta sp., Cercospors malayensis, Phyllosticta hibiscina): There are several leaf spotting organisms which attack okra. However, none have been shown to cause economic loss. No control is recommended.
Blossom and Fruit Blight (fungus - Choanephora cucurbitarum): Young fruit and blossoms are attacked by the fungus which gives them a "whiskery" appearance. Infected plant parts are reduced to a soft rotten substance. Disease development is favored by warm, humid weather. Spray with an approved fungicide.
Seedling Disease (fungus - Rhizoctonia sp.): This disease is more likely to occur if okra is planted before soils warm sufficiently in the spring. See the Seedling Disease Section for more information.
Virus: A whitefly-transmitted geminivirus was detected by Dr. Judith K. Brown in okra from Tamaulipas, Mexico (adjacent to the Lower Rio Grande Valley) in the fall of 1994. Fruit has irregular yellow areas which follow a longitudinal alignment. The initial symptom on young leaves is a diffuse, mottled appearance. Older leaves have irregular yellow areas which are interveinal. The disease is associated with the presence of whiteflies. The nature of the geminivirus has not been determined, nor has the presence of other viruses been ruled out. Preliminary experiments by Dr. Brown suggest the virus is not seedborne. There are no control measures. Submitted by Tom Isakeit, 5/7/95.
Cotton Root Rot (fungus - Phymatorichum omnivorum): Infected plants die suddenly from mid-summer to fall. Leaves usually remain attached to the plant. For more information see the Cotton Root Rot Section.
Charcoal Rot (Fungus - Macrophomina phaseolina): See the section on Charcoal Rot.
Southern Blight (fungus - Sclerotium rolfsii): See the Southern Blight Section.
Botrytis Leaf Blight (Blast) (fungi - Botrytis allii, B. squamosa and B. cinerea): Botrytis leaf blight or blast is a serious disease of onion in other onion producing states, such as New York and New Jersey, where it appears almost every season. In Texas, its presence is more sporadic, although during the last few years it has been occurring with alarming frequency. Several species of Botrytis occur on onion leaves; the disease caused by B. allii is usually called neck rot; that caused by B. cinerea is known as leaf fleck, and the one caused by B. squamosa is known as leaf blight. The disease is frequently associated with small whitish spots occurring along the entire length of the leaf. Most spots have greenish halos that at times appear to be water soaked. When the spots are numerous, the tip of the leaf may die back, giving the field a blasted appearance. However, other causes for these spots are suspected, such as cold rain, sleet, sandblasting and other fungal diseases. Most fungicides used for controlling purple blotch will also control Botrytis leaf blight.
Purple Blotch (fungus - Alternaria porri): The fungus affects leaves, seed, stems and bulbs. At first, small whitish, sunken lesions with purple centers are found on the leaves. These spots later enlarge and eventually encircle the leaf. Later, darkened zones appear on the surface of the leaves, retaining the characteristic purple color. Bulbs can be infected during harvest, curing, storage and transit. The disease will spread rapidly to other bulbs causing extensive damage. Recent research findings indicated that disease occurrence can be predicted accurately by measuring the number of hours that free moisture is present on the leaf surface. When 10 to 12 continuous leaf-wetness hours occur, the disease will develop when it is present. Fungicide application should be made when these conditions occur.
Downy Mildew (fungi - Peronospora destructor): Downy mildew is a relative new disease of onion in the Lower Rio Grande Valley, having made sporadic appearances during the last five years. When it does occur, it can cause severe economic damage to the crop. Symptoms consist of white to violet spots on leaves, later turning dark or almost black. Typical symptom of this disease as it occurs in Texas is a series of contiguous verticle spots up and down the leaves. A fuzzy growth is observed on the surface of the leaves, particularly during periods of high humidity. Fields should be monitored closely, particularly during prolonged cold wet weather, when the disease is more likely to occur. Fungicide applications should begin on plants two to three months old as soon as these conditions prevail. Downy mildew is seldom found in Texas as the season progresses into warmer weather.
Pink Root (fungus - Pyrenochaeta terrestris): Pink root is strictly a soilborne disease. Diseased roots turn pink in color, shrivel and die. As the plant sends out new roots, they become infected and die. Affected plants do not usually die, but bulbs are small and of poor quality. The disease affects only roots. Plants infected with the pink root fungus will also develop the tip blight condition discussed previously. The fungus usually is introduced to new land by transplants grown in infected soil. Once the soil becomes infested, the fungus remains in the soil for many years. Soil fumigation has been shown to be effective for the control of this disease. The cost, however, is high. The use of onion varieties resistant to pink root is the best control for this disease. Rotating fields will also reduce losses. Onions infected with the pink root fungus have also shown a higher incidence of Fusarium basal rot. Crop rotation is very important in attempting to control both of these problems.
Stemphylium Blight (fungus - Stemphylium vesicarium): The fungus causing this disease normally invades dead and dying onion tissue, particularly the tip blight and purple blotch lesions. This disease causes only minimal damage when plants are otherwise healthy. Control can be accomplished with the same chemicals used for purple blotch and tip blight control.
Neck Rot (fungus - Botrytis allii, Botrytis sp.): This disease is frequently unnoticed in the field because damage usually occurs during transit and storage. Diseased tissue at the base of the crown becomes sunken and watersoaked in appearance. A gray fungal growth later forms on the surface, which can be followed by other fungi and bacteria, causing decay. Careful handling of the crop at harvest and prompt drying of onions with heat and air ventilation are the best means of controlling this disease.
Basal Rot (fungus - Fusarium sp.): Disease occurrence starts at the basal plant of the onion bulb, causing the bulb to become soft. When onions are cut, a semi-watery decay is found, advancing from the base of the scales upward. The disease may remain unnoticed until transit, where the decay may continue until the entire bulb is destroyed. The fungus lives in the soil, invading the bulb through wounds or through root scars at the base. High soil temperatures favor disease development. No control practices are available, although crop rotation may be of help in avoiding soils that could have been infested from previous crops.
Black Mold Rot (fungus - Aspergillus niger): Black mold is generally destructive during storage and transit, although it may be observed on maturing onions in the field. The disease can be recognized by the presence of black powdery spore masses of the fungus on the outer scales. Observations indicate that high temperatures (85 to 95 degrees F) and moisture favor disease development. Bulbs should be protected from moisture in the field during and after digging and during transit.
Bacterial Soft Rot (bacterium - Erwinia carotovora): Bacterial soft rot isone of the more prevalent causes of loss in storage onions. The soft rot bacterium can enter the neck tissues as plants approach maturity. As the rot progresses, invaded scales become soft and foul-smelling. Onions with mechanical injuries, sunscald, or bruises are particularly susceptible to bacterial soft rot, especially if they have been held under warm, humid conditions.