Damping Off (fungi - Rhizoctonia, Pythium spp.): Small, emerging seedlings wilt and die soon after emergence. Root systems of surviving plants are damaged, resulting in stunted plants and poor yield. Use high quality seed that have been treated with a fungicide and plant on a well-drained bed.
Leaf Spot (fungus - Cercospora capsici): This disease rarely occurs in Texas. Spots on leaves are large and oval or somewhat oblong, with light gray centers. Spots may also be present on the stem. Severely infected leaves turn yellow and drop. The fungus does not live in the soil but is carried in the seed. Most field infections can be traced to infected seed. The spores of the fungus can be carried by wind and splashing rains. Control can be attained by using disease-free seed that are treated with a fungicide. Fungicide applications at 7 to 10 day intervals will check the disease in the field.
Phytophthora Blight (fungus - Phytophthora capsici): The disease is caused by a fungus that lives in the soil and may be carried in seed. Infection usually takes place at the soil line; diseased plants may be girdled at the base causing sudden wilting and death of plant. Diseased parts of the stem shows a dark green, watersoaked band extending from the soil line to several inches up the stem. This band later dies and turns brown. When peppers are grown with furrow irrigation, sometimes a single infected row is observed in the field. This is the result of the fungus being carried by water down the furrow from a diseased plant and resulting in the infection and death of several plants in the same row. Planting on a raised bed and avoiding excessive moisture in the plant bed are the best means of controlling this disease.
Southern Blight (fungus - Sclerotium rolfsii): The fungus attacks the stem of the plant at or near the soil line, causing the plant to wilt and die. A white, cotton growth is observed on the surface of the stem. Later, pink to brown bodies resembling radish seed appear in the fungal growth. Crop rotation and deep plowing are means of controlling this disease. Soil fungicides may be helpful where Southern Blight has been a problem.
Mosaic (virus): Several viruses are known to attack pepper. Often plants are infected by a combination of viruses, rather than by a single strain. Young leaves of affected plants show a greenish-yellow mottle and may be curved and irregular in shape. Leaves curl upward and under severe infection are bunched, very small and discolored. Plants are stunted and have a bunched appearance. Fruits are small, misshapen and of poor quality. Severe infection can result in the complete failure of the crop. There are no effective means of control. The virus overwinters in perennial weeds and is transmitted from weeds to healthy plants in the fields by aphids. Environmental conditions favoring aphid multiplication and migration into the field will result in severe outbreaks of the disease. Keeping fields clean of weeds around the edges and turnrows and controlling insects may help in reducing spread of the disease. Resistant varieties are now available in limited supply. Check with your county Extension agent on currently used varieties.
Late Blight (fungus - Phytophthora infestans): Large, watersoaked areas on leaves and stems. In damp weather, a whitish moldy growth appears on the underside of infected leaves. This fungus causes a firm rot of tubers in field and storage, later becoming a soft rot. Use a resistant variety when possible. Fungicide applications will help control the disease. Harvest should be delayed about 2 weeks when late blight is present until vines are killed by chemical treatment and infected tubers have decayed completely.
Pink Rot of Tubers (fungi - Phytophthora parasitica and P. cryptogea): Diseased tubers are spongy and initially discolored around the point of stolon attachment. Later, they become discolored around the buds and lenticels. The internal tissues appear cream-colored when cut, but turn salmon pink after 15-20 minutes. They gradually become darker, turning black after about 1 hour.
Diseased potatoes have been observed in the Texas High Plains primarily in furrow-irrigated fields with clay soils. Control suggestions are to select sites with good soil drainage and avoiding excessive irrigation late in the growing season.
Scab (fungus - Streptomyces scabies): Rough, corky areas that may be round or irregular occur on tubers. Injury does not extend far into the potato but the appearance is objectional. Scab is less prevalent in acid soils than in alkaline. Avoid alkaline soil amendments such as manure, lime, or ashes. Avoid severely infested fields. Apply heavy irrigation at the time of tuber or early root formation. Use a 4 to 6 year rotation under irrigation. A 3- to 4-year rotation is satisfactory under dry land conditions. Use certified seed of scab resistant varieties. Treat seedpieces (See in the chapter on "Seed Treatment"). Wire worm damage, commonly called "deep scab", is not a disease. It differs in that the pits are deep and tissue has been removed by insect feeding. Sulfur may be used in home gardens to make the soil more acid.
Blackleg (bacterium - Erwinia carotovora subsp. atroseptica): The lower parts of affected stems develop an inky black, slimy, foul-smelling, soft rot. Leaves near tops of vines curl upward and become stunted. Aerial tubers usually form in leaf axils. Severely affected plants die. Tubers from affected plants show black stem-end discoloration and sometimes a severe blacks soft rot in storage. Plant certified, healthy seed in well-drained soil. Cut seed tubers with a disinfected knife. Treat seedpieces and cure properly before planting.
Bacterial Wilt or Brown Rot (bacterium - Pseudomonas solanacearum): Stems discolor (at first only on inside) and plants wilt and die. Tubers with a dark, vascular ring may decay. Avoid infested fields and plant seed from northern states. Discard tubers with dark eyes or with sticky ooze on surface.
Ring Rot (bacterium - Corynebacterium sepedonicum): Resembles brown rot, but stem is not discolored. Use certified seed from areas where ring rot is not permitted.
Scurf and Stem Rot (fungus - Rhizoctonia sp.): Small, hard black bodies adhere to the surface of the tuber. Below ground part of stem turns brown. Aerial tubers are sometimes formed. Rotate crops. Use certified seed. Treat seed as for scab.
Curly Top, Several Mosaics, Leaf Roll, Spindle Tuber (viruses): Plants may be stunted and off-color. Foliage is mottled or leaflets tend to roll up. Tubers are often small, and in case of spindle tuber, elongated. Several viruses can infect potato without causing noticeable symptoms. However, yields can be reduced drastically. Use of seed certified to be true to variety and free of disease agents is the best way to control tuber-borne viruses. Control insects.
Spotted Wilt (virus - Tomato Spotted Wilt Virus): This virus is vectored by thrips insects. Early symptoms are dead spots on upper leaves and death of the upper plant parts. Early leaf symptoms may somewhat resemble early blight, but spotted wilt symptoms will usually be in the top of the plant rather than on lower leaves. Plant seed certified to be free of disease agents. Control broadleaf weeds in and around fields 4 to 5 weeks before planting.
Other Diseases: Potatoes are damaged severely by other diseases including
< HREF = ""> Verticillium Wilt,
Cercospora Leafspot (fungi - Cercospora cruciferarum and C. atrogrisea):
The fungus attacks the cotyledons, leaves, and petioles of the host plant.
Lesions are circular with gray, brown or slightly off-white centers with
slightly darkened margins. Severe infection can cause yellowing of
foliage and eventual defoliation. Young seedlings are killed whereas
later infection causes a reduction in size of the edible portion. The
fungus is soilborne and can live in several weeds or old volunteer plants.
Stromata are formed in the leaves and these overwinter in the fallen
foliage. When weather conditions favor fungal development, growth
resumes. Spores are formed from this tissue and they are carried by wind
or splashed onto the foliage by rain.
The disease organism is favored by cool temperatures (55 to 65 degrees F)
and free moisture on the leaves.
Deep plowing or removal of infected plant parts will help break the life
cycle of the organism. All volunteer members of the cabbage family and
cruciferous weeds should be destroyed in and around the area where
radishes are to be grown.
Bacterial Leaf Spot (bacterium - Xanthomonas campestris pv. campestris):
The bacterium attacks the leaves and petioles causing small tan to white
spots with narrow, yellowish, watersoaked zones on the leaves. The spots
on the leaf petioles are black, sunken and elongated. Severe infection
results in defoliation and, in extremely severe cases, death may occur.
The causal organism is carried over in the crop residue and in infected
seed. Once a plant is infected, further spread is by insects, rain, etc.
During warm spring days, lesions are visible four to five days after
infection. In cooler periods development is slower. The bacterium will
grow between temperatures of 41 and 94 degrees F., but is favored by
temperatures between 80 and 86 degrees F.
Field sanitation is important in preventing infection. Rotation will also
reduce the possibility of the disease becoming a problem. Use high
quality seed. Chemicals are not recommended.
Black Root (fungus - Aphanomyces raphani): The fungus attacks radishes at
any stage of plant development. Early infection results in seedling
death. Some escapes will occur which, although infected, continue to
develop. This often is visible as a deep crater type lesion.
Field sanitation and rotation will help reduce the occurrence of black
root. Cool wet soil favors development. Planting on a raised bed in well
drained soil will prevent losses. Fungicides are not currently recommended
for the control of black root.
Powdery Mildew (fungus - Erysiphe polygoni): Symptoms consist of a light,
grayish, powdery growth on the leaves, pods and occasionally the stems.
This powdery growth is easily rubbed off. When the disease is severe,
plants turn yellow and defoliate. Generally, powdery mildew does not
damage early planted peas. It can, however, be quite destructive on a
fall or late summer crop.
Rust (fungus - Uromyces phaseoli): Small, reddish-brown pustules appear
on both upper and lower leaf surfaces. Rust can develop rapidly,
resulting in severe leaf damage and defoliation. Check current clearances
of fungicides effective in controlling rust. Sulfur may be used.
Leaf-Spots (fungi - Cercospora sp., Aristastoma sp., Ascochyta sp.,
Colletotrichum sp., Stagnospora sp.): Various sized spots often yellowish
in color or with a yellow halo, others brown to purplish; these normally
develop first on lower leaves. They are most serious during periods of
prolonged moist weather and on late summer or early fall plantings.
Severe leaf spotting results in defoliation with subsequent yield
reductions. Practice crop rotation; avoid cultivating fields when foliage
is wet.
Ashy Stem Blight (fungus - Macrophomina phaseolina): Plants die quickly.
Lower stem is gray with internal black flecking of tissue. Most serious
when mature plants come under moisture stress. Same as charcoal rot. See
the section on charcoal rot.
Pod Spot (fungus - Cladosporium sp.): Irregular, purplish or blackish
spots on pods. Infection of young pods causes curling and shedding.
Spots also occur on leaves, characterized by a brownish to purple mold
growth on underleaf surfaces. Early defoliation may occur. The disease
causing organism is seed-borne. Obtain seed from healthy plants. Field
type peas generally are more resistant than garden varieties. Purple hull
varieties appear more resistant than blackeyes.
Seedling Disease (fungi - Rhizoctonia sp., Phythium sp., Fusarium sp.):
Seed may rot and young seedlings die. The condition is most common on
early plantings or when soil contains a large amount of nondecomposed
plant residue. Treat seed with an approved fungicide. Avoid planting in
wet, cold soils.
Bacterial Blight (bacterium - Xanthomonas vignicola): This
disease appears as tan to brown angular leaf spots with yellow margins on leaves,
pods, and stems. It may cause severe defoliation during periods of high humidity.
See bacterial blight of bean.
Mosaic (virus): Several viruses may produce a mosaic pattern on peas.
These viruses may be found singularly or in combination with others. They cause
irregular light and dark green mosaic patterns in the leaves. Some viruses cause
thickened, malformed leaves similar in appearance to damage caused by hormone
herbicides. The mosaic patterns are best observed on the younger foliage. Plants
may be stunted and fail to produce normal pods. It is best to plant healthy,
disease-free seed rather than saving seed from a crop that showed infection.
Early control of aphids may help prevent spread of virus diseases within the
crop. The pinkeye purplehull variety is highly susceptible to Blackeye Cowpea
Mosaic Virus (BlCMV). This virus can be seed borne and is common where pinkeye
purplehull is grown. Resistant purplehull varieties include Pinkeye Purplehull
BVR and Mopod. Crowder type peas have shown good resistance to BlCMV. See
variety listing.
Root Knot (nematode - Meloidogyne sp.): Relatively small galls
or knots develop on roots of affected plants. See the section on
root knot nematode for more information. Resistant
or tolerant varieties are available.
Marginal Yellowing and Burning of Leaves (physiological): In sandy soils
of East Texas, this may result from potassium deficiency in the soil.
Other causes may be drouth, or any factor interfering with normal root
functions.
Cotton Root Rot (fungus - Phymatotrichum omnivorum): See
Cotton Root Rot. Curly Top (virus): See Curly
Top. Southern Blight (fungus - Scelorium rolfsii): See Southern
Blight.
RADISH
Raphanus sativus
White Rust (fungus - Albugo candida): White, raised spore masses appear
on the underside of leaves. Even though the disease is confined usually
to the leaves, it may affect other parts of the plant. The host epidermis
is ruptured and powdery masses of spores are produced. Spots may range
from minute dots to one-half inch or more in diameter. With mild temperature and high humidity, the disease spreads quickly. Extreme hot or cold
temperatures and dry weather will slow the disease progress. The
following practices should control white rust: 1) destroy crop residue;
2) cultivate fields prior to planting to destroy all volunteer plants.
SOUTHERN PEA (BLACKEYE, COWPEA)
Vigna unoviculata
Wilt (fungus - Fusarium oxysporum f. sp. tracheiphilum): The lower leaves
turn yellow and fall. Death may be rapid after wilting is noted. Woody
tissue in the lower stem is dark brown due to fungal invasion. Wilt is
frequently associated with root knot nematode infestations. Control wilt
by planting tolerant varieties. Avoid deep cultivation that may injure
roots and increase wilt incidence.
cb@tamu.edu