Bacterial Wilt
This disease occurs over most of the United States. It
affects cucumbers and muskmelons and, less commonly,
pumpkins and squash. Individual leaves wilt during the heat
of the day, but recover overnight initially. As the disease
progresses, part or all of the vine wilts and dies. To test for
the disease, cut a wilted stem near the base of the plant.
Squeeze the sap out of the stem. If it is sticky and white and
forms a thread when the tip of a knife is touched to it and drawn away, bacterial wilt is probably
present. (This test works best with cucumbers.) The disease overwinters in the gut of cucumber
beetles, and is spread to plants as the beetles feed.
Prevention and Control
Controlling cucumber beetles is the key to prevention. Look for varieties of cucumbers and
squash that are resistant to cucumber beetles. Cover young plants with floating row covers.
Knock, shake, or hand pick beetles off plants and out of flowers, and clean up plant debris to
reduce the number of overwintering adults. Dust plants and flowers with a pyrethrum insecticide
as directed by the product label.
Septoria Leaf Spot
Tomato plants in many parts of the North America can suffer
from this fungal disease, which can appear at any time.
Symptoms first appear on oldest, lower leaves. Gray-brown
areas have gray centers and a darker border. The dark border
lacks the rings typical of early blight. The centers of the
discolored areas may have many small, black spores, and
sometimes a yellowish area surrounds the darkened areas. It usually slowly defoliates plants, but
a harvest is still possible.
Prevention and Control
To prevent septoria infection, use copper fungicides such as Bordeaux mixture or liquid copper
as directed by the product label, or spray aerated compost tea every 7 to 10 days. Grow ″potatoleaved″ (leaves that have no indentations on the borders) and rugose-leaved tomato varieties
(with a puckered leaf surface), which show better resistance.
Early Blight
This fungus disease attacks tomatoes and potatoes over most
of North America. Plants under stress or with a heavy load of
fruit are most susceptible. Dark brown spots with concentric
rings in them form on older leaves first. Infected leaves turn
yellow and die. Potato tubers are covered with brown, corky
spots. Tomato fruits may sometimes be infected; a black,
sunken, leathery spot fors at the stem end. Warm, moist conditions encourage disease
development. The fungus overwinters in plant residues in the soil.
Prevention and Control
Plant in well-drained soil where air circulation is good. Rotate crops and destroy any volunteer
potato or tomato plants. Don't wet foliage when watering. Amend soil with compost, and fertilize
plants judiciously to maintain plant vigor. Use certified disease-free seed potatoes and tomato
transplants. Tomato plants with early blight slowly lose their leaves, but unless the infection is
severe, you can usually harvest mature tomatoes. Use a copper-based fungicide as directed on the
product label at first sign of the disease.
Late Blight
Late blight strikes tomatoes and potatoes all over North America at
any stage of growth. Irregular gray spots form on leaves. White
mold grows on the undersides of these spots. Infected leaves turn
brown and dry up. Gray, water-soaked spots on infected fruits later
turn dark brown and corky. Infected tubers are covered with brown
spots where rot begins. Wet weather with cool nights and warm days favors the spread of the
disease. The fungus overwinters in infected plant debris.
Prevention and Control
Fertilize judiciously, as excess nitrogen favors the disease. Avoid overhead watering. Eliminate
infected plants with a flame weeder to prevent further spread of the disease. After potato vines
die at the end of the season, wait a week before harvesting to avoid infecting tubers. For early
harvest, first destroy plant residues with a flame weeder before digging tubers. Copper
fungicides such as Bordeaux mixture are effective as a preventive when applied as directed on
the product label, and may afford control if applied as soon as infection occurs. It's best not to
depend too heavily upon this method of control, as copper can build up in soil to levels toxic to
earthworms and microbes.
Blossom End Rot
Blossom End Rot (BER) is a physiological disorder
of tomatoes, peppers, and cucurbits caused by a
calcium imbalance within the plant. Fluctuations in
soil moisture, excessively wet or dry soil, excessive
nitrogen fertilizer, roots damaged by cultivation, very
high or low pH, or soils high in salts prevent all can
the roots from taking up enough calcium to satisfy the plant's rapid cell development. The result
is a water-soaked spot at the blossom end of the plant that enlarges, turning dark brown and
leathery. Rot may set in at the spot. BER is common when plants grow rapidly in the beginning
of the season, then set fruit during dry weather. As little as 30 minutes of water deficiency at any
time can cause BER.
Prevention and Control
Keep plants uniformly watered throughout the season. Water deeply; wet the soil at least 6
inches down. Apply mulch to maintain soil moisture. Keep soil pH around 6.5. Some older
varieties of indeterminates (vining tomatoes) and plum tomatoes are more susceptible to BER -make very sure they have adequate soil moisture.
Anthracnose
This fungus occurs worldwide. In North America it is
especially troublesome in the humid eastern part of the
continent. Beans develop round, black, sunken spots on pods
and stems. Veins on leaf undersides turn black. Cucumber
and muskmelon leaves develop yellow spots that dry up and
fall out; spots on watermelon leaves are black. Infected fruits
are covered with sunken spots with dark borders. Tomatoes develop sunken spots on ripe fruits,
the central parts of which turn dark. Wet weather encourages the disease to spread. The fungus
overwinters in plant residues in the soil.
Prevention and Control
Enriching soil with compost helps plants resist attack. Obtain disease-free seed and grow
resistant varieties. Don't work with plants while they are wet. Irrigate with drip vs. overhead
sprinklers. Rotate crops at least yearly (a 2- to 3-year rotation is better). Keep ripening fruits out
of contact with soil. Copper fungicides are effective as a preventive when applied as directed on
the product label. It's best not to depend too heavily upon this method of control, as copper can
build up in soil to levels toxic to earthworms and microbes.
Viruses
There are many kinds of viruses that can infect a wide variety of
plants all over North America, including beans, celery, corn,
cucurbits, peas, peppers, spinach, and tomatoes. Symptoms vary with
the specific virus, but include mottling, streaking, puckering, or
curling of leaves. An example is cucumber mosaic virus, which infects cucurbits, tomatoes and
peppers, causing green and yellow mottling of leaves, curled leaves, stunted plants, and fruits
with light splotches and warty bumps on them. Many viruses are spread from plant to plant by
sucking insects, such as aphids and leafhoppers, as they feed. Follow these links for additional
photos of tomatoes infected with Tobacco Mosaic Virus and Cucumber Mosaic Virus. Consult
your county Extension Service for help with a specific diagnosis.
Prevention and Control
Control insects that spread disease. Pull up and burn infected plants. Choose resistant varieties
when available. Buy certified virus-free seed potatoes. Control weeds that harbor viruses.