VEGETABLES AND SMALL FRUITS

Common smut of corn
Damping-off
Late blight of potato and tomato
Early blight of potato and tomato
Blackleg of potato
Scab on potatoes
Verticillium wilt of potato and tomato
Raspberries:  Cane blight and Verticillium wilt
Rotating crops to prevent disease
Tomato problems common in Montana

Common smut on corn

Symptoms
Symptoms of common smut on corn are very dramatic and impossible to miss. This fungal disease causes shiny silver-gray galls that may be several inches in diameter to appear on the ears. Later, the galls rupture and masses of black powdery spores pour forth.

Hosts
Corn. Mainly sweet corn, but also field corn.

Favorable conditions
Common smut spores survive the winter in the soil, on corn residues or in infected corn seed. Wounding by hail, wind, and insects or other factors is necessary for infection to occur. Although wet weather is needed for the fungus to start growing, dry weather must then follow for infection and disease development to occur.  Temperatures between 78 and 93 F favor common smut.

Controls
Resistant varieties are available. Remove and destroy the galls before they produce spores to decrease spore levels.  Do not use high amounts of nitrogen fertilizers. Minimize mechanical injury to corn whenever possible.

Culinary uses of corn smut
Mexicans consider the corn smut fungus to be an edible delicacy known as cuitlacoche.  To supply this discriminating market, some farmers grow especially susceptible sweet corn varieties to harvest this fungus. This is an interesting case where in certain locations, a diseased crop is more valuable than a healthy crop!
Warning: The fungus must be harvested in its early stages before the black spores are produced!

References
Howard, R.J et al. 1994.  Diseases and Pests of Vegetable Crops in Canada. Canadian Phytopathological Society and Entomological Society of Canada. Ottawa, Ontario, 554 pp.

Written by Martha Mikkelson, Plant Disease Diagnostician, April 1998

Damping-Off
Damping-off is a fungal disease commonly infecting seedlings planted in wet, cold soil. This disease is caused by fungi that rot the seedling stem right at the soil line. As a result, the infected seedlings can wilt and die literally overnight. Damping-off also can kill seedlings before they break through the soil. After a few weeks of growth, plant stems toughen and become resistant to damping-off fungi.

It is important to plant seeds when soil temperatures are favorable for plant growth. Optimal temperatures for growth vary with plant species. For example, spinach and pea seeds germinate and grow in cooler soils (at least 40 F) whereas corn and bean seeds require warmer soils (at least 55 -60 F). When starting plants indoors, grow in well-drained soil with plenty of light and air circulation. Avoid crowding of plants, low light, and heavy nitrogen fertilizers. A thin layer of sand or perlite that keeps stems dry at the soil line also can help. Plant seeds treated with a fungicide in areas where damping-off disease is common.

In greenhouses, fungicides containing a combination of thiophanate-methyl and metalaxyl such as DrenchPak (W.A. Cleary ) protect against the different fungi causing damping-off. Several biological fungicides recently have come on the market that are used as seed treatments, pre-plant applications, sidedressing, soil drenches, or foliar spraying. Examples include Promote Plus and Mycostop.

If, in spite of all your precautions, you find your garden seedlings toppled over one morning, all you can do is re-plant as soon as possible. Careful site preparation, good cultural care, and proper watering can all help prevent problems with damping-off.

References
Cleary, W.A. 'Technical Information" 1-800-524-1662.

Flint, M.L. 1990. Pests of the Garden and Small Farm. University of California. Oakland, CA 94608-1239. Pub. #3332. 276 pp.

Gilkeson, L., and P. Michalak. 1994. Controlling Pests and Diseases. Rodale Press, Emmaus, PA 18098. 159 pp.

Howard, R., J. Garland, and W. Seaman. 1994. Diseases and Pests of Vegetable Crops in Canada, An Ilustrated Compendium. The Canadian Phytopathological Society and the Entomological Society of Canada, Ottawa, Ont. Canada K2A 1Y8. 554 pp.
 
Written by Sherry Lajeunesse and Martha Mikkelson, Extension Urban Pest Management Specialist and Plant Disease Diagnostician. January 1998.

Late Blight of Potato and Tomato
 
A disease that affected millions
LATE BLIGHT is the most destructive disease of potatoes throughout the world. This disease was responsible for the Great Potato Famine in Ireland in the 1840's in which at least 1 million people died and another 1.5 million immigrated to the United States. The disease was first found in Montana in 1995. Late blight has the potential to devastate commercial potato crops in Montana when favorable weather conditions exist. It can spread rapidly in both commercial potato and tomato operations and in private gardens.
 
New threats
Because new strains of the late blight fungus are very aggressive and resistant to the very effective fungicide metylaxyl, late blight is a very real threat to seed potato producers in Montana. It is important for home gardeners to watch for the disease and to take potato plants with suspicious symptoms to their county Extension agent and be prepared to quickly destroy plants if the disease is confirmed.

Susceptible plants
Late blight infects crops and weeds in the family Solanaceae, including potato, tomato, and weeds such as the nightshades.

Symptoms
Symptoms usually appear first on older leaves soon after flowering, following warm and wet or humid weather. Dark green, water-soaked areas at leaf tips spread inward and become dark brown and brittle in one or two days. On undersides of these leaves a cottony film may be visible around the edges of the lesions. Lesions obliterate the pattern of leaf veination. (Another form of blight, early blight, does not obliterate patterns of leaf veination.)

Tubers near the soil surface can develop lesions that are irregular and sunken; the lesions usually appear in and around the eyes. Affected tissue is granular and reddish in appearance and can penetrate up to one inch into the tuber. If you suspect late blight, it is very important to report it immediately to your county Extension agent, listed in the phone book under county government, or to the Montana Department of Agriculture.
 
To control late blight;
1)   Never plant table stock potatoes. Plant only Montana certified seed potatoes. If you do buy out-of-state potatoes, buy only potatoes that are certified disease-free. This is required by law. Do not personally transport potatoes into Montana from other states since they can harbor the disease.

2)   Avoid highly susceptible varieties such as Bintje, Norchip, Norland, Norgold, Norkotah, Red Pontiac, Russet Burbank, Sangre, Shepody and Warba. If available, plant resistant varieties such as Brador, Chieftain, Desiree, Dorita, Epicure, Kennebec, Nooksac, and Sebago.
 
3)   Plant potatoes in a different part of the garden each year, but not close to tomatoes or in shaded areas.
 
4)   Plant tomatoes against a south wall where they can be protected from the rain by the overhanging eaves or by a temporary lean-to. For tomatoes grown in the open, be prepared to erect a temporary shelter of polyethylene or similar material above them to keep the rain off. Do not enclose the plants completely or the high humidity will encourage the disease.
 
5)   Do not overfertilize or overwater. Dense, lush foliage stays wet longer and is more likely to become infected.
 
6)   Water the ground only. Don't use sprinklers on either potatoes or tomatoes.
 
7)   Monitor your garden on a regular basis and quickly remove any suspicious leaves or shoots. Put them in a sealed plastic bag, keep them cool, and take them to your county extension office.

8)   For potatoes to be stored, remove tops two weeks before digging the tubers. Harvest in dry weather.
 
9)   Harvest all the tubers so that there are no "volunteer" plants that emerge the next year.
 
10)  Do NOT compost diseased potato residue!
 
Fungicides
Careful use of fungicides will help to ensure success in the battle against late blight. If you live in areas where seed potatoes are grown commercially, use preventative applications of fungicides such as maneb or chlorothalonil. Otherwise, no preventative fungicide treatments are recommended in home gardens. Because the disease is also spread by tomatoes, plant only tomatoes grown in Montana or certified to be free of late blight. Destroy and bury cull potatoes, diseased plants, and volunteer potato and tomato plants.
 
The product that you use must contain one of the following ingredients: chlorothalonil, copper oxychloride, tribasic copper sulphate, copper hydroxide, maneb, or zineb.
 
For best results, the first spray must be applied before any infections appear. For potatoes, start spraying when the plants are about six inches high. For tomatoes, start as soon as they are planted out.
 
The fungicide must be reapplied every 5 to 10 days. Read the label for rates of application and days to harvest limitations.
 
Spraying can be suspended during warm, dry weather in mid-summer but start once again before the onset of fall rains.
 
These funigicides are not systemic so residues can be washed off with warm water and a little detergent. Rinse in cold water and dry.
 
 

Early Blight of Potato and Tomato
 
Symptoms
Symptoms usually develop after flowering - not "early" as the disease name implies. Circular or irregular dark spots develop on older leaves. As these spots enlarge, they develop concentric rings, giving a target effect. There is usually a chlorotic (yellowish) zone around the spots that fades into the normal green of the leaves. Early blight can cause tuber rot on potato, but this is rare.
 
Favorable conditions
Early blight can occur over a wider range of weather conditions than can late blight. Warm weather with heavy dews or rain generally are favorable. Early blight is more severe when the plants are under stress from poor nutrition (especially low nitrogen), drought, or other pests.
 
Year-to year survival of disease
Early blight is caused by a fungus that overwinters in potato plant debris or tubers, soil, and in other members of the same plant family, such as peppers and tomatoes.
 
Control
- Plant potatoes or tomatoes in the same area of the garden only once in every 4 years.
- Maintain nitrogen fertility at optimum levels.
- When blight first appears on the lower leaves, remove and destroy affected leaves. Then, begin a a thorough spray program with fungicides containing such active ingredients as chlorothalonil, mancozeb 80% WP, or maneb. Apply according to label directions, repeating applications every 7 - 10 days.
- Minimize harvest injury to tubers.
- Store tubers at 38 - 40 F after curing.
 
 Blackleg of Potato
 
BLACKLEG is a bacterial disease that can infect potato stems and tubers.
 
Symptoms
The first symptoms often begin at flowering. Yellowish leaves that wilt during hot temperatures are common. The lower stem may be gray to inky black. Tubers may or may not be affected. Tuber infection usually begins at the stem end and appear as a creamy, odorless, soft decay that is sharply separated from the healthy tissue by a dark brown to black line. When other organisms move in, the rot can become very smelly.

Disease spread
The bacteria usually spread via infected seed tubers or plant residue, insects, water, rain splash, and equipment.
 
Favorable conditions
Wet soil and cool temperatures
 
Controls
Once a plant is infected, control is difficult. Prevention is best. Sometimes treatment with fixed copper fungicides can be effective. Purchase only certified seed potatoes to avoid problems with blackleg, phytophthora and other problems. Avoid over-watering and mechanical damage to plants. Limit early nutrient supply to keep top growth within reason.

1.   Buy certified disease-free seed
2.   Ideally, plant small whole seed. If you must cut the seed before planting, sterile the knife in rubbing alcohol between cuts. Planting in a warm, moist soil will promote growth and stimulate wound healing.
3.   Plant in moist, well-drained soil when temperatures are 10 C or higher.
4.   Do not overwater!
5.  Allow tubers to mature before harvest. Store freshly dug tubers at 12 C for 7 to 10 days to cure them. Then lower the temperature to 2 to 5 C.  Maintain relative humidity at 90% with adequate air circulation.  Do not wash tubers between harvest and storage.
 
Resistant cultivars
Russet Burbank has some resistance; all other commonly-planted cultivars are susceptible.

 

SCAB ON POTATOES

Scab is a potato disease that causes the skin to appear scabby and unsightly.  While the ugliness only goes skin deep and doesn't decrease yield, diseased tubers just don't look appetizing.  As with most diseases, a combination of long-term approaches must be used to control scab.

Symptoms
On tubers, lesions are somewhat circular and consist of raised, scablike lesions.  Damage generally does not extend much below the skin.  Ther are no above-ground symptoms.

Look-alike symptoms
Excessively wet soils can cause lenticels on the tubers to enlarge and become whitish and corky.  Lenticels are pores in the skin that allow gas exchange.  Black scurf is another common disease that extends no deeper than the skin.  The fungus involved with this disease looks like black dirt that won't wash off the skin.

Origin and persistance of scab
The bacterium Streptomyces scabies causes scab.  This pathogen generally is introduced into the soil via infected seed pieces.  Once the soil is contaminated, the pathogen can survive indefinitely on decaying plant material and manure so that ongoing control measures become necessary.

Favorable conditions
Soils with pH between 5.4 and 7.4 (the pathogen does not grow well below and above these pH's), soils with high levels or organic matter, and soils that are allowed to dry out all favor scab.

Controls
1) Always use seed potatoes that are certified disease-free.
2)  Use scab-resistant cultivars (see Table 1 below)
3)  Be careful not to let the soil dry out!  High soil moisture from one week before emergence until 8 weeks after emergence can really reduce scab severity.  The logic behind this is that the scab bacteria are inhibited in moist soils.  Keep the soil moist, but do not drown the plants since this can encourage other diseases such as blackleg and bacterial soft rot.
4)  Soils with a pH between 5.4 and 7.4 favor disease development.  Ammonium sulfate fertilizer can help reduce levels of scab and increase yields.  It is better to apply this fertilizer after potatoes emerge than before planting.  It is thought that this fertilizer  converts to evolve hydrogen sulfide in the soil.  This hydrogen sulfide may act as a germicide on the scab bacteria.
5)  Soils that are high in organic matter provide a good food base for the scab bacteria.  Avoid incorporating straw or manure where potatoes will be planted within 2 - 3 years.
6)  a 3 to 4 year crop rotation with non-susceptible crops should be established.
7)  Some crops harbor scab bacteria and negate the benefits of rotation.  Crops that should be avoided in the rotation include spinach, beets, turnips, rutabagas, radishes, carrots, and parsnips. Resistant crops that can be included in the rotation include corn, peas, beans, wheat, and alfalfa.
8) Fungicides labelled for seedpiece treatment include captan, mancozeb, thiophanate methyl, and thiobendazole (TBZ).  Read labels of these products carefully to determine application rates and other information.

Table 1.  Relative resistances of some potato varieties to scab.
 
Resistant Moderately resistant Susceptible
Nooksack Norkota Centennial Russet
Norgold Russet Kennebec Irish Cobbler
Russet Burbank LaRouge Katahdin
Redsen Norchip Red Pontiac
Norland Rosa
Red LaSoda Shepody
Sangre White Rose
Sebago Yukon Gold
Superior Ranger
Viking
 
 
 
 
 

 
Verticillium Wilt of Potato
 
 Symptoms
Symptoms include lower leaves and stems that die early on scattered plants. Areas between leaf veins often turn yellow then brown. Often only one stem or one side of the plant wilt, especially on hot days or when the soil is dry. Plants may recover at night or during wet, warm weather. When the stem at ground level is cut diagonally, the vascular rings may be brown.
 
Disease spread
Verticillium wilt is spread by infected or contaminated seed pieces and by infested soil, farm machinery, and irrigation water. The fungi involved can survive for many years in the soil. Other crops and weeds can harbor the fungi - even without symptoms. The fungus grows inside the water-conducting vessels and blocks water movement, thereby causing wilt.
 
Control
Maintain vigorous plants by proper fertilization and watering.
Plant potatoes and tomatoes in the same spot only once every four years.
 
The symptoms of this disease begin to appear late in the season, when older, lower leaves become yellow, curl and roll, and show tipburn, then die. The inside of the plant stem turns yellow or brown. This disease is caused by a fungus. Again, prevention is the best management tool, although applications of a sulphur fungicide every 7-10 days will help in control. To prevent the disease, plant resistant varieties, and do not plant in areas where other infected plants have grown.
 
PREVENTION OF POTATO DISEASES
To prevent many of the potato diseases, rotate crops in the potato family on a 4-year cycle (minimum) in your garden plot. Rotate garden crops, with a minimum 4 year rotation cycle, planting all crops in the same family as potato in a different part of the garden each year. For more information on rotating garden crops and on plant families, see the fact sheet in this series "Rotating Garden Crops to Prevent Disease". Clean up garden debris well in the fall and do not compost diseased plant material.
 
References
Agriculture and Agri-Food Canada, Province of British Columbia. 1994. Late Blight of Potato and Tomato. QP #17011.

Flint, M.L. 1990. Pests of the Garden and Small Farm. University of California.

Oakland, CA 94608-1239. Pub. #3332. 276 pp.

Howard, R., J. Garland, and W. Seaman. 1994. Diseases and Pests of Vegetable Crops in Canada, An llustrated Compendium. The Canadian Phytopathological Society and the Entomological Society of Canada, Ottawa, Ont. Canada K2A 1Y8. 554 pp.

Pacific Northwest Plant Disease Control Handbooks. 1997. Extension Services of Oregon State University. Corvallis, OR 97331-0817.
 
Written by Sherry Lajeunesse and Martha Mikkelson, Extension Urban Pest Management Specialist and Plant Disease Diagnostician. February, 1998.
 

Raspberries: Cane Blight & Verticillium Wilt
 
 Two common disease problems on raspberries are cane blight and verticillium wilt. CANE BLIGHT causes canes to wilt and die while loaded with fruit. Late in the season, brownish-purple areas appear on cut or injured parts of the cane. These discolored areas spread down the cane and encircle it. Cane blight cannot be cured. Remove and burn all affected stalks.

The second disease is VERTICILLIUM WILT. This disease causes lower leaves to develop an off-green or yellowish-bronze tinge in June or early July. The leaves curl upward, then turn brown and fall off, beginning at the base of canes and progressing upward. Sometimes a sudden wilt occurs in hot, dry weather. Canes show blue or purple streaks. Red raspberry is more resistant, symptoms show up later in the season, and discolored leaves turn upward. It is difficult to see cane streaks in red raspberries because of the natural color in them. Infected plants eventually die. Remove and destroy infected and surrounding plants. For future plantings, avoid heavy, poorly drained soils, and buy disease-free stock. Avoid planting new plants in soils which were formerly planted to other plant hosts of verticillium wilt.
 
References:
 Flint, M.L. 1990. Pests of the Garden and Small Farm. Univ. of California. Oakland, CA 94608-1239. Pub. #3332. 276 pp.

The Organic Gardener's Handbook of Natural Insect and Disease Control. 1992. B. Ellis and F. Bradley, Eds. Rodale Press, Emmaus, PA 18098. 534 pp.

Pacific Northwest Plant Disease Control Handbooks. 1997. Extension Services of Oregon State University. Corvallis, OR 97331-0817.
 
Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997
 

Rotating Crops to Prevent Disease
 The annual cycle for many disease organisms is to attack the crop in the summer, survive the winter as spores in the soil or in plant litter, then attack the new planting the following year. So if you plant the same crop in the same spot year after year, the pathogen populations can continue to build and the disease will become an increasingly serious problem. You can break the cycle by moving the susceptible plant to another spot. Over time, many pathogens die back, and then it is safe to plant the original crop in that spot again. This is the idea behind rotating crops to prevent plant diseases.
 
The first step in planning crop rotation is to make a list of the crops you intend to plant. Most diseases tend to attack plants within in the same botanical family, so group your crops together in these botanical groups. Our common vegetables usually belong to one of ten families. Tomatoes, potatoes, peppers, and eggplants are in the Nightshade Family (Solanaceae); cabbage, radishes, turnips, broccoli, cauliflower, and brussels sprouts are in the cabbage family (Cruciferae), and are also called cole crops. Spinach is in the family Chenopodiaceae, and lettuce and other salad greens are in the Lettuce Family (Compositae). Peas, beans, and lentils are in the Legume Family (Leguminosae), and carrots, parsley, and dill are in the Umbelliferae Family. Cucumbers, the squashes, and melons are in the Melon Family (Cucurbitaceae), and onions, leeks, garlic, and chives are in the Lily Family (Liliaceae). Corn is in the family Gramineae, and strawberries are in the Rose Family.
 
The second step is to divide your garden into the same number of sections as the number of plant families you will have. For example, if you have six plant families, divide your garden into six sections. It works best if all the sections are the same size to ensure each section of soil is rotated from year to year.
 
Step three is to assign a plant family to each section, making notes of the assignment on a permanent map of your garden. (It is difficult to remember the order of rotation after two or three years without permanent records.) In each subsequent year, move each plant family to the next section in the garden, always following the same order. If you have 6 groups of plants, 5 years will pass before the same crop is planted in the same section of ground. A cycle of at least 4 years is most effective.
 
Crop rotation works best for controlling soilborne diseases that attack only a few species of plants. Most of the diseases caused by soilborne fungi are particularly easy to control this way. Added bonuses of crop rotation include control of certain insect pests and prevention nutrient depletion in the soil in particular spots in your garden.
 
References
 Controlling Pests and Diseases; Rodale's Successful Organic Gardening. 1994. Rodale Press, Emmaus, Pennsylvania 18098. 422 pp.

Ellis, B.W. and F. Marshall. 1991. The Organic Gardner's Handbook of Natural Insect and Disease Control: A Complete Problem-Solving Guide to Keeping Your Garden and Yard Healthy Without Chemicals. Rodale Press, Emmaus, Pennsylvania. 418 pp.

Flint, M.L. 1990. Pests of the Garden and Small Farm. Univ. of California. Oakland, CA 94608-1239. Pub. #3332. 276 pp.
 

Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997
 

Tomato Problems Common in Montana
 
 From a plant disease's point of view, years with cool, wet weather are very good years. Some of the more common tomato diseases we see in Montana include septoria leaf spot, early blight, and a variety of symptoms that are caused by environmental factors rather than a disease organism.
 
SEPTORIA LEAF SPOT, also known as SEPTORIA BLIGHT, is caused by a fungus that produces brown, somewhat circular spots that appear first on the lower leaves, then begin moving up the plant. The spots are quite small, ranging in size from 1/16" to 1/8" in diameter. As the spots mature, they sometimes become gray or white, surrounded by a dark margin.
 
To help control septoria leaf spot, stake plants to improve air circulation and to encourage quick drying of moisture on plant surfaces. Water only at the base of the plant, to avoid wetting the foliage. Remove weeds, which can harbor the disease, and remove badly infected leaves from the plant and destroy them. Clean up and remove all plant debris in the fall to minimize re-infection next year. Little is known about varieties that are resistant to this disease.
 
After discovering the disease, apply fungicides containing chlorothalonil, such as Ortho's Multi-Purpose Fungicide or Daconil 2787. Fungicides with the active ingredient maneb will also help, but do not apply within 5 days of harvest.
 

Another problem we see is EARLY BLIGHT, another fungal disease. This disease will also attack potatoes. Symptoms of early blight can be found on leaves, stems, and fruit. Small, irregular brown spots appear on older leaves first, enlarging until they are 1/4 to ½ inch in diameter. Dark rings of velvety spores commonly form in a bull's eye pattern inside the lesions. The surrounding tissue turns yellow. Often, the entire leaf turns yellow if the infection is severe. Dark, leathery spots may appear on the fruit.
 
High soil fertility reduces the severity of early blight, so keep plants properly fertilized and growing vigorously, and control weeds that might harbor the disease. When purchasing tomato plants, ask for varieties that are resistant.
 
If you find early blight, begin applying fungicides that contain the active ingredient chlorothalonil, such as Ortho Multi-Purpose Fungicide or Bravo. Fungicides that contain mancozeb or maneb will also work, such as Dithane or Maneb.
 
Most of the non-pest problems we see in tomatoes are caused by environmental factors such as COLD, WET WEATHER, symptoms of which include rolled and deformed leaves, especially the lower ones, and yellowing leaves with reddish veins. There is not much to be done about these problems, obviously. Another condition, called OEDEMA, is caused by water-logged soil and high humidity. This can result in whitish-tan raised areas that have a corky texture. Improving drainage, if possible, might help. SUNSCALD and SCORCH are two other examples of environmentally-caused symptoms.
 
 Because tomatoes can also spread late blight, a very serious disease of potatoes, it is extremely important to plant only Montana-grown or certified late blight-free tomatoes. The Great Potato Famine in Ireland in the 1850s was caused by late blight. For more information on late blight in potatoes, see the fact sheet in this series "Potato Diseases".
 
References
 Controlling Vegetable Pests; Environmentally Friendly Gardening. 1991. C. Putnam, Projec t Ed. Ortho Books, Chevron Chem. Co., San Ramon, CA 94583. 160 pp.

Flint, M.L. 1990. Pests of the Garden and Small Farm. Univ. of California. Oakland, CA 94608-1239. Pub. #3332. 276 pp.

Howard, R., J. Garland, and W. Seaman. 1994. Diseases and Pests of Vegetable Crops in Canada, An lllustrated Compendium. Canadian Phytopathological Society & Entomological Society of Canada, Ottawa, Ont. K2A 1Y8. 554 pp.

Pacific Northwest Plant Disease Control Handbooks. 1997. Extension Services of OSU. Corvallis, OR 97331-0817.
 
Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997