Pacific Northwest Vegetable Extension Group

of Washington State University, Oregon State University, and University of Idaho

Newsletter Archives

June 2001 Newsletter

Lindsey du Toit and Debra Inglis, editors
WSU Mount Vernon NWREC
16650 State Rte 536, Mount Vernon, WA 98273-4768
360-848-6140 (tel), 360-848-6159 (fax)

WSU Vegetable Pathology Team Newsletter

Welcome to the June 2001 edition of Washington State University's Vegetable Pathology Extension Team. This newsletter follows the team's June conference call when team members discussed current vegetable diseases, diagnoses and control. Please use this information in your own program newsletters and activities.

Update about Vegetable Pathology Team Website
Please view our new website! Considerable work has gone into its design and development, and it is expanding continuously. The new crop rotation publication is now posted. The home page can be accessed at:
We would appreciate receiving comments about the site: contact
For additions/deletions to the vegetable events calendar: contact

Another Useful Vegetable Disease Team Website
Michigan State University's Vegetable Advisory Team newsletters can be found at: They present information for muck and upland vegetable pest management including potato production, soil nutrition, and potato late blight management.

Need to Know Current Status of Pathogens & Pests on Washington Vegetables?
Review the crop profiles now being assembled by WSU's Pesticide Information Center. Each crop profile in this growing collection tells how a commodity is produced, with emphasis on critical pest management needs -- including the role of pesticides in integrated pest managament (IPM) and resistance management programs. The development of crop profiles is coordinated in each state by the land-grant university liaison to the regional Pest Management Centers. Their development involves extensive collaboration among land-grant faculty and growers.

The carrot profile is at
The lettuce profile is at
The pea (dry) profile is at
The sugar beet profile is at
The cabbage seed profile is
The spinach seed profile
The table beet seed profile

Vegetable Diseases/Issues and Places to Find Information

Questions to ask and steps to follow when diagnosing vegetable diseases.

1. Identify the symptoms. Are leaves yellow or brown? Is the discoloration worse on the leaf edges or in the middle of the leaf? Are there lesions or cankers on leaves, stems or roots? Are the spots of uniform size, or are they of many different shapes and sizes? Is only the upper or lower surface of leaves showing symptoms? Are plants wilting? Note: Spots uniform in size and distribution may indicate some type of chemical injury. Discoloration on leaf edges might indicate fertilizer toxicity. Mottling and mosaic patterns may indicate a virus infection. Lesions and cankers are generally caused by bacteria or fungi.

2. Determine if there is a pattern to the symptoms. Are many plant species in the same area affected? Does the problem seem worse on the field's edge or are symptoms uniform throughout the field? Are all plants in a particular row or field affected? Are all of the leaves or shoots on the plant affected? Are affected plants in a low spot or where there is poor drainage or compacted soil? Note: Damage caused by pathogens or insects often shows up as scattered areas on plants or in fields and is usually crop specific. Damage that has a uniform pattern or appears suddenly may be caused by physical, chemical, mechanical or environmental factors.

3. Trace the problem's history. When were the symptoms first noticed? What rates and types of fertilizer were used? What were the weather conditions like before the problem was noticed? Has the field recently been treated with herbicide? How much mulch has recently been applied?

4. Examine the plants carefully. Look for the presence or signs of insects feeding on leaves, stems and roots. Sometimes it's easier to find insects early in the morning or toward evening. Pull up plant and examine roots for signs of feeding. If you find an insect, it doesn't necessarily mean it caused the problem--you still need to get the insect identified. For diseases, look for yellow, discolored, or dead areas on roots, leaves, stems and flowers. The signs of a plant pathogen (fruiting bodies, slime, cysts, etc) might be present. Are the plants stunted or wilted even though there is available soil moisture? This could be from a root rot, wilt disease, or salt injury. Are the plants stunted or do they have obvious growth malformations; are they discolored or mottled? This might be due to infection by a virus.

5. Could plants be lacking essential nutrients? See table below.

6. Could there be a nutrient toxicity? Soluble salt injury may be seen as wilting of the plant even when the soil is wet. Leaf margins may appear scorched or browned, usually resulting from excessive fertilizer application. Burning on leaf edges can be a sign of chlorine damage. Also, if wood ashes were applied to the field, using more than 5 pounds per 100 square feet could increase the soil pH and cause nutritional deficiencies and/or toxicities.

7. Could the problem be related to the soil? Soil problems such as compaction and poor drainage can severely stunt plants and make them susceptible to seedling damping-off and root diseases.

8. Could a pesticide injury be the cause? What pesticides were applied and when were they applied? Copper and sulfur fungicides sometimes can burn plants when applied in hot weather. Also, broadleaf herbicides can cause abnormal growth or kill some plants. Be aware of mulches that may have residual herbicide. Obtain information about the field's history i.e., crops planted and herbicides applied in prior years.

9. Could the damage be caused by environmental conditions? Have temperatures been excessively high or low? Plants that die practically overnight may have been affected by frost. Has it been very dry or wet for extended periods? Have there been strong winds? What about air pollution? Ozone levels may rise as hot, humid weather settles in for long periods of time, and flecking from damage may be concentrated in specific areas of leaves. Look for irregularly shaped spots that may look similar to feeding damage of mites and certain leafhoppers.

Some Nutrient Deficiency Symptoms


light green or yellow older foliage


stunted plants and purplish leaves


stunted plants, stubby roots, causes blossom end rot of tomatoes, tip burn of cabbage, brown heart of escarole, celery blackheart, carrot cavity spot


yellowing between veins of older leaves


yellowing of new leaves, stunted plants


growing points die back and leaves are distorted


yellowing of leaves which become thin and elongated; soft onion bulbs with thin scales


light green or yellow foliage on youngest leaves


rust colored spots on seed leaves of beans, green and yellow striping of corn, yellowing of beet leaves


mottled yellow areas appearing on younger leaves first. In beets, foliage becomes dark red


distorted, narrow leaves, yellowing of older leaves, whiptail leaf symptoms in cauliflower

-Adapted from American Vegetable Grower, February 1998

10. Seek the advice of a professional. If you need assistance with plant disease diagnoses, you can submit a sample to the WSU plant clinics. Find out how at: Submission Procedures

Is this mildew downy or powdery?

Downy mildew and powdery mildew are two foliar diseases which are sometimes confused with one another, but actually are distinct and relatively easy to tell apart. The pathogens causing both types of mildew are obligate parasites, and require an association with host tissue for long term survival. However, the downy and powdery mildew organisms belong to different groups of plant pathogens. This means that different types of fungicides are required in their control – an important fact often overlooked. Both mildews occur in Washington although downy mildew is more common in the western part of the state while powdery mildew generally is more common on vegetables in eastern Washington. The following comparisons might be useful for distinguishing between the two diseases since only a few crops (beet and green pea) can be affected by both.

Downy mildew Powdery mildew
Common vegetable hosts in Washington, and the mildew species affecting them Beet (Peronospora farinosa f. sp. betae)

Broccoli (P. parasitica)

Cabbage (P. parasitica)




Cauliflower (P. parasitica)

Green pea ( (P. viciae)

Lettuce (Bremia lactucae)

Onion (P. destructor)


Radish (P. parasitica)

Spinach (P. farinosa f. sp. spinaciae)

Swiss chard (P. parasitica)


Beet (Erisiphe polygoni)



Cantaloupe (Erysiphe chichoracearum and
Podosphaera fuliginea

Carrot (Erysiphe heraclei)

Cucumber (as for cantaloupe)


Green pea (Erysiphe pisi)



Potato (Erysiphe chichoracearum)




Squash (as for cucumber)

Typical symptoms

Light green spots develop on upper leaf surfaces; under moist conditions, a light to dark moldy (downy) growth appears on the lower leaf surfaces; young petioles or stems can also be affected and in severe outbreaks even the crown of the plant, leading to misshapen plants and systemic infections; black streaking in leaves and stems may be apparent in cruciferous hosts like broccoli

Small, discrete white patches on upper and lower leaf surfaces; patches may coalesce

Spots sometimes remain green while the rest of the leaf turns yellow (green island effect)

Premature defoliation may follow severe infections

On seed crops, infections on pedicels and florets may result in premature death of these plant parts

Typical signs

The gray or brown moldy or downy growth of the pathogen on foliage which is primarily on the lower leaf surface

White mycelium and spores, and in some cases, dark pinpoint-size fruiting bodies called cleistothecia (on both the upper & lower leaf surfaces)

Generalized disease cycle

The pathogen survives in infected crop residues or in wild or volunteer crop species; for some crops it can also survive on or with seed to a limited extent; primary infections result in lesions which can sporulate profusely; the wind and rain-splashed spores cause repeated secondary infections

Overwinters on infected seed, dead plants or weeds; disease spreads during the crop season by wind blown spores

Conditions favoring ections

High humidity, fog, drizzle and heavy dew; night temperatures of 46 to 61 degrees F and day temperatures up to 75 degrees F

Can be serious in some greenhouse and field-grown winter crops

The fungus tolerates a wide range of environmental conditions; spore germination enhanced by high humidity but limited by surface water on leaves

Cultural management strategies

Host resistance in some cultivars is available against certain races of the pathogen on some crop species; eradicate weedy hosts and wild species; manage irrigation to reduce periods of high humidity; avoid poorly drained soils; manage plant density; destroy crop refuse and cull piles

Host resistance in some cultivars is available against certain races of the pathogen on some crop species

A 2-year rotation cycle is often suggested

Chemical management strategies

A wide range of fungicides are registered on specific crops. Check labels and the PNW Plant Disease Management Handbook. Remember that downy mildews are grouped with the water molds so fungicides used against oomycete fungi are often effective.

A wide range of fungicides are registered on specific crops. Check labels and the PNW Plant Disease Management Handbook. Fungicides used against oomycetes are not effective.

Register Now for the Second International Vegetable Soybean Conference
The conference will be held August 10-12 at the Sheraton Tacoma Hotel.
E-mail: or for more information.

Check out the Vegetable Pathology Team's calender of upcoming vegetable events:

Our pages provide links to external sites for the convenience of users. WSU Extension does not manage these external sites, nor does Extension review, control, or take responsibility for the content of these sites. These external sites do not implicitly or explicitly represent official positions and policies of WSU Extension.

WSU Mount Vernon NWREC, 16650 State Route 536, Mount Vernon, WA 98273-4768, 360-848-6120
Contact Us: Lindsey du Toit and Carol Miles