Pacific Northwest Vegetable Extension Group

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

Newsletter Archives

August 2003 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


IN THIS ISSUE

HELLO!

Welcome to the August 2003 edition of Washington State University's Vegetable Pathology Extension Team newsletter, the final edition of the 2003 growing season. If you have suggestions for next year's growing season newsletters, please let us know.

If you havequestions or comments about vegetable diseases and pests, or WSU's Vegetable Pathology Team, contact Debbie Inglis (dainglis@wsu.edu) or Lindsey du Toit (dutoit@wsu.edu).

 

PEST ALERT!!!

Field Days

High Plains Disease Caused by the High Plains Virus
Photos courtesy of Gary Q. Pelter

There have been reports this season of severe outbreaks of high plains disease (HPD) in sweet corn fields in several counties in the Columbia Basin. HPD is caused by the High plains tenuivirus (HPV). The disease was first detected in Colorado, Kansas, and Texas in 1993; in Utah in 1994; and in Washington in 1998.

Hosts: HPV can infect field corn and sweet corn, wheat, barley, yellow and green foxtail, and witchgrass.

Photo of severe mosaic symptoms on sweet corn
Fig. 1. Severe mosaic symptoms on sweet corn caused by the High plains virus.

Photo of infected (lower) and health (upper) plants
Fig. 2.
Infected (lower) and healthy (upper) plants in a sweet corn crop infected with high plains disease caused by the High plains virus.

Symptoms: Mosaic symptoms are usually first evident on leaves emerging from the whorl, although older leaves may also develop mosaic symptoms (Fig. 1 and 2). Infected plants may be stunted and yellowish (Fig. 3). Yellow or red-purple (depending on the genetic background of the corn cultivar) bands ¼ to ½" in width and running parallel to the veins may be observed on intermediate to fully-developed leaves. These bands later turn necrotic. Infected plants may have stunted or weakened root systems.

Transmission: HPV is transmitted by: 1) the wheat curl mite (Aceria tosichella), a tiny eriophyid mite which also transmits wheat streak mosaic virus (WSMV); and 2) corn seed. Corn plants can be infected with both HPV and WSMV. The mite, which is barely visible with a 10x magnifying lens, can be blown between fields. As a result, a distinct disease gradient is sometimes evident on corn crops planted immediately adjacent to infected wheat crops. Seed transmission is thought to be very low, although the pattern of symptom development in some outbreaks of HPD in sweet corn crops have suggested higher incidences of seed transmission.

Photo of severely stunted sweet corn plant
Fig. 3. Severely stunted sweet corn plant infected with the High plains virus.

Management: Management options for HPD are limited. Avoid seeding crops late. Control grassy weeds and volunteer wheat, which may serve as reservoirs of HPV and on which the wheat curl mite may persist. If possible, avoid planting sweet corn crops immediately adjacent to wheat crops. The objective is to break the 'green bridge' effect and prevent spread of the HPV in the mite vector. As resistance screening efforts progress, plant resistant corn cultivars. Limited information is currently available on the resistance of sweet corn hybrids to HPD, but known susceptible hybrids include: Ambrosia, Challenger, Crisp 'n Sweet, DMC 20-3, DMC 20-10, Double Gem, Extra Sweet, Honey and Pearl, How Sweet It Is, Native Gem, Phenominal, Shasta, Style Sweet, and 710. Resistant or tolerant hybrids include: Delectable, Empire, Gemini, Imaculata, Incredible, Platinum Lady, Silver Queen, and 711.


UPCOMING VEGETABLE EVENTS AND ANNOUNCEMENTS

New Book on Genetically Modified Crops

The World Scientific Publishing of the Imperial College Press in London has published "Genetically Modified Crops" by Nigel G. Halford. The book describes the history and development of the science of biotechnology. It also features the GM crops that are grown commercially around the world, and the new varieties that are currently being developed. More information can be found at http://www.wspc.com.sg/books/lifesci/p284.html

New Bulletin from Skagit Veg Trials Available

A new bulletin "Temperature Modification Techniques for the Growing Environment in the Puget Sound" prepared by the volunteers of the Skagit Veg Trials and Dr. Andy Anderson, Skagit Veg Trials Director, is now available. Information on transplanting, raised beds, solar mulch, cloches, plastic mulches and combined strategies for modifying soil temperature is included. For copies, contact andersonw@wsu.edu

New Organic Agriculture Resource Guide

OrganicAgInfo is an on-line database of research reports, farmer-to-farmer information, and outreach publications on organic agriculture. The database is being hosted by North Carolina State University, and was funded by grants to the Scientific Congress on Organic Agricultural Research and the Organic Agriculture Consortium from the Initiative for Future Agriculture and Food Systems through USDA-CSREES. See http://www.organicaginfo.org

Onion Field Day

The 2003 Columbia Basin Onion Field Day will be held Thursday, August 28 from 9:00 am until noon at Grigg and Sons Farm near Quincy, WA. Forty-six onion cultivars and lines from nine seed companies will be compared and contrasted, and gray mold research explained. For more information, contact Gary Pelter at 509-754-2011.

Plant Disease Diagnosis

For a comprehensive guide to plant disease diangosis see the APSnet Education Center Introductory Topic article written by M. B. Riley, M. R. Williamson and O. C. Maloy. It can be viewed at http://www.apsnet.org/education/IntroPlantPath/Topics/Default.htm

Searching for Plant Science Images?

Check the Plant Management Network Image Collections at http://www.plantmanagementnetwork.org/images/ The site contains more than 1,500 high quality photographs, graphics, charts, graphs, maps, biological drawings, chemical structures, diagrams and disease cycles focused on plant science. Subscriptions are $38. See http://www.plantmanagementnetwork.org/about/subscribe/

WSU Cooperative Extension Has On-line Publications

You can download hundreds of WSU publications free of charge at the new WSU publications catalog and sales site at http://pubs.wsu.edu/. Should you choose others, ordering is very easy. Temporary discounts of up to 25% on some publications are being offered.
 

NFORMATION ABOUT DIAGNOSING LEAFY VEGETABLE DISEASES

Foliar diseases on leafy vegetables in northwestern Washington

The following is an excerpt from the WSU-Vegetable Pathology Team's Leafy Vegetable Workshop, presented by Lindsey du Toit, Jenny Glass and Debbie Inglis on July 24, 2003 at WSU-Mount Vernon.

Foliar diseases on leafy vegetable crops in Washington can be caused by both obligate and non-obligate parasites. Obligate parasites are organisms that can grow and multiply only in or on living tissue, and include Peronospora, an oomycete causing downy mildew; Erysiphe, one of the fungi causing powdery mildew; Albugo, an oomycete causing white rust; and, Uromyces and Puccinia two fungi which cause common rust. Erisphe belongs to the ascomycetes, and Uromyces and Puccinia are basidiomycetes. Some examples of non-obligate parasites include spinach leaf spot fungi (Cladosporium, Stemphylium, and Colletotrichum); the beet and chard leaf spot fungi (Phoma, Ramularia, and Cercospora), and crucifer foliar pathogens (Alternaria and Mycosphaerella). These fungi are ascomycetes or deuteromycetes.

Correct diagnoses of leafy vegetable foliar diseases are important because they can lead to appropriate fungicide selection, improve the timing of fungicide applications, assist with the selection of cultivars with higher levels of resistance, and determine appropriate cultural practices for more effective disease management.

The downy mildews are some of the most commonly encountered foliar diseases on leafy vegetables in the PNW. They are also relatively easy to diagnose (see photos in table below). Favorable conditions for downy mildew outbreaks include cool temperatures ranging from 50 to 60 °F and high moisture related to low cloud ceilings, dew, fog and rain.

Downy mildews on leafy vegetables.

Symptoms

Disease and pathogen

Comments

Photo of Peronospora farinosa f. sp. spinaciae

Peronospora farinosa f. sp. spinaciae on spinach

Note chlorosis on upper leaf surface, and grayish brown felt-like mildew growth on leaf undersurface

Photo of Peronospora farinosa f. sp. betae

Peronospora farinosa f. sp. betae on Swiss chard

Note grayish green felt-like growth and twisted leaf

Photo of Peronospora farinosa f. sp. betae

Peronospora farinosa f. sp. betae on beet

Note pinkish felt-like growth on leaves and twisted seed stalk

Photo of Peronospora parasitica

Peronospora parasitica on cabbage

Note chlorotic spots on upper leaf surface, and grayish white crust-like growth on leaf undersurface

Photo of Peronospora parasitica

Peronospora parasitica on broccoli

Note dark streaking in vascular tissue due to systemic infection


 

Leaf spot diseases on leafy vegetables are quite common in the PNW, although they can be somewhat more challenging to diagnose than downy mildews. Often the pathogens need to be cultured from the diseased leaf tissues, and the spores from the fungal colonies identified with the use of a microscope before the diagnosis can be made.

Three fungal leaf spot diseases of spinach.

 

Cladosporium leaf spot

Stemphylium leaf spot

Anthracnose

Pathogen name

Cladosporium variabile

Stemphylium botryosum

Colletotrichum dematium

Leaf spot symptoms

Distinct, 1-3 mm spots, develop dark margin

Diffuse, rapidly expanding

Distinct, coalescing, water-soaked in humid conditions

Spores in lesions

+

+

+

Dispersal

Wind, seed

Wind, seed

Splashing water, seed

Overwintering

Volunteers, seed

Woody spinach debris, seed

Volunteers, seed

Favorable conditions

Moist, cool

Moist, warm, pollen on leaves

Wet, cool

Host range

Chenopod species, others?

Spinach

Spinach


Photo of Cladosporium leaf spot and Stemphylium leaf spot Photo of Anthracnose

Fig. 1. Cladosporium leaf spot (left) and Stemphylium leaf spot (right) on spinach. Photo courtesy of Mike L. Derie.

Fig. 2. Anthracnose on young spinach leaves caused by Colletotrichum dematium. Photo courtesy of Mike L. Derie.

Three fungal leaf spot diseases of beets and chard.


Cercospora leaf spot

Ramularia leaf spot

Phoma leaf spot

Pathogen name

Cercospora beticola

Ramularia betae

Phoma betae

Symptoms

Circular leaf spots, red-brown margin, on older leaves;

Also causes crown lesions

Light brown leaf spots, angular & larger, on older leaves

Round leaf spots, concentric rings on perimeter, dark margin;

Also causes seedling black leg & crown rot

Spores in leaf spots

Minute black dots (stromata)

Silvery gray to white spore masses

Black fruiting bodies (pycnidia), also in crown lesions

Seedborne

+ (external)

+ (?)

+

Dispersal

Splashing water, wind, insects, seed

Wind, seed

Splashing water, insects, seed

Overwintering

Weeds, debris, seed

Debris, seed

Soil, roots, debris, weeds, seed

Favorable conditions

Warm, moist

Cool, moist

Cool to warm, moist

Host range

Beet, chard, Chenopod. weeds

Beet, chard

Beet, lambsquarter


Photo of Cercospora left spot Photo of Ramularia leaf spot Photo of Phoma leaf spot

Fig. 1. Cercospora leaf spot on sugar beet.
Photo courtesy of North Dakota State University (http://www.ext.nodak.edu/extpubs/plantsci/
rowcrops/pp1244w.htm
)

Fig. 2. Ramularia leaf spot of chard. Photo courtesy of Debra A. Inglis.

Fig. 3. Phoma leaf spot on beets. Photo courtesy of Lindsey J. du Toit.


Two fungal leaf spot diseases of crucifer crops.

 

Alternaria leaf & pod spot

Ring spot

Pathogen name

Alternaria brassicicola &
A. brassicae

Mycosphaerella brassicicola

Symptoms

Black circular to irregular lesions, necrotic center;

Circular to elongated black spots on pods and racemes

Circular lesions (up to 2 cm diameter), definite margin & chlorotic halo, concentric zonation, on older leaves

Spores in leaf spots

Naked spores

Black fruiting bodies (pycnidia &/or perithecia) in concentric rings

Seedborne

+

-

Dispersal

Wind, insects, windblown rain

Splashing water, insects

Overwintering

Debris, biennial seed crops, seed

Debris, biennial seed crops

Favorable conditions

Moist, warm

Moist, cool

Host range

Crucifers

Crucifers


Photo of Alternaria leaf and pod spot Photo of Ring spot

Fig. 1. Alternaria leaf and pod spot caused by A. brassicicola & A. brassicae. Photos courtesy of Lindsey J. du Toit.

Fig. 2. Ring spot of cabbage caused by Mycosphaerella brassicicola. Photo courtesy of Lindsey J. du Toit.


Bacterial foliar diseases of crucifer crops.

Black rot

Xanthomonas leaf spot

Peppery leaf spot

Pathogen name

Xanthomonas campestris pv. campestris

X. campestris pv. raphani

X. campestris pv. armoraciae

Pseudomonas syringae pv. maculicola

Symptoms

Yellow leaves, wilting, black veins, V-shaped lesions from leaf margin

Circular, water-soaked leaf spots, chlorotic halo; dark lesions on petioles

Circular to angular spots, chlorotic halo

Systemic

+

-

-

Seedborne

+

+

+

+

Dispersal

Splashing water, seed, insects

Overwintering

Debris, cruciferous weeds, soil

Favorable conditions

Warm to hot, moist

Cool to warm, extended wet periods

Cool, wet

Host range

Crucifers (including weeds)

Cabbage, broccoli, cauliflower, kale, radish, tomato, pepper

Cabbage, broccoli, cauliflower, kale, radish, horseradish

Cabbage, broccoli, cauliflower, Brussel sprouts, turnip


Photo of Black rot Photo of Xanthomonas

Fig. 1. Black rot of cabbage showing blackened veins, V-shaped marginal lesions, chlorosis and wilting. Photo courtesy of Lindsey J. du Toit.

Fig. 2. Xanthomonas leaf spot of crucifers caused by X. campestris pv. raphani & X. campestris pv. armoraciae. Photo courtesy of Lindsey J. du Toit.


 

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