Pacific Northwest Vegetable Extension Group

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

Newsletter Archives

July 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




Welcome to the July 2003 edition of Washington State University's Vegetable Pathology Extension Team newsletter, the third edition of the 2003 growing season. We hope you will enjoy it. Be sure to view the excellent photos of carrot diseases at the end of the article!

If you have questions or comments about vegetable diseases and pests, or WSU's Vegetable Pathology Team, contact Debbie Inglis ( or Lindsey du Toit (


Growing conditions in western Washington have been unusually dry and hot. As a consequence potato and tomato growers have been irrigating their crops. Early blight, caused by Alternaria solani, is now being reported in some areas of western Washington. The weather is likely contributing to disease occurrences this year even though early blight is not often a serious problem in western Washington. Generally, early blight is found primarily under sprinkler irrigation; premature senescence resulting from drought or other stresses also favors disease development. Crop losses due to early blight can be significant if extensive foliar infections occur before or soon after flowering. Potato tubers and tomato fruits can also become infected. For information on cultural and chemical practices for controlling early blight, see the Pacific Northwest Plant Disease Management Handbook ( or contact

Early blight lesions on potato leaflet Early bilight lesions on potato tuber

Early blight lesions on potato leaflet. Note the dark colored "target" spots with surrounding chlorotic zones. Photo courtesy of Babette Gundersen.

Early blight lesions on potato tuber. Note the circular sunken spots about 0.5 inch in diameter. The underlying tissue will be brown and corky. Photo courtesy of Gary Pelter.



2003 WSU Carrot Cultivar Field Day
The 2003 WSU Carrot Cultivar Field Day will be held on Friday, August 8th at 9 am at Klaustermeyer Farms in a field located near Othello, WA. The event will feature a variety of carrot types including 17 slicers/cellos, 23 cut & peels, 12 dicers from commercial seed companies, and 40 "nutritionally-improved" carrots from USDA breeders. The field is located on Yeisley Road, east of Booker Road and south of Highway 26 and north of Highway 17. For a map, contact Eric Sorensen at The field day is sponsored by WSU Extension, Benton-Franklin Counties, and the Pacific Northwest Vegetable Association.

Onion Field Day
The WSU Onion Field Day is on Thursday August 28th at Grigg & Sons Farm near Quincy, WA. The contact person is Gary Pelter or (509) 754-2011 ext. 413.

Potato Association of America
The 87th Annual Meeting of the Potato Association of America, "Back to Tradition", will be held August 10-14 at the Red Lion Hotel at the Park in Spokane, WA. For more information:

WSU-Mount Vernon Potato Field Day and Cucumber Twilight Tour
The WSU-Mount Vernon Potato Field Day and Cucumber Twilight Tour will be held Thursday, August 21 from 4 to 5:30 pm, and from 6:00 to 7:30 pm, respectively. For further information contact Debbie Inglis at and Tim Miller at


The Washington State Department of Agriculture (WSDA) has a new Web site address at WSDA has designed the site to be more compatible with Web browsers and user-friendly.

To view a listing of those who are doing plant disease diagnoses for commercial plantings view WSU's online publication, "Analytical Laboratories and Consultants Serving Ag in the PNW" at

Commercial Vegetable Production Guides are available at Although written for Oregon commercial vegetable growers, they contain a great deal of useful information. The guides were adapted by the Oregon State University Extension Service from Oregon and Pacific Northwest publications, publications of other public institutions, published and unpublished research, and the professional experience of OSU faculty and some colleagues in agribusiness.

A new title in organic pest management, Plant Protection Challenges in Organic Production edited by D.M. Suckling and M.R. Butcher can be ordered through the New Zealand Plant Protection Society. Contact .

Other websites to check:
Certified Crop Advisor website:
USDA Crop Profiles: Interregional Research Project (IR-4) Assists Growers of Minor Crops

Carrot Seed Disease Management
Lindsey J. du Toit, WSU Vegetable Seed Pathology, Mount Vernon, WA; and Gary Q. Pelter, WSU Cooperative Extension, Ephrata, WA
(Click here to go to the figures at the end of the article)

Three of the most important seedborne pathogens of carrot in the Pacific Northwest are Alternaria dauci (the fungus causing Alternaria leaf blight), Alternaria radicina (the fungus causing black rot), and Xanthomonas campestris pv. carotae (the bacterium causing bacterial leaf/umbel blight). Alternaria leaf blight is seldom observed in Washington and, correspondingly, Alternaria dauci is rarely detected on carrot seed produced in the PNW. However, black rot and bacterial blight are observed regularly, although sporadically, in carrot seed crops in the PNW; and the associated pathogens, A. radicina and X. campestris pv. carotae, have been detected on carrot seed lots produced in this region.

What are the recommended practices for the carrot industry to manage these seedborne pathogens?

1. The first step in effective regional management of seedborne pathogens is to make it a routine practice to have all stock seed lots tested for seedborne pathogens of concern in this region. Seed testing should be completed prior to planting stock seed. However, if circumstances prevent this (e.g., limited time from cleaning/shipping of stock seed to planting), it is still valuable to have a sample of the stock seed assayed after planting to identify the potential risks of diseases developing as a result of seedborne inoculum. This information will also help determine appropriate in-season production and disease management practices needed. Only seed lots determined to be free of seedborne pathogens should be used as stock seed for organic seed crops.

2. If seedborne pathogens are detected on stock seed lots, infected seed lots should be treated appropriately prior to planting to reduce, or even eradicate, seedborne inoculum. Hot water, chlorine, and fungicide seed treatments can be used. Fungicide seed treatments do not eradicate seedborne inoculum, but prevent or reduce transmission of some fungal pathogens from the seed to developing seedlings. Not all fungicide seed treatments are effective against all seedborne pathogens. For example, Rovral (iprodione) and Maxim (fludioxonil) help prevent transmission of seedborne A. dauci and A. radicina, but do little to control seedborne X. campestris pv. carotae. Treatment of seed with chlorine alone will eradicate inoculum located on the seed coat, but will not eradicate inoculum located within the seed. However, hot water treatment can eradicate internal seedborne inoculum when carried out precisely to allow thorough penetration of the water (heated to a specific temperature) into bags of seed for the specific duration determined for the seedborne pathogens. For example, X. campestris pv. carotae can be eradicated by placing infected seed in water heated to 125oF (52oC) for 25 minutes. Alternaria radicina can eradicated from carrot seed using hot water treatment at 122oF (50oC) for 30 minutes, or hot sodium hypochlorite (0.1 to 1.0%) at 122oF for 30 minutes. Treated seed should be re-assayed to determine whether the pathogen was eradicated or the population decreased adequately as a result of the treatment.

3. The initial disease management steps of assaying and treating stock seed lots for pathogens also apply to steckling crops. Alternaria radicina and X. campestris pv. carotae have been detected on stecklings sampled directly out of shipping crates upon arrival in Washington, highlighting the potential for stecklings to serve as the initial source of inoculum. Stecklings should be grown in areas where bacterial blight and black rot are less prevalent, e.g., bacterial blight is widespread in the Central Valley of CA, but seldom a problem in the Imperial Valley of CA. If there is reason to suspect stecklings may be infected, the roots could be dipped in a chlorine solution. Alternaria radicina can continue to develop and spread from infected to healthy stecklings during cold storage, so it is important to inspect stecklings for evidence of black rot prior to cold storage.

4. Cultural management practices recommended for seedborne pathogens apply to direct-seeded crops and to steckling beds. An adequate crop rotation must be maintained. If a field has a record of A. radicina infestation, a rotation interval of six years may not be adequate since this pathogen has been shown to survive in soils for up to eight years. In comparison, X. campestris pv. carotae does not survive in the soil after carrot debris has decomposed. Therefore, a minimum rotation interval of three years, combined with incorporation of carrot residues into the soil immediately after harvest (to speed up microbial degradation of the debris), is recommended for management of bacterial blight.

5. In-field fungicide applications (e.g., Rovral, Kocide, etc.) may be beneficial if black rot is present on the crowns/foliage. However, foliar fungicide applications will not eradicate infection established on roots beneath the soil-line. Development and spread of bacterial blight can be minimized with foliar copper applications (e.g., Kocide, C-O-C-S, or ManKocide), but the applications have limited efficacy once bacterial blight is well established in a seed crop.

6. The increase in use of overhead irrigation systems for carrot seed production has particular ramifications for bacterial blight. Wet foliage and splashing water are highly conducive to the spread and development of bacterial blight. If a stock seed lot is infested with X. campestris pv. carotae, grow the crop using furrow or drip irrigation instead of overhead irrigation, to minimize splash-dispersal of the pathogen. Similarly for stecklings infected with bacterial blight. If a crop is under overhead irrigation, complete each irrigation earlier in the day (where possible) to allow the foliage to dry more rapidly compared to irrigating at night or later in the day.

7. Volunteer carrots and related Umbelliferous weeds can serve as reservoirs of carrot pathogens. Scout and rogue volunteers and related weeds regularly, both within and adjacent to carrot seed crops.

8. Xanthomonas campestris pv. carotae has been detected in airborne dust and debris generated during threshing, and as far as a -mile from seed crops being threshed. Where possible, direct-seeded crops should not be located immediately adjacent to mature crops of the previous season, to minimize the risk of inoculum spreading from the mature crop during threshing onto young seedlings of the new crop. This recommendation is most pertinent when planting direct-seeded crops 'early' (e.g., in early August in Washington), and when planting direct-seeded crops under overhead irrigation.

Implementing appropriate production practices and management recommendations will minimize the risk of introducing damaging carrot pathogens in stock seed or stecklings, and will reduce the incidence of harvested seed lots infected with these pathogens. Scout fields carefully and regularly, and implement disease control measures immediately when a pathogen is detected in a crop. Know the crop history before determining the specific field in which to plant a carrot seed crop.

Fig. 1. Alternaria leaf blight caused by Alternaria dauci Photo of Alternaria leaf blight
Photo of spores of alternaria dauci Fig. 2. Spores of Alternaria
(100x magnification)
Fig. 3. Black rot (crown rot) caused by Alternaria radicina. Photo of Alternaria radicina
Photo of foliar blight and crown rot Fig. 4. Foliar blight and crown rot caused by Alternaria radicina.
Fig. 5. Superficial russeting
of carrot stecklings caused by Alternaria radicina.
Photo of superficial russetting of carrot stecklings
Photo of spores of Alternaria radicina Fig. 6. Spores of Alternaria radicina (55x magnification).
Fig. 7. Bacterial leaf blight with ooze caused by Xanthomonas campestris pv. carotae.

Photo of bacterial leaf blight


Photo of Umbel blight Fig. 8. Umbel blight caused by Xanthomonas campestris pv. carotae.

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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