Volume 64 Number 19 Date 11/07/2019
SWEDE MIDGE - A DATCP vegetable pest survey in 32 community garden and CSA farms resulted in the first capture of swede midge, Contarinia nasturtii, in Dane and Milwaukee counties. The flies were collected on June 17 and July 1 on sticky traps placed in broccoli. Initial determination of the specimens was made by a DATCP specialist, with verification by an authorized USDA identifier. The positive identifications represent the first detection of invasive swede midge (SM) in Wisconsin and a new state record. This non-native insect is expected to significantly impact brassica production in the state as it becomes more widely established.
In response, vegetable growers--particularly in Dane and Milwaukee counties--are encouraged to increase vigilance for signs of this pest in 2020. Damage symptoms caused by swede midge larval feeding include puckered leaf tissue, brown scarring at the growing points, distorted, twisted leaves, and broccoli and cauliflower plants with blind heads. The single most effective control is rotation to a non-cruciferous crop for 2-3 years. DATCP is planning an expanded survey for swede midge in 2020.
VARIEGATED CUTWORM - Migrant cutworm moths arriving in May produced damaging infestations by mid-June. Several vegetable growers, mainly in southern and eastern Wisconsin, observed severe cutworm feeding on a wide variety of crops. On June 27, a Milwaukee County CSA grower reported experiencing "the worst cutworm infestation in 20 years" of vegetable production. The caterpillars invaded her hoop-houses and attacked seedlings under lights, chewing plants down to the stems. Based on photos received by the DATCP Entomologist, the variegated cutworm was the primary species of concern.
-- Krista Hamilton, DATCP Entomologist
LATE BLIGHT - Disease pressure increased significantly in 2019 due to wet weather, with detections in 18 counties, compared to four counties in 2018. The state's first infected commercial potato field was confirmed in Wood County on July 17 by the UW Plant Pathology Department. The next find came from a CSA tomato field in La Crosse County surveyed by DATCP on August 2. Additional cases of late blight on both potato and tomato were detected in the following counties: Adams, Barron, Crawford, Green Lake, Jackson, Juneau, Monroe, Pierce, Polk, Portage, Sauk, Shawano, St. Croix, Vernon, Walworth and Waushara. All samples tested by UW from Wisconsin were the US-23 pathogen genotype.
CUCURBIT DOWNY MILDEW - Cucurbit downy mildew (CDM) was diagnosed by the UW in Dane and Vernon counties on August 20 and in Buffalo County on September 5. Infected host plants were butternut squash, cucumber, pumpkin and watermelon. Although cucurbit downy mildew (CDM) produces no direct symptoms on cucumber fruits, the disease increases the risk of sunscald and causes secondary fruit decay. CDM spreads into the northern U.S. in summer on airborne sporangia from infected plants in other states.
BASIL DOWNY MILDEW - This aggressive foliar disease was confirmed in four counties: Dane, La Crosse, Sheboygan and Waukesha. Basil downy mildew (BDM) spreads via wind-dispersed spores, rapidly infecting entire fields and causing complete plant loss. BDM is often present on greenhouse-grown basil in garden centers in the spring, though it may not progress until late summer. Purchasing disease-free plants, promoting airflow, and frequent monitoring of the crop so harvest can occur quickly once mildew symptoms appear are all important controls. Use of fungicides is not recommended.
-- Sam Fieweger, DATCP Plant Industry Lab
VEGETABLE DISEASES - Unusually wet conditions this season were very conducive for development of certain bacterial and fungal diseases. Surveys for vegetable diseases in 32 community gardens and CSA farms generated 162 symptomatic plant samples that were at tested at the DATCP Plant Industry Lab. Results were as follows:
Twenty-three of 35 (66%) tomato samples were infected with Septoria leaf spot, which was ubiquitous on tomatoes throughout the state. Early blight (Alternaria solani) was diagnosed on tomato from Polk, Portage and Pierce counties, and leaf mold (Fulvia fulva) was detected in hoop house grown tomatoes in La Crosse County.
Bacterial diseases on peppers were another common problem. Ten of 14 (71%) pepper samples tested positive for bacterial spot (Xanthomonas vesicatoria or X. euvesicatoria) and Syringae seedling blight and leaf spots (Pseudomonas syringae).
Other notable diseases diagnosed were purple blotch (Alternaria porri) on garlic, leek and onion from La Crosse County; garlic rust on Dane County garlic; and black rot (Xanthomonas campestris) on a variety of cole crops (broccoli, cauliflower, kale, kohlrabi and red cabbage) from Dane, La Crosse, Pierce and Washington counties.
Lab testing found no evidence of cucumber green mottle mosaic virus (CGMMV), a disease of concern for crop exporters, in cucurbit samples. In addition, 28 samples from Dane, Kewaunee, La Crosse and Polk counties tested negative for potato spindle tuber viroid (PSTVd) and tomato chlorotic dwarf viroid (TCDVd), two more diseases of regulatory significance that are highly transmissible by seed and mechanical means such as touching infected plants.
-- Anette Phibbs, DATCP Plant Pathologist