West central Michigan vegetable update – July 3, 2019

High rainfall and warm temperatures this week could favor Phytophthora capsica. Take action to protect your cucurbits.

Foggy morning drive
This morning’s drive to the Hart MSU Extension office featured temperatures in the 60s and fog; high overnight temperatures and long dew periods are favorable for disease development. Now is a good time to buckle down on fungicide programs in many vegetables. Photo by Ben Werling, MSU Extension.

Weather

Overall, we have seen a big change in the weather recently. The last time we had a string of warmer-than-normal weather like the past week was April. To find the last time we had six days without precipitation, we have to look back into March.

For the short term, most of the state north of I-96 should be dry today, July 3. Tomorrow, July 4, we will have an upper air disturbance bringing a threat of thundershowers in the afternoon into Friday, July 5. Most of the state will remain dry over the weekend. A high pressure system will bring cooler, more seasonable weather Sunday through early next week, with highs in the 70s and low 80s.

Our summer-like weather this past week is due to a ridge in the Jetstream. However, the Jetstream guidance is forecasting a return to troughing by mid-July, which will bring cooler temperatures. Temperatures are predicted to be near-normal with above-normal precipitation for July 8-12. For July 10-18, the forecast predicts cooler-than-normal weather with normal to above-normal precipitation.

Crops

Asparagus plantings that are ferning out should be protected with chlorothalonil; chlorothalonil is preferred over mancozeb for purple spot given our weather conditions. Michigan State University Extension TomCast sensors will be placed out over the next week.

For carrots, an aster leafhopper sample taken June 26 from Oceana County had 0% infectivity.

Celery harvest has started on farms with frost-protected, early plantings. An aster leafhopper sample taken June 26 from Allegan County had 0% infectivity.

For cole crops, diamondback moth caterpillars have continued to be present in an Ottawa and Kent County location MSU interns are scouting, with some imported cabbage worm as well. Note that diamondback moth is a "light feeder" compared to imported cabbage worm and loopers. Plants can take minor feeding on leaves before head or floret formation; the bigger concern is often quality (not quantity) issues. Also note that in other crops, onion thrips have been present and they may move into other crops from wheat as it begins to dry down.

Swede midge has been causing damage this spring in eastern Michigan, see last week’s report from eastern Michigan. The east Michigan location also had extremely high sticky card captures of this pest (more than 500 on a trap on a single date); here in west central Michigan, MSU interns have only trapped a single swede midge the whole season. This pest seems to cause the most damage in organic operations. For organic growers, rotation is the single best control option as swede midge is a weak flyer. Organic growers in eastern Michigan (the spot in the state where this pest was first detected and caused major losses) have gone so far as to rotate brassicas between different farms in different years, which greatly reduced the issue.

Cucurbit powdery mildew was not present in one pumpkin field we scouted last week, but it's that time of year. I did find powdery mildew at a garden site in Muskegon County and it has been detected in Ohio. I highly recommend checking out page 130 of the Midwest Vegetable Production Guide; it has research-based ratings of products for powdery mildew control. Recent work in Ohio has assayed powdery mildew for sensitivity to fungicides. In this assay, the Ohio powdery strains were susceptible to Rally, Aprovia Top, Procure, Inspire Super and Quintec. Sensitivity was reduced for some other fungicides. Read more in “Cucurbit Powdery Mildew – Start Scouting Now” from Ohio State University. There is also a nice article summarizing efficacy of OMRI-approved products for powdery mildew, also by Ohio State University.

Cucurbit downy mildew spores have not been detected in MSU traps as of June 24. Melon and pickle growers should keep an eye on spore counts. This yea, counts distinguish between hop downy mildew (not a threat) and cucurbit downy mildew.

Our wet weather combined with high temperatures mean we are likely going to see an uptick in root and crown rot in cucurbits due to Phytophthora capsici in fields with a history of this disease. Disc under affected areas plus a firewall of healthy plants to slow the spread of the problem. For fields that are drip irrigated, it is possible to protect against root rot. Orondis Gold, Presidio, Ridomil Gold and Zampro are labeled for drip application. Unfortunately, foliar sprays are not effective at preventing root infections. Crops at highest risk of root and crown rot include yellow summer and acorn squash. Butternut, spaghetti, zucchini and some processing varieties are less susceptible, but can still have issues.

Some early-planted squash on plastic has set fruit—these should be protected if grown on farms with a history of Phytophthora. In research trials, the two actives that have been most effective at reducing fruit rots include mefenoxam and oxathiapiprolin. Mandipropamid (Revus) and fluopicolide (Presidio) could be alternated with these as "B-team" products. Formulations include:

  • Mefenoxam: Ridomil Gold Bravo and Copper (all cucurbits) and Ridomil Gold MZ (summer squash, melon, cukes) describe foliar applications on their label. Ultra Flourish (a generic) and Ridomil Gold describe soil directed applications. All have a five-day preharvest interval (PHI). In areas of the state with a long history of Ridomil, use resistance can be present.
  • Oxathiapiprolin: Formulations include Orondis Opti (pre-mix with Bravo) and Orondis Ultra (pre-mix with Revus).

Onions need to be protected with fungicides given the weather. MSU plant pathologist Mary Hausbeck suggested tailoring programs for Stemphylium, which is a persistent problem, while keeping an eye out for downy mildew, for which we have also had favorable weather. Fluopyram (e.g., Luna Tranquility) is our most effective Stemphylium product, and Hausbeck suggests it is worthwhile to apply now. For protectants, consider using chlorothalonil in place of mancozeb. In Stemphylium trials, mancozeb was not at all effective, but chlorothalonil did help prevent Stemphylium. Remember, chlorothalonil is not a good tank-mix partner with Movento.

For tomatoes, peppers and potatoes, early blight symptoms have developed in southwest Michigan. Consider applying chlorothalonil as a protectant spray on a 7-10-day schedule. Bacterial leaf spot has also been detected in peppers. Late blight has not yet been detected in the Great Lakes region.

Sweet corn scouts have detected higher than normal European corn borer activity in the earliest plantings and corn earworm has already been captured in pheromone traps. Overall, delayed field corn plantings mean that early sweet corn may be larger, and much more attractive, to moths looking for a good host. This might mean we will see higher than normal early-season caterpillar activity this year, including corn borer and true armyworm. Be on the lookout for damage.

For European corn borer, once tassels emerge, the caterpillars will leave the whorl and can bore into the developing ear. Prompt treatment at row tassel with a pyrethroid can be effective when caterpillars are active at row tassel, as it can contact and kill larvae as they move down the plant.


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