Michigan vegetable crop report – Aug. 4, 2021

Pepper and onion harvest is ramping up. Pumpkins are starting to color on some farms.

Asparagus with stripped and white ferns
Asparagus beetle larvae or slugs strip the fern of leaves and cuticle, making it look white.

Weather

Watch the weather update here.

The past week saw temperatures 5 degrees Fahrenheit cooler than normal. Precipitation was variable, with higher totals to the north. Degree day totals are still a few days ahead of normal. Soil moisture overall is now close to normal, with drier conditions in the southern lower and the tip of the Thumb.

The weather forecast calls for:

  • Fair and warm most areas Wednesday. A few isolated showers possible this afternoon and early evening.
  • Mostly dry and warmer Thursday. Scattered showers and thunderstorms developing northwest late afternoon spreading southeast on Friday. Precipitation over the next week is predicted to total 0.5-1 inch north, decreasing to less than 0.25 inches to the south, with most falling Friday or late Sunday into Monday.
  • Warmer and more humid this weekend with scattered showers and thunderstorms Saturday and Sunday continuing into early next week.
  • Maximum temperatures gradually increasing into the mid-upper 80s this weekend. Minimum temperatures mostly in the upper 50s/low 60s Thursday, warming to the mid-upper 60s this weekend.
  • Medium range guidance strongly suggests a return of warmer than normal temperatures next week into mid-August with normal to above normal precipitation totals.

General crop management

We remain within a good window to plant a variety of late summer/early fall cover crops and get good growth before frosts/winter. See comments on cover crop options for late July/August in a previous vegetable crop report. As we get later in the season, the number of species options and performance of cover crops will narrow. By late September, cereal rye may be your only good option.

Crop updates

Asparagus

Growers on some farms were making their second fungicide application last week. Between 5-8 disease severity values (DSVs) accumulated between July 27 and Aug. 2 at nine MSU and industry sensors in Oceana County. A spray would have been indicated Monday at these sites if nothing had been applied since July 18-23. This coming week, we will see higher dewpoints and temperatures, so keep an eye on TomCast data.

Adult asparagus beetles, eggs and larvae were present in some locations; the larvae are the main defoliators, and strip the fern of leaves and cuticle, so it looks white.

Japanese beetle was also present in some fields. Adult beetles do the damage and feed on leaves and stems. Fern has a “gray” cast after Japanese beetle damage. Both beetle pests are often worst along field edges.

Grayish asparagus fern
Japanese beetles leave fern with a grayish cast as they feed on leaves and stems.

Carrots and celery

For celery, scouts have found leaftier larvae in multiple fields, but so far caterpillars are staying in the top of the plant. When it gets hot, caterpillars may move down to the hearts of plants. Aphids are present here-and-there in our growing region. Captures of variegated cutworm moths went up this week in Ottawa County. This pest typically emerges in south to north “waves” as it develops. Aster yellows is present here and there this year in fields; it has been a number of years since it has been present even at lower levels.

Aster yellows symptoms were also observable in west Michigan Carrots across multiple counties this week. Symptoms include bronzed foliage (which can happen for other reasons) but also hairier than normal roots and distorted new growth. It is normal to see symptoms this time of year, but the “here and there” is more extensive in some fields this year compared to normal.

aster yellows
Aster yellows symptoms are becoming more visible now, with bronzed foliage, distorted new growth and hairier than normal roots.

Disease lesions were present on the foliage and petioles of carrots at one location this week. The upcoming weather could be more favorable for disease development as dewpoints and temperatures rise again this weekend. Watch TomCast data; if you see a string of values of 3 or higher, it is a sign that it could be worthwhile using a highly effective product-such as Merivon or a Luna product-in your next application, but don’t save these for last as they are most effective when applied before widespread symptoms develop.

Leaf lesions on carrot
Disease lesions from either Cercospora or Alternaria were present on leaves and petioles in this carrot field.

Cole crops

Diamondback moth continued to be present-no surprise-in cole crops this week. Downy mildew and Alternaria is present in some fields with high weed pressure and with tall windbreaks like hedgerows and field corn. This poor air flow can maintain high humidity levels in the crop for longer periods of time.

Cucurbits and pickles

Downy mildew continues to infect cucumber plantings. Powdery mildew and gummy stem blight are starting to show up in hard squashes. Powdery mildew is a yield reducer because it defoliates crops, reducing fruit size and increasing sunburn on fruit. It also weakens handles for pumpkins but does not affect fruit directly. Gummy stem blight defoliates and can cause a wilt, but also infects fruit directly and causes a hard brown and spreading target-like spots on fruit.

There are several effective fungicides for conventional management of powdery mildew, including Gatten, Prolivo, Quintec, Rally, Torino and Vivando. For conventional fungicide management of gummy stem blight, a regular schedule of cheap protectants like Bravo or Mancozeb works well for prevention. These are often included in tank mixes for management of other diseases.

For organic fungicide management of powdery mildew, Cornell University’s Meg McGrath mined the Plant Disease Management Reports database and summarized biochemical pesticides and microbial-based pesticides to make very handy metatables across multiple crops. Among the products that were tested for powdery mildew, Actinovate, coppers, Regalia, Serenade and Sonata had more than 10 trials performed with more than half of those trials reporting efficacy. Other products were effective as well but have had fewer than 10 studies conducted in the database.

For organic fungicide management of gummy stem blight/black rot, fewer studies have been performed. From the small dataset available, it appears that AVIV and Timorex ACT provided some control of gummy stem blight in watermelon. AVIV is a strain of Bacillus subtilis. There are several other products containing different strains of Bacillus subtilis, including Cease, Serenade, Sonata and Taegro. Studies have not been conducted to show whether they are more or less effective against gummy stem blight than AVIV, or other products.

Wilts are difficult to diagnose but are often a process of elimination. Several wilt-causing issues can be found right now.

Phytophthora continues to be present in west Michigan, causing plant death in zucchini. Give our recent Vegetable Beet podcast a listen for tips from Michigan State University’s Mary Hausbeck, and check out a fact sheet for summer squash and zucchini. Hausbeck shared that intervals need to be tighter than 14-days to maximize efficacy. Also, reduce the burden placed on fungicides by promptly disking under areas where plants are dying plus a firewall of asymptomatic plants. Growers on the east side with a history of Phytophthora have successfully managed a healthy crop this year despite several periods of heavy rain with the fungicides recommended in the linked resource above.

Fusarium wilt is showing up in watermelon fields with a history of the soil disease. Effective fungicides are limited for managing this pathogen, and it is best managed with a combination of lengthy crop rotations and resistant varieties. For more information, read this article, “Managing diseases caused by Fusarium: The case of watermelon wilt”. Proline (seven-day PHI) is one effective fungicide when banded at the soil and crown of young plants or through drip irrigation of plants at any stage.

Bacterial wilt in vine crops is one of the more confusing wilts and is caused by cucumber beetles carrying a bacteria in their gut. A small percentage of beetles carry this bacteria in the beginning of the season. As they “gang up” on weak plants, the disease gets moved to the plant, and then other beetles can pick it up from there. From a few plants the newly inoculated beetles spread it to other plants in a patchy pattern.

Bacterial wilt does not discolor the vascular tissue, does not affect the color or number of roots, and does not rot the fruit on the ground side like some other soil pathogens. Fungicides are ineffective, and the beetle must be managed with insecticides to prevent the disease. Organic insecticides include azadirachtin and spinosad. The efficacy of pyrethrins and spinosad can be increased by mixing it with CideTrak D. Pyrethrins can also be improved with piperonyl-butoxide, and is available as a premix called Evergreen, but it is not approved for organic use. There are several other conventional insecticides that work well, that can be found in the Midwest Vegetable Guide.

Onions and garlic

The onion harvest process has begun. Crops are being uprooted and flipped for field curing, with some being marketed fresh. Early Highlander and set onions were also being harvested at larger scales.

Stemphylium remains the disease driver in onions, but there are others to keep an eye out for, especially on storage onions. Downy mildew first infects older leaves, occurring as pale, elongated patches that may have a grayish-violet fuzzy growth appear early in the morning during moist periods. Infected leaves become pale green then yellow and can fold over and collapse. Lesions can be violet purple in color; affected leaves become pale green then yellow and can fold over and collapse. Symptoms of the disease are best recognized when dew is present in the morning.

Anthracnose has been present in some locations and produces distinct symptoms, as it causes lesions on the necks of onions, not just the leaves. Strobilurins have efficacy, but do not work for Stemphylium. If you are targeting anthracnose with strobilurins, don’t leave out an effective Stemphylium product. Since Stemphylium became a big player, newer fungicides have arrived that have not been tested against anthracnose.

Anthracnose on onions
Anthracnose produces lesions on both onion leaves and necks. Stemphylium remains the “big dog” in the house, but this pathogen can also be destructive.

Onion thrips populations continued to be high at two locations Michigan State University Extension visited, at two to three per leaf. Scouting will tell you if you have high or low populations. Remember that as bulbing progresses, the impact of thrips populations on final yield will lessen. Lannate spiked with Warrior could be used after Radiant to suppress populations. Alternatively, Exirel could be used if you have not already used Minecto Pro.

Resistance management for thrips is a bit confusing! In an ideal world, use the same mode of action in back-to-back applications to reduce risk of resistance. For example, follow Radiant with a second Radiant application the next week, if another is needed. This way you are only targeting one generation of thrips with that mode of action (one generation takes about two weeks). Applying Radiant now and then again three to four weeks later could expose two generations of thrips to selection. This is a long-term game, so deviating for one year is probably OK!

Potatoes

Late blight was confirmed in Portage, Wisconsin as well as Georgia last week. It has not been reported in Michigan to date.

Harvest of new potatoes is on-going. Be careful with transferring loads and washing because new potato skins are thin and easily damaged, which decreases display appeal.

The Late Blight Forecast is medium risk for most of Michigan, based on DSVs. Areas of high risk may consider using full fungicide rates and reducing the application interval to five days depending on individual field conditions.

Root crops

Cercospora leaf spot is present in sugarbeets, with levels increasing. Rhizomania, a disease caused by a tag-teaming fungus and virus, has been detected in Michigan table beets over the past few years. This disease causes excessive development of fine roots, sometimes referred to as hairy root, as well as excessive top growth and zoning.

Sweet corn

Only a few corn earworm moths were captured in Lapeer, Saginaw, Ottawa and Oceana County traps over the past week, but check your traps Friday morning. Insectforecast.com is predicting a low risk of migration into the southern half of Michigan on Aug. 5.

Lesions caused by rust are becoming more noticeable. This disease causes raised reddish powdery lesions. There may also be chlorotic rings around the pustules. Grower experience has suggested that rust should be managed if sweet corn has not yet tasseled. Tebuconazole is an effective and inexpensive fungicide for rust in sweet corn and is widely labeled on other vegetables.

Tomatoes, peppers and eggplants

Field pickings are coming fast now, with eggplants and green bell peppers entering markets as well. Small hot peppers are ripening to full color.

The TOM-CAST model from the Network for Environment and Weather Application (NEWA) for early blight, Septoria leaf spot and anthracnose has accumulated between 70 to 84 DSVs since May 15 in our region. That is 8-11 more DSVs since last week. The effective DSV threshold between fungicide applications averages 18, depending on the products used. If accumulations exceed two weeks without crossing the DSV threshold, and rain is in the forecast, it is worth applying a fungicide. For growers fighting bacterial diseases, the TOM-CAST model should not dictate your spray program.

Verticillium has showed up in eggplant at some locations. This vascular disease shows up in tomatoes and eggplant as V-shaped, dying areas, on the margins of leaves as it clogs the plant’s plumbing. Maintaining a good rotation between Solanaceous crops may help prevent disease buildup. Cornell University maintains a list of disease resistant varieties for tomatoes and eggplant.

Verticillium on eggplant
“V” is for Verticillium, which causes V-shaped dead areas to form on eggplant and tomato leaves as it clogs the plant’s plumbing. Crop rotation is an important preventive measure, there are resistant tomato varieties, but less for eggplant.

Events

Food safety

Need your water tested for the Food Safety Modernization Act (FSMA)? Check out the Michigan Ag Water Lab Map.

On-Farm Readiness Reviews are designed to help fresh produce growers feel prepared and ready for a full implementation of the Food Safety Modernization Act (FSMA). They allow growers to walk through their operation with a small team of food-safety experts and discuss how to meet the new FSMA Produce Safety Rule requirements. Go here to sign up for one.

Farm Supervisor Produce Safety Courses are being hosted through the MSU Online Learning Portal - D2L. They are self-paced and free. It will be available for farm supervisors starting July 19. Register here.

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