West central Michigan vegetable update – July 10, 2019

Disease control and irrigation are the focus as we have transitioned from cool and wet to hot weather.

Purple spot in a young, ferned out asparagus
Purple spot in a young, ferned out asparagus field. Fields should be covered with chlorothalonil as soon as plants are up and have branched. Photo by Ben Werling, MSU Extension


For the past week, we have seen a continuation of warmer-than-normal weather with unusually warm overnight temperatures. Most parts of the state saw 0.5 inch or less of rain except for some southern counties.

For the short term, the best chance of rain for the next week should be this evening, July 10. Chances will be highest in the northern lower and decrease to the southeast. Thursday, July 11, will bring cooler air, but high temperatures and humidity will return Saturday/Sunday that will ramp up as we head towards the middle of next week. Rainfall after Wednesday evening will likely be hit and miss, with 0.25 inch totals predicted for the next week.

In the medium range, outlooks call for a broad ridge in the Jetstream for July 15-23 with warmer than normal mean temperatures.

Irrigation will be important right now. One impact of our cool, wet spring could be shallow, under-developed root systems. This means plants may not have a deep root system to draw from during our current hot weather.


Asparagus plantings should be protected with chlorothalonil for purple spot. Given our conducive disease weather, it makes sense to get asparagus covered even if it is not fully ferned out.

Note that purple spot problems during harvest are best prevented by a rigorous spray program during the preceding fern season. Good control in the fern stage this year will limit inoculum that could overwinter to infect spears next spring.

For carrots, an aster leafhopper sample taken from Oceana County on July 5 had 0% infectivity.

Celery harvest has been ongoing. An aster leafhopper sample taken from Allegan County on July 2 had 0% infectivity.

Cucurbit downy mildew spores were captured for the first time in Michigan State University spore traps on June 28 in Allegan County. Melons and cucumbers could be protected with Bravo, Zing! or Gavel. While spores have bene trapped, the disease has not been detected in Michigan production fields. Read more in “Vegetable disease alert: Act now to protect cucumbers and onions” from MSU Extension.

Phytophthora capsici was reportedly causing plant death in cucurbit fields. When plant death occurs, consider disking under affected areas plus a firewall of healthy plants to limit spread. Sprays made for protection of petioles, leaves and fruit should occur on a 10-14-day schedule to be effective. If a very heavy rain is forecast, consider an application on a shortened interval ahead of the storm. Remember, infection occurs when rain splashes infested soil onto plants and fruit.

In research trials, the two actives that were most effective at reducing foliage and fruit infections were mefenoxam (in areas without resistance) 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 5-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).

Squash bug was present in one pumpkin field I visited this week, but eggs had not been laid yet and populations were low. The threshold when plants are blooming is one egg mass per plant. Sprays are most effective at killing newly hatched nymphs, so keep an eye on egg masses and spray when they are just hatched. Multiple pyrethroids including esfenvalerate (Asana), permethrin (Perm-Up) or bifenthrin (Brigade) are labeled.

Squash vine borer was laying eggs in one northwest Michigan location this week. A pyrethroid application directed to the crown of the plant can kill larvae before they hatch and bore into the stem. This pest is typically most problematic in small plantings, though I have seen damage in larger, organic processing squash plantings. Fields where there were cucurbits last year will be most at risk.

Striped cucumber beetles continue to be present in pumpkin fields I’m scouting, but plants are large and damage should be minimal unless populations exceed five beetles per plant for squash and pumpkin. The threshold for melons and cucumbers is lower at one beetle per plant.

Onions need to be protected with fungicides given the weather. Products with fluopyram (Luna Experience or Tranquility) have consistently been most effective. Consider leading off your spray program with a high rate of a Luna product plus Bravo. The following week you could use Tilt plus Bravo. You could then rotate back to a Luna material the following week. Note, Luna Experience and Tilt both contain a group 3 fungicide, which limits the number of sequential applications to two. Luna Tranquality does not.

Onion thrips populations were well below threshold after two Movento applications at a Kent and Newaygo County field I’ve been checking. If thrips populations are below one per leaf following Movento, skip this week’s spray and reassess the next week. Either Agri-Mek (active ingredient abamectin) or Minecto Pro could be sprayed during the two-week slot following Movento if thrips exceed one per leaf.

Sweet corn plantings that got in early were tasseling and silking in most areas I visited this week.

MSU earworm traps were placed out at a Kent and Oceana County location, and captured one moth per night over the past few days. At this trap catch, it is worth protecting silking corn; a five-day spray interval should be sufficient. Note, this earworm season may be different as field corn is way behind. This could mean that sweet corn will not benefit from the pull that silking field corn has on moths for quite some time. Overall, it may be a year to make sure even early-planted corn is well-protected.

European corn borer larvae were feeding in whorls at a Kent County location I visited this week. A pyrethroid application at row tassel can help protect corn against this pest before earworm sprays kick in. Look for feeding damage in the whorl, as not all locations will have this pest. Fields where there was non-Bt corn last year will be most at risk.

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