West central Michigan vegetable regional report – June 26, 2013

Warm, humid weather may favor disease in vegetable crops. Aphids have also been active relatively early.

While asparagus harvest ceased on some fields last week, harvest on other fields has continued through the start of the week of June 24 in Oceana County. Major growers expected harvest to continue through today (June 26), while others may continue through the end of this week.

Asparagus rust uredospores were detected in an Oceana County location on June 17. The uredospore stage is the “repeating” stage of this asparagus pathogen, which can rapidly multiply when conditions are right and kill asparagus fern if not controlled. Applying protective fungicides to young fields where harvest has ceased – and fern has grown and expanded – will help keep rust disease in check. A rotation of Bravo and Folicur is the mainstay of a rust prevention program.

Accumulation of disease severity values (DSVs) at a TomCast sensor near Hart, Mich., did pick up over this past weekend with warmer, more humid weather. The TomCast weather model can be used to time application of fungicides to protect against purple spot, another disease of asparagus. Application of herbicides to fields where harvest has ceased will likely continue through next week.

In celery, significant aphid populations were observed in a southwest Michigan location. Michigan State University Extension specialist Zsofia Szendrei has shown that the insecticide Movento (a.i. spirotetramat) provides excellent aphid control, outperforming all other tested compounds, if it is applied with a non-ionic, penetrating surfactant such as Silwet. However, if not applied with a penetrating surfactant, it provides virtually no control. To see results of Szendrei’s trials, see “Michigan Vegetable Insecticide Evaluation Studies 2012.”

In cucurbits, aphids have been abundant in some crops already this season, which suggests we could see a higher incidence of viruses they transmit to cucurbits this year. Note that aphid-transmission of viruses is so quick that insecticides will not kill aphids before they probe and transmit viruses to plants. Importantly, not all aphids entering a field will carry the virus, so the presence of aphids does not guarantee a problem will develop. Conservation of beneficials through judicious use of insecticides will help reduce problems.

Continue to scout for striped cucumber beetles; thresholds can be used to determine if an insecticide application is necessary in fields where a seed treatment or at-plant drench of a neonicotinoid insecticide (e.g., Admire or Platinum) was not used; neonicotinoids should provide two to three weeks of control at the season’s start. Growing degree day models using Enviro-weather data from the Fremont, Hart, Hudsonville and Sparta Enviro-weather stations suggest that squash vine borer adults either are already or will become active within the next week. Experience suggests this pest is often worse in smaller plantings of cucurbits typical of growers selling at farm or farmer’s markets. Pheromone traps are available for monitoring of this pest and should be placed out now if they will be used.

Identification of squash vine borer can be difficult, as these traps also collect related moths. Timely control of this pest is critical; an insecticide needs to be on the plant at the time moths are laying eggs so that hatching larvae ingest it and die. Once larvae enter the plant, they are protected from foliar applications. Contact a Michigan State University Extension educator if you need help identifying moths in squash vine borer traps.

Finally, growers planting zucchini may wonder whether or not they still need protection against seed corn maggots that feed on germinating seedlings and can reduce stands. Once maximum daily soil temperatures are above 70 degrees Fahrenheit at a 4-inch depth for a sustained period, the risk of seed corn maggot damage greatly declines; soil temperatures can be checked on the Enviro-weather website.

In general, maximum soil temperatures have been above 70 F at 4 inches for about two weeks. However, if your soil has been amended with manure recently or a cover crop has been incorporated less than three weeks before planting, resulting high amounts of organic matter could still make your planting attractive to this pest. Finally, research has shown that thiomethoxam, the insecticide contained in the seed treatment FarMore FI-400, can provide protection against seed corn maggot populations when pressure is low to moderate.

In onions, iris yellow spot virus has been detected at low incidence in two onion fields in Michigan; this is relatively early for detection of this virus, which is transmitted by onion thrips. Symptoms this year have not been as distinct as in past years. Typically, lesions are diamond-shaped and straw-colored; however, symptoms have not been as distinct this year. If you think you have onions displaying symptoms of this disease, contact your local MSU Extension educator.

Iris yellow spot virus
Iris yellow spot virus typically produces diamond-shaped, straw-colored lesions. However, early symptoms this year have not always been this distinct. Photo credit: Howard F. Schwartz, Colorado State University, Bugwood.org

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