West central Michigan vegetable regional report – August 17, 2016

Cucurbit downy mildew is in west central Michigan, and corn earworm numbers may be increasing.

Photo 1. Rust uredospore lesions on asparagus fern. This is the rapidly reproducing phase of this pathogen. If you see it, prompt action will help prevent epidemics. Photo: Ben Werling, MSU Extension.

Photo 1. Rust uredospore lesions on asparagus fern. This is the rapidly reproducing phase of this pathogen. If you see it, prompt action will help prevent epidemics. Photo: Ben Werling, MSU Extension.

Asparagus rust uredospore lesions were present in research plots and one grower’s field I visited last week (Photo 1). Michigan State University Extension suggests looking for rust on branches on the interior of the fern; I walked by a rust infestation that was right under my nose until a colleague pointed it out and we took the time to look. Rust lesions are “blistered” and raised above the plant, unlike purple spot lesions. Tebuconazole (4-6 ounces per acre for 3.6 formulations) remains the best labelled material for rust. It has some “back action,” but this is limited to very new infections, visible infections cannot be killed. It can penetrate plant tissue and has a longer residual than protectants like chlorothalonil, but its movement in the plant is very limited, so good coverage is still key.

Asparagus miner mines are visible at the base of plants, and adult flies are active (Photo 2). Right now, this pest is in its second generation; offspring of the flies you see now will overwinter. MSU entomologist Zsofia Szendrei is researching ways to control this pest, which is of concern as a potential carrier of Fusarium to new plantings.

Rust uredospore lesions on asparagus fern.

Photo 2. These brown areas at the base of asparagus shoots are caused by maggots of a tiny fly, the asparagus miner. Photo: Ben Werling, MSU Extension.

In carrots, an aster leafhopper sample from Oceana County taken Aug. 9 had 0 percent of leafhoppers infected with aster yellows.

There were isolated celery plants with colonies of aphids in fields again this week. Hoverflies were extremely abundant in Hudsonville, Michigan, this week (Photo 3). These bee mimics lay eggs on plants, which hatch into maggots that feed on aphids. One of our industry’s celery scouts has seen multiple hover fly larvae attacking aphid colonies on celery. These beneficials can help keep aphids in check. Movento, used with a penetrating surfactant, is an effective chemical control. Scattered plants also had aster yellows and celery anthracnose symptoms. So far this year, scouts have noticed relatively little foliar disease. Aster leafhopper numbers remained very low through early this week.

Hoverfly adults.

Photo 3. Hoverfly adults were easy to find this week. Their larvae are predators of aphids. Photo: David Cappaert, Bugwood.org.

Cucurbit virus symptoms were present in one Oceana and Ottawa County vine crop planting I visited last week. Symptoms of virus infection can include distortion and mottling of leaves (Photo 4), distortion of fruit and color breaking on yellow fruited squash (e.g., green speckling). Most viruses are aphid-vectored, but unfortunately insecticides are not effective for preventing viruses. Aphids transmit the virus in seconds before insecticides can kill them, and winged aphids can recolonize fields, so a local population is not needed. There are virus-resistant varieties of some vine crops available.

A zucchini leaf showing symptoms of virus infection.

Photo 4. A zucchini leaf showing symptoms of virus infection. Viruses can cause discoloration and distortion of fruit. Green discoloration is problematic on yellow fruited squashes. Photo: Ben Werling, MSU Extension.

Cucurbit downy mildew has been confirmed from 11 Michigan counties, most recently in Muskegon County. Spore traps in Muskegon and Allegan counties have been consistently catching downy mildew spores since the end of July, with some very high counts, suggesting the pathogen is in the area and spreading. Preventive action is critical for pickle, cucumber and melon growers.

Early onion varieties were going down naturally or being rolled in Ottawa County this week. Note, if you are looking to use Lannate to finish the season, research from Cornell University has shown that mixing it with Warrior provides better control than Lannate alone; Warrior by itself is not effective.

Risk of potato and tomato late blight development is high for each of the next five days across most Lower Michigan MSU Enviro-weather stations (Aug. 17-21).

Early this week, sweet corn traps in Ottawa and Oceana counties had moderately high catches for the first time this year (Figure 1). In areas with higher pressure, tighter spray intervals could help protect your crop. For example, as captures approach 50-100 per night in wire traps, a two-to-three day spray schedule can help minimize earworm problems. If catches are lower than this, a five-day schedule can suffice, or whatever schedule you find helps your corn meet your buyers’ standards. This shows why having your own trap can be helpful.

Graph of corn earworm captures

Figure 1. Corn earworm catches increased in an Oceana and Ottawa County trap this week.

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