West central Michigan vegetables regional report – Aug. 14, 2013

Continue to scout for diseases and insects despite the cool weather.

In asparagus, disease severity values (DSVs), calculated from weather sensors placed in growers’ fields, have continued to accumulate over the past week despite cooler weather. Growers should continue to apply protectant fungicides (e.g., Bravo, a.i. chlorothalonil) at 15DSV intervals, which have been shown to be effective for control of purple spot. Inclusion of Folicur (a.i. Tebuconazole) every three weeks will help prevent rust problems as well.

In field visits this week, asparagus rust teliospores and uredospores were present. In the past, these teliospore lesions, which are black and much the same shape as uredospore lesions, have been mistaken for dead uredospore lesions “burned down” by fungicide application. However, these spores are viable and represent the stage that enables this disease to overwinter on dead asparagus fern. A picture of teliospore lesions is below. Uredospore lesions could become more abundant again once weather warms.

Asparagus rust lesions
Asparagus rust teliospores lesions are elongate and black, and
have appeared in some locations with recent, cool weather.
Photo credit: Ben Werling, MSU Extension

Early symptoms of aster yellows were visible on foliage of scattered plants in a Mason County carrot field yesterday (Aug. 13). Foliage of these plants had a “bronzed,” burgundy color. Aster yellows is a disease vectored by aster leafhoppers, and is caused by a microorganism. Other symptoms include distorted foliage and the development of hairy side roots on carrots.

Aster yellows
Early symptoms of aster yellows include bronzing of carrot
foliage, which gives them a burgundy color.
Photo credit: Ben Werling, MSU Extension

Aphid populations in celery have developed in some locations and require control. Movento (a.i. spirotetramat, pre-harvest interval of three days) has proven to be very effective for aphid control, but only if used in combination with the labeled rate of a penetrating surfactant (e.g., Silwet L-77). Aster leafhopper numbers have continued to be low to date.

Cucurbits (cucumbers, pumpkins, summer squash, winter squash). Cool, nighttime temperatures may have slowed the development of diseases such as Phytophthora capsici and powdery mildew, although disease development could pick up as the weather warms. Weather this Monday and Tuesday (Aug. 12-13) were favorable for continued spread of cucurbit downy mildew. Powdery mildew is also present on foliage of some cucurbit plantings. Powdery mildew can cause damage by defoliating plants, but can also cause problems by infecting the “handle” of jack-o-lantern pumpkins. Use of tolerant varieties slows, but does not eliminate, disease spread, which is faster in humid, warm conditions.

Sprays in pumpkins can be timed using scouting and thresholds. To scout, walk a field once a week and examine both surfaces of five older leaves at nine locations spaced throughout the planting. The threshold is one leaf with powdery mildew per 45 leaves. For control of powdery mildew, inclusion of a systemic fungicide in your rotation is important as powdery mildew can form colonies on the underside of leaves where protectants may not reach. Effective fungicides include Pristine 38WG (a.i. boscalid), Quintec 2.08SC (a.i. quinoxyfen), Torino 0.85SC (a.i. cyflufenamid), Topsin M 70WP (a.i. thiophanate-methyl), Fontelis 1.67SC (a.i. penthiopyrad) and Flint 50WG (a.i. trifloxystrobin). A rotation of compounds with different FRAC codes can be used to provide control. View a factsheet on powdery mildew.

Powdery mildew on squash
A summer squash leaf with a powdery mildew colony indicated.
Photo credit: Ben Werling, MSU Extension

Symptoms of bacterial infection have continued to be present in onions. Michigan State University researchers have detected one primary bacterial species associated with water-soaked lesions of outer leaves. The species in question is a new report for Michigan; this species is of concern for causing storage rot, although much remains to be learned about when and how it can cause these problems. Thrips control has become less of a concern with cool temperatures and the start of onion harvest. Onion harvest was continuing as of Aug. 13 in the Hudsonville, Mich., area.

A regular program of fungicide applications, whether conventional or organic, will help you avoid losing potato and tomato plantings to late blight or other diseases. To date, late blight has been detected in one Allegan County potato field, three potato fields in St. Joseph County, and a Kalamazoo County tomato planting. In farm visits yesterday (Aug. 13), growers reported that tomato ripening has slowed due to cool weather.

If your tomatoes will be grown into September, consider including a fungicide in your rotation that specifically targets oomycetes, which is the group of organisms that includes the late blight pathogen. A variety of ooymycete materials are now available. Two examples of such products are Curzate (a.i. cymoxanil, PHI: 3) and Ranman (a.i. cyazofamid, PHI: 0). Some products containing active ingredients specific to oomycetes will indicate this on the pesticide label under “general information” or “product information.” Specifically, these labels may state the product is for use against late blight, early blight and downy mildews, which are oomycetes, but will not list many other (non-oomycete) pathogens. Oomycete materials and other fungicides are listed in recommendations based on Michigan State University research.

In sweet corn, over the period Aug. 1-8, I captured 15 corn earworm moths in one trap in Hart Township, Oceana County; zero European corn borer adults in three traps in Hart Township; and three western bean cutworm adults in one trap in Hart Township and one in a second trap in Weare Township, Oceana County.

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