West central Michigan vegetable regional report – August 3, 2016
Home gardeners can help our pickle growers by keeping an eye out for cucurbit downy mildew.
Asparagus growers should be aware that disease severity value (DSV) accumulation ramped up over the week ending last Wednesday, July 27. Foliar disease may continue to develop in asparagus despite dry weather, so fungicide covers will be important (see graph). Chlorothalonil or mancozeb can be applied every time 15 DSVs accumulate to protect against purple spot.
DSVs can be calculated from in-field weather sensors to time fungicide applications for asparagus purple spot, as seen in the graph. DSV accumulations ramp up when foliage remains wet and air temperatures are warm. DSV accumulations accelerated starting July 16 at the Oceana Tomcast sensors shown in the graph due to high dew points and overnight temperatures. Foliar disease will likely start to develop now too.
Carrot growers should be aware that DSV accumulation ramped up over the week ending last Wednesday, July 27. Foliar disease may continue to develop despite dry weather, so fungicide covers will be important. Chlorothalonil alternated with a strobilurin such as azoxystrobin can provide protection against foliar disease. Chlorothalonil currently remains the backbone of foliar disease control in carrots.
For onions, Stemphyllium is causing problems in some areas. Luna Tranquility is now labelled for onions and provides good control of this disease.
Risk of late blight development for tomatoes and potatoes was high for four out of the five days, Sunday, July 31 through today, based on the weather forecast at the Holland, Grand Rapids, Kalamazoo and Muskegon Enviro-weather stations.
For sweet corn, there is a risk of scattered corn earworm flights into sweet corn for the middle of this week. Western bean cutworm activity has heightened this year, and moth activity has recently peaked. Consider scouting your pre-tassel corn for egg masses. Growers who normally do not apply insecticides this time of year can be caught off guard by this damaging pest. A long-lasting pyrethroid (e.g., bifenthrin) applied as soon as silks start to emerge can help protect against these western bean cutworms. If you want to target it specifically (e.g., outside of a normal earworm spray), you can estimate when caterpillars will hatch based on egg color and make an application around that time. “Managing western bean cutworm in field corn” by MSU entomologist Christina DiFonzo is an excellent fact sheet.
These caterpillars initially feed on the tassel, but move down the stalk once the tassel pokes out. During their journey to the ear they can be killed if they contact a pyrethroid with good residual activity. Published thresholds for western bean cutworm vary: Purdue University Extension recommends treating when 4 percent of scouted plants have egg masses or larvae, while Cornell University recommends a lower threshold of 1 percent. To scout, examine the top surface of the top four leaves on 20 plants at five locations. This is where females tend to lay eggs.