Ask a Master Gardener: Discoloration of leaves, Japanese beetles and picking the right herbicide

Q. I have noticed a gray-ish discoloration spreading across some squash leaves. It is worse on the older leaves but now is on the newer ones too. What is it, and should I do anything about it?

A. This is likely powdery mildew, which is known to afflict plants in the melon/cucumber/squash family (cucurbits). It often is worse as we go into the fall, but with the rainy and sometimes cool weather this year, it is has been popping up even mid-summer. If uncontrolled, older leaves will die off quicker and the plant's growth will be weaker, but they often survive and continue growing. Of course, with a weaker plant you'll get less fruit.

Synthetic broad-spectrum fungicides easily control it. Spectracide Immunox and Daconil Fungicide both are very effective and should give control when sprayed at about 10- to 14-day intervals, depending upon ensuing rain and temperatures. A natural product that gives about five to seven days of control is baking soda. Cornell University worked with it specifically on cucurbits and found that a combination of horticultural oil and baking soda together worked best. For this, per gallon of water, add 1.25 tablespoons of horticultural oil and 1.5 tablespoons of baking soda. Apply as well as possible to top and bottom of leaves until runoff. Do not apply in the full sun; waiting until dusk is the best. Also, the plants should not be stressed from dry soil.

Q. This wild grape plant volunteers up onto my deck. I used to like it, but now these Japanese beetles are always swarming around. If I get rid of it, will these annoying bugs go away?

A. Probably. Grapes are very attractive to Japanese beetles, and some wild grapes highly so. However, there is a number of other popular garden plants they like, such as roses, raspberries, flowering crabapples, American linden and hollyhocks. So make review of other plants around your deck for feeding damage.

If you can tolerate these beetles for about six weeks each year, then you could keep the plant. However, they tend to peak around July 4. For this year, by the end of July you shouldn't be seeing them much anymore, if at all. You are not alone in finding them annoying.

Q. I'm getting too many weeds around my vegetables, flowers and fruit plants. I haven't ever used herbicides, but am thinking I might need to or they'll take over, and I'm not getting any younger. What are a few of the easier and more effective options?

A. By far the most versatile and effective is the active ingredient glyphosate (made popular by Roundup). For use around both fruit and vegetable plants, find Roundup Weed and Grass Killer Super Concentrate; other formulations may not be labeled. The mode of action is very simple to explain: you spray it; you kill it. So one has to learn the careful art of spot-treating plants without having any spray drift onto desirable plants. For this purpose, spray when there is no wind. It is also best if the plant you treat does not touch a desirable plant. Since the active ingredient is systemic, it can transmit from foliage rubbing for about two days after applying. Read the label carefully; there are several tips and applications easily overlooked. For instance, since seedlings are easily killed, avoid spot-spraying nearby desirable seedlings.

There is an herbicide labeled for killing grass with flowers and many veggies and fruit plants (again, read label carefully). The active ingredient sethoxydim is available as Hi-Yield Poast and Fertilome's Over the Top Grass Killer II, and is effective at killing grasses but not broadleaf plants. For edible plants, the time from applying the product to when a plant can be harvested may be quite long, so pay attention to this.

Left out for easy control is the difficult weed yellow nutsedge, also sometimes called watergrass because it likes wet areas. While there is a highly effective product readily available, called Sedgehammer, it is only labeled for use in turf, around established ornamental shrubs or trees, and in fallow areas. With all the rain we are having this summer, yellow nutsedge will likely grow aggressively.

The local Master Gardener hotline is staffed from 12:30-4:30 p.m. Tuesday and Friday afternoons; call 634-2824 then or anytime to leave a message. The Central Missouri Master Gardeners are a volunteer group of 191 members, 122 of whom are Cole County residents, who maintain 11 beautification sites in Cole County. Master Gardeners must complete a basic training program of at least 30 hours of horticultural training including landscaping, lawns, vegetables, flowers and fruits, as well as 30 hours of volunteer service.

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