Help! I have a Thompson seedless grapevine that's in its fifth year. Each of the past few years, it has produced bunches of grapes that never mature. The grapes grow to approximately 5 millimeters, stop growing, and eventually harden and die. I'm about to the pull the darned thing out, but I've decided to ask for your advice first.

– Don Pimentel, Sacramento

According to UC Master Gardeners Bill Pierce and Mary Griggs, you are describing the classic problem that occurs with all varieties of grapes that have not received sulfur treatment after the bunches form.

One possibility could be powdery mildew, which causes leaves to become scarred, and berries to become scarred and crack open as they grow.

Both dusting and wettable forms of sulfur are used. Dust the vines with sulfur, starting in the spring when weather is favorable. Re- apply every seven to 10 days through the spring and continuing until hot weather sets in.

If temperatures remain hot after late June, spraying is less necessary, but mildew can spread if cooler spells persist. If rain occurs, re- apply the sulfur as soon as possible after the foliage dries. Vines grown in partial shade or adjacent to lawns that are watered with overhead sprinklers will require additional dustings.

An additional problem may be from heat damage to berries. This usually occurs when berries are pea- sized. Berries exposed to sun are most susceptible, especially if maximum temperatures reach 100 degrees and above, and the weather preceding was mild (70 to 80 degrees). Vines under water stress and with bare reflective soils are more susceptible.

Water your vine deeply. For example, a fully trellised vine on a warm day in the Central Valley requires about 8 to 10 gallons of water per vine per day. Dig down in your soil to see if your watering schedule is adequate to wet the soil to a depth of 18 to 24 inches. Maintain an even level of soil moisture to avoid cracking.

For additional information, send a 44-cent self- addressed, stamped business-size envelope to UC Cooperative Extension, 4145 Branch Center Road, Sacramento, CA 95827 and request "The Home Vineyard" Leaflet 7036, and Garden Notes 105 and 106.

I have a patch of Aaron's beard (Hypericum calycium) that is not looking well. I water it three times a week. It gets morning and early- afternoon sun but is shaded during the hottest part of the day.

It looked great this spring with lots of bright yellow flowers, but in recent weeks the leaves on the lower parts of the stems are dying. The leaves get rust-brown spots on them and then turn completely rust-brown and die. The lower parts of the stems are yellow and brown, but the outer leaves are nice and green.

What's wrong?

– Ron Hills, Fair Oaks

According to UC Master Gardener Bill Pierce, your hypericum – commonly called Aaron's beard and St. Johns wort – is most likely infected with a rust disease, either Melampsora hypericorum or Uromyces triquetrus. These diseases are favored by wet foliage, so overhead watering should be avoided or done so that the foliage dries before nightfall.

Vigorous plants can withstand moderate infestations. Two approaches to ridding your plants of the problem would be to rake out the diseased leaves so that the spores on them don't re- infect other leaves, or mow the area and remove all the debris.

Mowing every few years will keep this ground cover free of old woody stems and reduce rust infestations.

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