HIGH COUNTRY VEGETABLE NEWS
Roland E. Roberts
Extension Horticulturist

Vegetable Crop Field Observations/Activities
July 5, 1996

Corn earworm was seen feeding on ears of sweet corn cv. Illini Early Extra Sweet south of Lubbock on June 21. Infestation was close to 100% of ears. Damage extended down ear one to two inches from tip. Preventive control had not been attempted.

Squash bug adults were found on lower part of stems of yellow summer squash in Lubbock on June 22, and on June 24, adults and egg masses were found on pumpkin cv. Prizewinner south of Floydada. Where resistance to commonly used insecticides Sevin or Asana has occurred, pumpkin growers should know that endosulfan (Thiodan, Phaser) is labeled for squash bug control on pumpkin. Squash bugs are elusive. Aggressive scouting is required to get a realistic estimate of populations and activity.

Potato scab has infected tubers of the cv. Yukon Gold in two fields. The infection is slight to moderate, but of great concern. Surface and pitted scab was found in the tuber skin surface. Infected tubers are culls. Potato scab is transmitted in seed tubers, and can overwinter in soil. Growers who plan to succeed with Yukon Gold must have disease free seed and plant where no potato crop has been planted in at least 4 or 5 years.

Spider mites infesting tomato plants in Lubbock are causing mottled yellowing of older leaves. If allowed to go uncontrolled spider mites will kill tomato plants. Experiments using highly refined petroleum oil and vegetable oil for mite control are underway and results will be reported later. Check underside of oldest leaves on tomato plants where spider mite infestations get started. A magnifying glass is required to clearly see spider mites.

Yellowing of lower leaves on tomato plants is not the defining symptom for tomato early blight! Tomato early blight, caused by the fungus Alternaria solani, appears as dark brown spots with dark concentric rings on the oldest leaves. A yellow area can occur around the dark spot, but the uninfected part of the leaf usually remains green. Spotted leaves may die prematurely, resulting in substantial early defoliation, fruit sun scald, and poor fruit color. When symptoms are first noticed spray to completely cover both surfaces of all leaves with chlorothalonil fungicide (Daconil 2787 or Bravo 720 or Bravo Elite). Obey label directions for application rate, procedures, and timing.

Yellowing of lower leaves of tomato plants accompanied by wilting of the upper (younger) leaves is a strong indication of either fusarium wilt or verticillium wilt (diseases caused by invasion of the root system and water conducting cells by these two fungal organisms); or sometimes the result of root knot nematode infestation in the roots. If the leaf yellowing and wilting is caused by nematodes, there will be bead-like knots on the roots. If cutting the stem crosswise at an angle exposes a gray to brown colored ring inside the stem instead of a pearly white ring of water conducting cells, the yellowing/wilting is probably caused by the soil borne fungi, fusarium or verticillium. Fusarium is more troublesome in hot seasons that begin with abnormally warm springs.

Blossom End Rot of tomato fruit results from drought, wind, low humidity, and low soil moisture stresses felt by the tomato plant. These stresses result in a water deficit in the blossom end cells of young tomato fruit within the first few days after fruit set. At this time, the fruit are no bigger than the tip of your little finger. These stressed cells in the fruit die from dehydration. Tiny, newly set tomato fruit will suffer damage when water escapes from the tip cells faster than from nearby leaves. These stressed blossom end cells will also be deficient in calcium even in tomatoes grown in our naturally calcium-rich soils of the Texas High Plains. The reason for low calcium concentration in water deficient fruit is that calcium ions have a relatively very low solubility in water, so there is a relatively low calcium concentration in the water stream flowing upward inside the plant even when soil moisture is optimum and the air is still. The high soluble salt concentration of some municipal water supplies antagonizes uptake of calcium by tomato plants.

Calcium sprays recommended by well-meaning people will prevent blossom end rot only in tomatoes growing in calcium deficient soils where soil moisture is optimum and the plant is not stressed by low humidity and high winds. To prevent blossom end rot of tomato fruit here on the Texas High Plains: irrigate often enough and with sufficient volume, use windbreaks to deflect wind around tomato plants, maintain a 4 to 6-inch layer of mulch on the soil at least 3 to 4 feet outward from the main stem, and supplement irrigations with rain water if it is possible to collect and store this precious resource.