neighborhood garden for july 16, 2016




                                      by george edgar     


            With the extreme high temperatures we have been experiencing in Nebraska, we can expect some problems with vegetable fruit set and quality. The ground around green beans may be littered with aborted flowers. Flower and fruit abortion are common responses to heat stress as a result of the general failure of successful pollination and fertilization. Many people are concerned most about their tomatoes and cucumbers. Another problem can be a lack of female flowers, often noticed in summer squash.  As the heat increases, the ratio of male to female flowers changes to more, or exclusively, male flowers. When cooler weather prevails the plants will shift back to a more normal ratio.

          A lot of research has been done on tomatoes and especially FRUIT SET in tomatoes. Day temperatures over 90 degrees F and night temperatures over 70 degrees F reduces fruit set.

          Another problem related to heat stress in tomatoes is LEAF ROLL. During mild spring weather, top growth is more vigorous than root growth. When drier summer weather occurs, the abundant foliage transpires water faster than the root system absorbs it. The plant compensates by rolling its leaves to reduce surface area. Also the leaves may become thickened. The symptoms may occur after cultivation, heavy rains, or a sudden change in weather. This condition is usually temporary and the plant will recover on its own. To prevent leaf roll, keep soil evenly moist (not too wet, not too dry) and avoid cultivation that damages roots.

          Also tomato problems related to high temperature are SUNSCALD and SOLAR YELLOWING (yellow shoulders). Sunscald occurs when previously covered fruit are exposed to direct sunlight, e.g. when foliage wilts or a branch breaks off in the wind. Cultivars with sparse foliage may be more prone to sunscald, or plants that have lost foliage to any of the numerous tomato foliage diseases including early bight and septoria.

          Solar yellowing or yellow shoulders is a type of sun scald and occurs when lycopene (the red pigment in tomato fails to develop and leaves only the carotene (yellow) pigment to show at the shoulder. This occurs when tissue temperature rises about above 86 degrees F. 

          Heat stress can also cause bitterness in many vegetables. For example, broccoli, zucchini, and lettuce form bitter compounds as a result of heat stress. Generally, fruit that sets and develops later, after the more moderate temperatures return, are ok. Also, fruit that became fully sized prior to heat stress may not develop bitterness. Muskmelon may also turn bitter but this is more closely related to bacterial wilt or other diseases that affect healthy leaf development.

          The last problem I want to mention is BLOSSOM END ROT. Blossom end rot causes sunken, water soaked lesions near the blossom end of the fruit that eventually turns black. It is most often seen on tomatoes but can also be a problem on other vegetables such as squash and cucumbers. It is caused by a shortage of calcium in rapidly developing fruit. This problem can occur even though the soil has an abundance of calcium. It is associated with rapid plant growth and quick fluctuations in soil moisture. In Nebraska it is typically most severe during periods of hot daytime temperatures, low humidity, and windy conditions. The disease is also more serious when excess nitrogen fertilizer has been applied. During rapid growth the calcium in the soil is used by the plant for growth and does not provide enough for the fruit.

          This problem usually happens to the first fruit. The condition is temporary and the plant will recover and produce good fruit later.

Copyright 2016