Issue: April 28, 1997
Black leaves on pear tree in springQuestion:
Enclosed are leaves taken from my pear tree. (They are dry, and blackened around the edges.) When they first appear, the seem to be healthy. Then the edges begin to turn brown. The tree gets adequate water. This problem was first noticed last year and now is occurring again. The tree appears to be healthy otherwise and fruit production is not affected.Answer:
When observing black leaves on pear trees, it is important to rule our fire blight. The symptoms do not appear to be those of fire blight, so we will rule it out first. That is good. Fire blight is a bacterial disease which is difficult to manage.
A common problem observed in trees in New Mexico each spring is blackening of leaves of pear, cottonwood, and many other trees. This can be due to slight frost damage or by wind damage. The frost damage may have occurred on a night in which temperatures at location of most thermometers registered temperatures above freezing; the frost may have been localized to small pockets.
Young leaves in the spring are often very tender and subject to damage from the winds which develop during our warm desert days. Temperatures can rise from very pleasant temperatures to rather warm mid-day temperatures. These warm days following cool nights, when combined with our frequent spring winds, can cause rapid desiccation of the leaves. The leaf margin is the most likely injured part of the leaf. This desiccation can occur even if the soil has adequate moisture because the wind can draw water from the leaves faster than the tree can move it from the soil into the leaves. Very young trees may not exhibit symptoms because they are smaller and closer to the ground and more protected from the wind, and also because they have less distance to move water through the plant to resupply the leaves. Newly transplanted trees, with limited root systems, and those which have just begun rapid growth after a couple of years establishment following transplanting may be the most likely to show the symptoms. However, under the right conditions this spring wind desiccation injury can occur in larger trees as well.
As long as the tree appears to be healthy and fruit production is normal, there should be little to worry about. New Mexico's climate can provide some harsh environmental conditions which cause cosmetic defects in many of our landscape and garden plants.
One other problem to consider in New Mexico, but which did not appear to be a problem in the samples sent, is salt damage. Our soils are often salty, containing sodium and other salts, which can accumulate to excess levels in leaves. Older leaves will develop chlorosis, yellowing, while new tender leaves will turn black. Use of poor quality irrigation water, high in dissolved salts, will aggravate the problem unless sufficient water is applied during irrigation to leach excess salts below the root zone of the crops grown.
For more information on proper irrigation and other cultural methods to minimize salt problems, contact your local county Extension office.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: firstname.lastname@example.org, office: 505-865-7340, ext. 113.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!www