1 - Identifying the problem with leaves from a boxwood plant and hawthorn plant.

Yard and Garden August 4, 2012

Q.

Enclosed are a couple of leaves from my boxwood plant and hawthorn plant. Can you identify the problem with these plants?

Edie H.

Las Cruces

A.

The samples of leaves (hawthorn and boxwood) that you sent me appear to suffer from an environmental problem, not insect or disease.

From the limited sample, I was not able to identify the species of hawthorn, but it appears to be one that retains its leaves through the winter as does the boxwood plant. The symptoms your leaves exhibited could be suffering from winter damage (desiccation and wind damage). This would be the case if these leaves are old leaves that formed last year and overwintered. In the heat of summer, these leaves begin to show discoloration and eventually drop from the plant.

If these are newer leaves that were formed this years, the problem may be due to heat, drought, mineral salt accumulation (salt burn), and wind damage. This summer has provided all of these.

Whether the hawthorn leaves were old or new, the problem appears as salt burn. The ends of the leaves have turned brown while the base of the leaf remains green. Salt burn is common in many plants grown in New Mexico because our soils have a higher mineral content than soils in many other parts of the country. Our water is also high in minerals. Together these facts can result in salt accumulation in the leaves and damage to the leaves. Fertilization may increase the problem because fertilizer contains necessary nutrients in the form of mineral salts. If salt accumulation is the problem, try irrigation periodically to leach excess salts below the root zone. Organic mulches around the plants may also help maintain constant moisture and soil temperature, reducing the frequency of needed irrigation. Frequent irrigation results in accumulation of additional salts (from the water) around plant roots.

The symptoms I could see in the boxwood leaves was breaking of the tip of the leaf along a straight line and browning of the youngest leaves. The breaking of the leaf appears to be wind damage. If the leaf browned before breaking, salt accumulation as mentioned above may be involved. The browning of the newest leaves could be salt burn damage, or desiccation due to heat and drying winds as the new leaves were developing.

Irrigation and organic mulch to maintain constant moisture in the root zones of these plants will help minimize these problems. Direct rainwater from the roof of structures to the root zones of these plants to provide water with lower dissolved salt content to reduce addition of salt to the soil and to leach salts below the root zone of these plants.

If a significant number of leaves on these plants are showing symptoms, you should contact your local NMSU Cooperative Extension Service office to have someone familiar with local conditions examine your plants and provide additional, more specific, recommendations.

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: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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