Issue: September 13, 2003
My dwarf mugo pines are dying. I planted a dozen mugo pines in a new bed with fresh topsoil and peat last Spring. Half are now dead and several of the rest are turning yellow and look like their dead siblings before they died. The nursery said it might be spider mites. What should I do? Mike Byrnes RuidosoAnswer:
If your mugo pines are infested with spider mites, a frequent washing with a strong stream or mist of water will help. This washes away the mites and their protective webbing. Rains often do this for us, but in our climate with long periods without precipitation, the spider mite population increases to damaging levels. To confirm that spider mites are the problem, look for a lint-like webbing that coats the needles and twigs. If you see the webbing, look closely for very small mites crawling along the webbing. This does not look like spider webs. It is a fine webbing that coats the needles and traps dust. That is why it looks like lint. There are also some pesticides labeled for control of mites. Read the labels before purchasing to determine the one best for your mugo pines, then read again and follow the directions when treating the plants.
There are other problems that could also be causing the problem. The top-soilmay have a high concentration of manure and related salt-minerals. These salts can damage plant roots and result in the symptoms you have described. As the plants die and you remove them from the soil, investigate the appearance of the roots. If the roots were killed by salt, they will be dark brown and black. If the roots are healthy, the ends of the roots will be white or a lighter brown. Healthy roots will also be firm but flexible. Dead roots will be rotting and mushy or dried and brittle. If you wait too long after the plant dies, the roots will indeed be dead, so this must be done as soon as the plants appear to have died.
Root rotting can also be caused by too much water, keeping the soil soggy around the roots. Irregular watering (allowing roots to dry then waterlogging them) can cause rotting. Soil conditions (in addition to the manner in which you water) determine if this is the problem. Clay soils hold water for a long time and are easily over-watered; once they dry, they are difficult to re-moisten. Sandy soils do not hold much water and must be watered more frequently, but even in sandy soils it is possible to over-water. However, sandy soils are much more likely to cause the second problem - drying and over-watering.
In your area there may be Master Gardeners trained by the NMSU Extension Service who can help you. They can take a look at your soil and irrigation conditions and better determine the exact nature of your problems. To contact Master Gardeners in your area call your local Cooperative Extension Service office.back to top
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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, office: 505-865-7340, ext. 113.