ISSUE 4 May 24, 2001
The cool, wet weather this week in southeastern North Dakota will probably spur another round of ash anthracnose, causing defoliation similar to that seen in the previous two to three years. See the Plant Pathology section of this report for more information about this disease.
Since spruce needles are currently elongating in most of the state, the first application of a chlorothalonil product (ex. Bravo, Ortho Multipurpose Fungicide, Daconil, etc.) for this year may be applied at this time to spruce trees infected with this disease. See last weeks Crop and Pest Report for additional information about this disease.
SPRUCE SPIDER MITE
The wind-driven rains may help to reduce the numbers of spruce spider mites in some areas; however, continue monitoring for these mites after the rains stop and the foliage dries. Symptoms include speckling or bleaching of affected foliage. Needles may turn brown and fall off from heavy infestations. Severe infestations may kill branches or trees.
As with most spider mites, a good test for spruce spider mites is to place a white piece of paper under needles which are believed to be infested and tap the branch. Mites appear as mobile specks on the white paper. Generally, if ten or more mites are found per sample, some type of control may be necessary. Syringing and chemical controls can be used in controlling spruce spider mites. Spraying mites with a forceful jet of water (syringing) can be an effective method for controlling mite populations in home landscapes while maintaining natural predators. Insecticidal soaps can be used to manage spruce spider mites in warm weather, while horticultural oils (1-2% rate) may be used during the summer and dormant oils (3-4% rate) can be used to kill mite eggs and adults during the spring and fall. Horticultural oils can injure conifers if applied when temperatures are not appropriate. Read the labels carefully. Dicofol or spinosad may be sprayed when adults are active with a follow-up spray 7-10 days after the first spray to control later hatching nymphs.
FOREST TENT CATERPILLAR
Reports of forest tent caterpillars (FTCs) in eastern North Dakota have intensified over the last week. We are passing the point where Bt will keep the insects from causing substantial defoliation of infested trees; therefore, carbaryl, permethrin, pyrethrins or other insecticides will provide the greatest relief at this time. Always follow pesticide labels. Itís recommended that people do not fertilize trees during FTC outbreaks since fertilizers may reduce levels of defensive compounds in trees and improve the nutritional value of the trees to the forest tent caterpillars (larvae have been shown to grow faster on fertilized trees). For additional information about this insect see the May 10, 2001 issue of the Crop and Pest Report.
DROUGHT AFFECTS ON TREES
Western parts of North Dakota havenít seen much rain this spring. Many of these areas had a fairly dry fall and winter as well. In addition to direct damage to trees (smaller leaves, branch dieback, etc.), research has shown that many opportunistic insects and diseases cause more tree injury when trees lack adequate water. Proper water amounts are especially important for building food reserves during early and midsummer. The water needs of a tree to maintain health generally declines as the summer progresses. Therefore, if a tree uses 1.5" of moisture per week in June, it may use 1" in July, 0.75" in August, and 0.5" in September. If trees are lacking water, and good quality water (not saline, etc.) is available, now is an important time to water them. Many tree roots are deeper in the soil than turf roots. Generally, trees need to be watered less often than turf but require more water in a given area than turf during each watering.