NDSU Crop and Pest Report
Horticulture


ISSUE 5  May 30, 2002

 

TOP DIEBACK IN SPRUCE

The most consistent concern on spruce tree problems this spring has been described as top dieback where the branches at the top of the tree are brown or bare. Some of the possible causes of this symptom include drought, main stem cankers, or an insect pest called yellow-headed spruce sawfly. Despite the generally wet springs we’ve experienced in many parts of the state in the last 2-3 years, there have been periods of drought in the latter half of the summer that have adversely affected many plants including spruce. Spruce have shallow root systems so extended periods where there is no rain in the heat of the summer may act as a drought-type of condition on spruce. In shelterbelts where spruce may be planted close together, there is great competition for the available water. In yards, watering to keep the grass green is rarely sufficient to meet the needs of large trees. Drought induced dieback may not appear until spring, and it may take a year or two of late season dry weather for top dieback to be expressed. Thorough, deep watering in the heat of the summer, and into the fall if rainfall is not adequate, around the drip-line of the tree will relieve drought stress

Main stem cankers can be caused by several different fungi including Cytospora. Hail storms are a source of wounding that leaves the tree open to infection. While Cytospora canker generally starts in the lower branches of a spruce, it may begin any place in the tree where a wound exists. If the main stem or trunk is wounded by hail, infection may result in a dieback of all parts of the tree above the canker. Examine the tree for signs of sunken, swollen, or discolored areas along the main stem just at the point the tree is turning brown. Also check for sticky sap being extruded from the main stem at the point where the branches are turning brown. The only way to eliminate this pathogen from the tree is to prune off the infected area, making the cut about 8-12 inches below the symptomatic area of the tree. Topping trees is rarely a desirable solution, however in this case, it is the only means of stopping disease progression in this tree and neighboring trees. If more than one third to one half the tree has been killed, it may be more reasonable to remove the tree entirely.

Another possibility for these symptoms is an infestation of the yellow headed spruce sawfly. The problem was identified in some areas of ND in June of 2000. See website:

http://www.ag.ndsu.nodak.edu/aginfo/trees/problems/pr061500.htm

All native and introduced spruce species are hosts to this pest, Pikonema alaskensis. Larvae of the yellow-headed spruce sawfly first feed on new needles in the spring, leaving jagged-edged brown stubs. Older needles will be chewed only after the new growth is devoured. By July, trees may appear ragged and yellow-ish brown, especially near the tops.

Top dieback in spruce

The yellow-headed spruce sawfly emerges in the spring as adults just as the buds are beginning to swell. Females deposit eggs into the base of new needles. Within 2 weeks, usually the first or second week of June, larvae emerge and begin feeding on new needles. These larvae will be fully grown (about 20 mm) by mid-July, when they fall to the ground to overwinter in cocoons in the soil. There is only one generation each year. Young larvae are quite small with have a yellow-ish body and a reddish-brown head. When disturbed, they arch both their head and rear an s-shape. The mature larvae are dark green with lighter stripes down the sides, and still have a reddish-brown head.

yellow-headed spruce sawfly

Trees may withstand light defoliation associated with larval feeding but extensive defoliation or consecutive years of defoliation will kill trees. On isolated trees, it is possible to simply knock the larvae to the ground or blast them out with a stream of water, where they will be attacked by natural predators. Chemical insecticides are quite effective if the yellow-headed spruce sawfly is determined to be the cause of the symptoms. Acephate (Isotox and Orthene), carbaryl (Sevin), and cyfluthrin (Tempo) are all labeled for use against this sawfly. This pest feeds in groups so treatment need only be done on clumps of feeding larvae. Since most yellow-headed spruce sawflies are believed to overwinter very near the soil surface, removing the duff beneath infested trees may reduce the impact of this insect. This should be done after the larvae have finished feeding, but before the spruce buds swell in the spring.

Chemical treatment is not warranted unless the larvae of the yellow-headed spruce sawfly are identified on the tree. To try to confirm this insect pest, look for chewed and jagged needles still on the tree. It is important to note that the symptoms will also appear in July. If the tree in questions looked good all summer and only now show the browning symptoms, the cause is likely not the sawfly pest. Bare branches where the needles of the tree are completely gone, or brown needles that are intact and clinging to the branches are also not indications of yellow-headed spruce sawfly. This is more typical of drought or a canker, possibly Cytospora canker. We are trying to define areas of yellow-headed spruce sawfly infestation so please call or email the lab if you believe you that this is the case in your spruce trees.

Cheryl Biller
NDSU Plant Diagnostician
diaglab@ndsuext.nodak.edu


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