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Nursery & Forest
Volume 57 Number 16 Date 08/02/2012

VIRUS SURVEY OF ORNAMETALS - A spring survey for viruses of ornamentals conducted at 29 Wisconsin greenhouse, nursery and retailer locations found a wide range of plants to be infected. One hundred and ten plants displaying virus symptoms were collected during the period of March 2-June 21 and diagnosed at the DATCP Plant Industry Laboratory. Samples with unusual virus-like symptoms were forwarded to the UMN Plant Disease Diagnostic Clinic for further testing. Of the 110 plants tested, 47 (43%) were infected with one or more viruses.

The most frequently detected virus was tobacco rattle virus (TRV), found in 27 of 90 (30%) plant samples. Nearly all of the bleeding heart (93%) and barrentwort (100%) samples submitted for testing were infected with TRV. Several other viruses were also detected, namely cucumber mosaic virus, diagnosed in 10 of 103 (10%) samples, and hosta virus X, found in seven of 14 (50%) samples. Another eight ornamental hosts tested by virologists at the UMN were positive for virus particles, including a 'Black Jack' fig grown in California which was infected with fig mosaic virus and a clematis plant with tombusvirus. The results of the survey are summarized in the table below.

Most viruses are readily transmitted through contaminated pruning and propagating tools that move sap from one plant to another, and natural vectors such as aphids, thrips and nematodes. To limit the spread of these destructive plant diseases to garden plants and crops, DATCP requires all virus-infected nursery stock to be removed from sale and destroyed.

--Anette Phibbs, DATCP Plant Industry Laboratory

GYPSY MOTH - The state moth count as of August 1 is 61,432, based on examination of 11,046 of the estimated 19,000 traps deployed in 50 counties. Of the checked traps, 3,245 traps were positive for moths. Trap checks are complete in 20 counties and removal is scheduled to begin in the southern areas during the week of August 6.

--Nkauj Vang, DATCP Gypsy Moth Program

PHOMOPSIS BLIGHT - This evergreen disease is reported to be infecting several varieties of juniper and boxwood in Ozaukee and Washington counties. Plants with phomopsis blight develop yellow spots at the shoot tips of young needles that progress to the stems, causing gradual dieback of new growth and eventual death of the infected branch. Its occurrence can be reduced by pruning out symptomatic branches and twigs 4-6 inches below the diseased area, disinfecting pruning shears between each cut. Maintaining adequate spacing and airflow between plantings will also help to prevent it from spreading.

CYTOSPORA CANKER - Colorado blue spruce trees in Ozaukee County nurseries are exhibiting cankers typical of this disease. The canker-causing fungi invade the bark of twigs, branches or trunks of woody plants that are physiologically stressed due to drought, flooding, insects or mechanical injury. Diseased branches and twigs should be pruned out during periods of dry weather, while cankers that develop on the trunk require removal and destruction of the entire tree. Many other trees are susceptible to this disorder, including apple, ash, aspen, birch, cottonwood, elm, maple, poplar and willow.

GUIGNARDIA LEAF BLOTCH - Nursery inspectors observed symptoms of this common leaf blotch disease on horsechestnut and buckeye trees in Ozaukee County in the past week. Diagnostic characteristics include irregular shaped, reddish-brown leaf blotches with yellow margins that twist and distort affected foliage as they increase in size and severity. The impact of this leaf blotch on tree health is minimal. Fallen leaves should be collected and disposed of in the fall to reduce inoculum levels and suppress disease development.

PINE NEEDLE SCALE - Light infestations of this scale insect were found on Scots pine in Ozaukee County. At low densities, pine needle scales inflict little damage, but larger populations cause needle discoloration and potentially branch death. Controls are most effective in spring when applied against the crawler stage shortly after egg hatch. This event corresponds with first bloom of common lilac, usually around early to mid-May. The proper timing of insecticidal treatments should be determined by monitoring infested pines for newly emerged crawlers.

CHRISTMAS TREES - Christmas trees are showing the effects of this summer's unprecedented drought. The problem is particularly evident in the southern counties where extensive losses of as many as 5,000-30,000 seedlings have been reported from some tree farms. Conifer seedlings are planted to a depth of about eight inches and their roots are less capable of absorbing moisture than mature trees, making them more susceptible to moisture stress. Many growers will need to plant twice as many next year to compensate for the damage.

--Ellen Hermanson, DATCP Nursery Inspector