BLUEBERRY SCORCH VIRUS

PEST ALERT AND FACT SHEET

Robert Martin¹, Gene Milbrath², Jan Hedberg²

¹USDA Horticultural Crops Laboratory

^{2Plant Division, Oregon Department of Agriculture

Blueberry scorch virus (BlSV) was first observed in a Berkeley blueberry planting near Puyallup, Washington in 1980 and has been observed in western Oregon and Washington (near Puyallup and in Clark County), but not northern Washington or in the Fraser River Valley of British Columbia. A recent outbreak of BlSV with more severe symptoms was reported in British Columbia during the summer of 2000. Based on symptoms, it closely resembles the New Jersey strain of BlSV, known there as ‘Sheep Pen Hill Disease’. The identification of this disease in British Columbia has the potential to have a devastating effect on blueberry production in western Oregon and Washington production areas. The New Jersey strain of BlSV causes symptoms in all cultivars except ‘Jersey’, whereas the west coast strain is symptomless in ‘Bluecrop’ and ‘Duke’ as well as several other cultivars (Table 1).
Since BlSV was found throughout the main blueberry production areas of BC, all planting material from BC should be considered suspect and potentially infected with the virus. Growers should not obtain blueberry plants from BC until a certification program has been established to ensure virus-free planting stocks.

Symptoms
The symptoms of scorch on infected blueberry cultivars range from complete browning of the flowers and some of the leaves leading to eventual death of the infected bushes to cultivars that are completely symptomless. The cultivars that remain symptomless do carry the virus and serve as a source of inoculum for infection of other bushes. For cultivars that exhibit symptoms, there can be a 1 to 2 year latent period between the time of infection and appearance of symptoms. Thus, early detection of infected plants cannot be based on the presence or absence of symptoms. The disease starts on 1 or 2 branches before it spreads throughout the bush in succeeding years. The flowers on the infected plants turn brown and fade to a greyish color before they fall off the plants. With the west coast strain of BlSV, the dried flowers can be retained on the bush for more than a year. Flowers are not retained on the bush with the New Jersey strain. However, it is not clear at this time if flower retention is strictly strain dependent or if environmental conditions play a role. The symptoms of the disease are expressed in infected plants every year and plants do not recover as they do when infected with blueberry shock ilarvirus, the production begins to drop off rapidly and the need for replanting becomes evident.

Fig. 1 Bush of BlSV infected 'Berkeley' blueberry flanked by healthy bushes

Fig. 2. Close up of 'Berkeley' showing flower and tip necrosis

Causal Organism and Disease Cycle

The virus is spread from one geographic location to another over long distances through infected planting stock. This group of plant viruses is spread by aphids feeding on infected plants, picking up the virus on their mouthparts, and then moving to a new location and feeding on another plant. In the process of feeding, the virus is transmitted to the uninfected bushes. The rate of spread through a planting can be rapid and depends upon the number of aphids available to spread the virus. Aphid transmission occurs over relatively short distances, (less than a mile). There is evidence to show that most infections occur from mid-April to early-August with the west coast strain.

Cultural Control
The following recommendations are made in order to reduce the potential detrimental impact of BlSV on blueberry plantings in the Pacific Northwest region:}

1. For any new planting of bushes obtain the plants from local reputable nurseries who grow quality plants and have a virus testing program in place, ask for documentation.

2. Buy bushes that were propagated from bushes grown in BlSV-free areas (at least ten miles from any known infected bushes).

3. Have the plants tested for the presence of BlSV before planting any new acreage for commercial production.

4. If plants are found infected with BlSV, the plants should be sprayed for aphids and then remove from the planting immediately and initiate an aphid-control program for two years, removing any new plants that develop symptoms or test positive for BlSV.

5. Replant with virus-tested plants preferably of plants that are tolerant of BlSV.

6. Apply sound integrated pest management strategies to control the populations of potential insect vectors.

Table 1. Reaction of blueberry cultivars to BlSV. All cultivars can be infected.

Reaction	West Coast Strains	East Coast
____________________	__________________________________________________	_______________
Symptomless	Bluechip, Bluecrop, Bluegold, Bluetta, Burlington, Concord, Coville, Duke, Ivanhoe, Lateblue, Nelson, Northblue, Northcountry, Northsky, Nui, O’Neal, Puru, Rancocas, Reka, Sierra, Sunrise, Toro, Washington,1613-A (Hardiblue), U254	Jersey
Marginal chlorosis	Olympia, Stanley, Washington, N51G (Eberhardt)
Flower and leaf necrosis and general plant decline	Atlantic, Berkeley, Bluehaven, Bluejay, Blueray, Collins, Darrow, Dixi, Earliblue, Elizabeth, Elliott, Gem, Herbert, Meader, Northland, Pemberton, Rubel, Spartan, Weymouth	All others

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The only cultivar with known tolerance to the New Jersey strain of BlSV is ‘Jersey’

Fig. 3. Marginal leaf chlorosis in BlSV infected 'Stanley' blueberry

A cooperative research program between the USDA-ARS, Washington State Univ., BC Ministry of Agriculture Fisheries and Food, and Agriculture and Agri-Food Canada is being undertaken during 2001 to obtain more information on the management of this disease in the Pacific Northwest.

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