Rutgers Agricultural Research and Extension Center, 121 Northville Rd, Bridgeton, NJ 08302 USA

Background and objectives
Fusicoccum canker is a serious disease that causes both direct and indirect crop loss. The fungal pathogen, Phomopsis amygdali (syn. Fusicoccum amygdali), infects the current season's shoot growth in the fall and again during the following spring [1]. The resulting fungal cankers eventually girdle and kill these fruiting twigs during the subsequent summer. The young fruit lost on these blighted twigs represents a direct crop loss. Shoot death also results in indirect crop loss by reducing the total amount of photosynthetically active tissue necessary to grow the remaining fruit.

Fusicoccum canker was first observed in New Jersey in 1934 and major epidemics occurred during the 1940's and 1950's, causing many growers to halt peach production [2]. Today, most commercial peach orchards in New Jersey still harbor significant levels of the disease, and many growers have at least one severely infected block. Although crop reductions of more than 50% have been mentioned in the past [2], no attempts have been made to quantitatively determine the loss. Thus, the objective of this study was to ascertain the potential for crop loss by surveying and estimating the yield loss in severely infected peach orchards.

Materials and methods
Ten commercial peach orchards in southern NJ were chosen for the study. At each orchard, from one to three heavily infected peach blocks were selected. The total number of blocks or sites in the study was 21 and eleven different varieties were represented. Six trees in each block were used for observations by randomly selecting two trees from rows that were 25%, 50%, and 75% distant from the first row. On each tree, four limbs were selected at 90 degree increments around the periphery so that two limbs were perpendicular and two parallel to the row. The total number of healthy shoots, infected shoots, and fruit were counted on each limb. Assuming that blighted shoots would have yielded equal to healthy shoots, the number of fruit lost to disease was calculated from the estimated number of fruit per healthy shoot and the number of infected shoots observed. These estimates were then converted to percent yield loss based on the estimated total number of fruit possible in the absence of disease. Assessments were made at each site during June and July of 1996 and 1997.

Results and conclusions
The overall yield loss average across all sites and varieties was 23.9% in 1996 and 31.0% in 1997. The minimum to maximum yield losses for any given site ranged from 14.8% to 52.9% in 1996 and 11.4% to 58.8% in 1997, respectively. The 7.1% increase in yield loss between these years can be directly attributed to an increase in the number of diseased shoots from 3.8 to 4.5 per tree limb. Furthermore, in an analysis of variance of yield loss, the main effect for year had a Pr>F of 0.0001 thus indicating the 1996 to 1997 increase was statistically significant. Although the site and variety effects were also highly significant, no inference can be made given the unbalanced design within each year (not all varieties were present at all sites). The number of sites assessed for each variety and their average yield loss, calculated over both years, were: Biscoe 2, 49%; Autumnglo 2, 38%; Harbrite 1, 28%; Jerseyglo 4, 25%; Jerseyland 1, 25%; Encore 2, 24%; SunQueen 1, 23%; Redhaven 5, 23%; Loring 1, 22%; Cresthaven 16%; and Harcrest 1,13%. Given an average crop value of $5,440/A in 1996 and $4,862/A in 1997 [3], these yield losses represent from $707/A to $2,665/A dollar loss in 1996 and from $632/A to $2,382/A dollar loss in 1997. These results clearly show the considerable potential for significant crop loss at high levels of disease incidence.

1. Guba EF, 1958. Plant Disease Reporter 42:481A92.
2. Dames RH, 1974. NJ State Hort Soc, Horticultural News 55(4):12-13.
3. Polk D, Schmitt D, Rizio E, Petersen K, 1997. NJ State Hort Soc, Horticultural News 78(1):3-10.