Development of a Model for Predicting 

 Flyspeck Risks in Blocks of Apple Trees 



Arthur TYittle, Christopher Bergweiler, James Hall, Lisa Reisner, Steven Christie, 

 Wesley Autio, and Daniel Cooley 

 University of Massachusetts 



For several years, we have been working toward 

 the elimination of summer fungicide applications in 

 apple orchards. The positive economic and 

 environmental impacts of achieving this goal are 

 considerable. Unfortunately, in the absence of 

 fungicides, the severity of flyspeck disease, and to a 

 lesser extent sooty blotch, can be significant. In apple 

 trees which are not sprayed in the summer, flyspeck 

 incidence varies dramatically, from barely existing in 

 some blocks of apple trees to infesting more than half 

 the fhiit in others. 



How do we decide which trees need spraying and 

 which do not in a given year or month? We know that 

 the flyspeck fungus needs very high relative humidity 

 (97-100 %) to develop. By tracking leaf wetness, 

 rainfall, relative humidity, and temperature we can 

 estimate when specks will first show up in unsprayed 

 trees in or near an orchard. We can also estimate when 

 spray residues will be removed from apple trees by 

 rain, thanks to studies performed by Dave Rosenberger 

 at Cornell University's Hudson Valley Laboratory. It 

 remains a challenge, however, to estimate severity of 

 symptoms at harvest for a given block of trees. 



Certain characteristics of blocks of apple trees, 

 such as slope, relative altitude in the orchard, and 

 spacing of rows and alleys are likely to influence air 

 drainage and relative humidity in the blocks. The size 

 and openness of tree canopies will also affect the 

 humidity surrounding an apple. The consensus among 

 plant pathologists working with apples is that the 

 inoculum for flyspeck disease overwinters on the waxy 

 cuticle of alternate host plants like blackberry, oak, 

 grape, and maple in wooded or shrubby borders near 

 the apple trees. Within the orchard block, flyspeck 

 does not colonize apple twigs. Flyspeck that grows on 

 fruit is removed at harvest or decays over the winter 

 on drops. The orchard border is home to over 100 

 species that maintain waxy cuticle suitable for flyspeck 



and sooty blotch over a 12-month period on first year 

 growth. 



Many relationships involving the block and the 

 borders seem worthy of investigation. Number and 

 size of borders around a block, distance between a block 

 and its borders, density of alternate host plants in the 

 borders, and density of the fungus on those hosts might 

 all have significant impacts on summer diseases in fruit 

 at harvest. We study these factors and their 

 relationships to flyspeck disease development in order 

 to create a predictive model to help growers safely 

 reduce fungicide inputs. 



We reported on the first part of this study in the 

 Spring 1996 issue of Fruit Notes. This experiment 

 took place in six orchards over the 1995 and 1996 

 growing seasons. In each orchard, pairs of similar 

 blocks of apple trees were chosen. Some orchards 

 dedicated as many as 13 pairs of blocks to this 

 experiment. At each orchard, one block received no 

 fungicide after primary scab season (approximately 

 June 15), while the other block was managed according 

 to the grower's preferences using standard first-level 

 IPM. Flyspeck incidence was recorded weekly by 

 examining 200 fruit in each block from late-July 

 through mid-September. For each block, the following 

 orchard site characteristics were evaluated and 

 compared statistically to the flyspeck incidence or 

 severity data: slope of the ground, relative elevation 

 of the block compared to other blocks in the orchard, 

 closeness of shrubby or wooded borders to the apple 

 trees, density of a major alternate host plant in the 

 borders (blackberry), severity of flyspeck infestation 

 on those host plants, and density of apple tree canopies. 



Table 1 lists the orchard site factors that had the 

 greatest effects on the flyspeck counts that were done 

 in the in 2-week period leading up to harvest in 1995 

 and 1996. Unless otherwise noted, analyses for this 

 report were performed on data from the blocks which 



Fruit Notes, Volume 67, Winter, 2002 



