April 18, 2000

Stage of Bud:   Approaching Half Inch Green  at UVM HRC   

Apple Scab Ascospore Maturity:

We are monitoring degree days (base 32F) from Green Tip at 6 sites around Vermont. Based on DD estimates, the following are the estimates for Cumulative Percent Ascospore Maturity as of April 16th:

As you know, ascospore maturity is only part of the information needed in determining the importance of a potential infection period. The following are four factors that impact the number of lesions that will develop (i.e., how much infection will occur) from an infection period:


1. The amount of inoculum (ascospore density) in the orchard
2. The amount of susceptible tissue on the trees
3. The level of susceptibility of the cultivar(s)
4. How long the leaves and fruits remain wet after the minimum hours for infection have been met


We are fortunate this year in that I suspect many orchards in Vermont are entering the spring with potentially lower amounts of overwintering inoculum than “normal” because the dry weather last year was not favorable for scab development. If you did have time last autumn to evaluate your PAD (potential ascospore dose) and had determined that your orchard was a
“low risk” orchard, then the action there should is to apply your first fungicide spray until after 3 infection periods ( but before the fourth infection period) or a Pink bud stage, which ever comes first. [Note: Some areas of Vermont did have more rain than others and had higher levels of apple scab. If you were not able to determine you level of foliar scab in the autumn, caution is advised.]