The BIG FOUR Factors :
Determining the Importance of a Primary Infection Period
Once you have determined that the minimal conditions have been met for a primary scab
infection period (see chart on next page), then the question that is often asked is:
"How important was that 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
Information about each of these factors will help to categorize the severity of an
infection period and help to determine the risk of not having preventive or retroactive
(kickback) fungicide protection against scab infection.
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 threshold 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.
Since the 2000-2001 New England Apple Pest Management Guide is not yet available, I have
included in this newsletter the next table which indicates the hours of leaf wetness
needed at various temperatures for infection to occur and the estimated days it will take
before lesions are first visible.
|
