supplemental label must be in the grower's
possession before use.
[!]To keep iron insoluble, maintain the pH of the water
between 6.5 and 7.0. If the water is acid, less than pH
6.0, then a buffer should be added to raise the pH to
6.5 to 7.0. Your County Agricultural Agent or
Rutgers Specialist can provide specific
recommendations based on the pH of your water.
[!]Drain and refill the water in the hydrocooler and
dumptank on a regular basis daily, if possible. Be
sure to remove all dirt, leaves, and other debris after
draining.
[!]
Sources of chlorinating chemicals and equipment may
be obtained from your County Agricultural Agents.

Table 11.5. Minimum Requirements For
Apple Scab Leaf Infection (Revised Mills)

Diseases Of Apples

AppleScab.This disease, caused by the fungal
plant pathogen Venturia inaequalis, is the most
important apple disease in our area. The fungus
overwinters in fallen leaves on the ground. Ascospores
(sexual spores) are released from these fallen leaves when
they become wet during spring rains. The critical period
for spore release is from the time green tissue is first
visible through third cover. Any infections which occur
during this period result in primary scab since the
ascospores are the initial inoculum for the growing
season.
Primary scab infection periods can be predicted by
gathering data on wetness period duration and average air
temperature during the wetness period. The table below
shows the minimum hours of wetness needed at various
temperatures in order for infection to take place. For
example, if the average wetness period temperature was
55.4F and the period duration was 11 hours, then an
infection period occurred since only 8 hours of wetness are
needed at this temperature.
Recent research has shown that most ascospores are
released onlyduring daylight hours (dawn to dusk).
Thus, if a rainfall begins at night, the infection period
[duration of wetness] does not begin until sunrise.
However, if a rainy period begins late in the day and
continues into the night, then the night-time hours of
wetting need to be included in the wetness duration, since
initial wetting [and spore release] would have occurred
during daylight.

Wetness durations and average air temperatures can
be determined by visual observations and use of a
min/max thermometer. However, a variety of mechanical
and electronic devices can be purchased to help automate
data gathering. Contact your agent for details.
A variety of fungicides are available for control of scab
during the primary period. However, whether or not a
fungicide is prone to the development of resistant scab
influences how it is used (see Apple Spray Schedule -
Delayed Dormant). If a fungicide is selected that is not at-
risk to resistance, then it can be used alone (e.g., Captan
or Ziram). If a material is selected that is at-risk, then it
should be mixed with a fungicide that is not at risk. For
example, Nova or Rubigan should be used in
combination with another non-risk fungicide, such as
Captan. And when used in combination, the non-risk
fungicide is applied at half the standard rate.
Resistance development is not the only factor which
determines use patterns of fungicides. The EBDC
fungicides (see Apple Spray Schedule - Delayed Dormant)
can be applied up to bloom at full rate when used alone.
However, they can be used in an extended program,
through second cover, when they are applied at half-rate in
combination with another non-EBDC fungicide.
In general, if scab is properly controlled with
fungicides during the primary scab period, then no further
disease control is needed for the remainder of the season.
However, if field observations at the end of the primary
period indicate the presence of primary scab lesions, then
additional sprays will be necessary. These lesions
produce asexual spores, called conidia, which can
continue to cause infection throughout the summer.
However, unlike the primary ascospores, conidia can be
released during any period of the day when rain or heavy
dew occurs.
Strains of the apple scab fungus that are resistant to

1998 New Jersey Commercial Tree Fruit Production Guide

113