Agriculture Reference
In-Depth Information
40
30
20
Severe
Moderate
Light
10
50
60
70
Temperature in Degrees Farenheit
Figure 18.1. The relation of wetness duration, temperature and degree of apple scab infection
as originally published by Mills (1944). Scab warning systems based on these criteria have
survived several decades.
Mills made two assumptions in drawing up his minimum curves for primary apple
scab infection. These assumptions were well known to those who helped him
validate his curves in the 1940s but were never presented in refereed journal articles.
First, he used the onset of rain as the trigger mechanism for initiating the models
because he desired a starting point that could be easily identified (available data at
the time indicated that the onset of ascospore discharge, unlike the release of
conidia, was often delayed compared to the onset of rain). Second, he assumed it
would take at least three hours longer in the field than in growth chambers for
infection from ascospores. In the growth chamber studies, ascospores were applied
directly to the plants but in the field ascospore release is delayed until the spore
discharge mechanisms of the pseudothecia are activated. Mills also reasoned that it
would take more time for airborne ascospores than waterborne conidia to reach
susceptible tissue in sufficient quantities to lead to a significant level of infection. It
was the difference in the time of inoculation, not possible differences in infection
rate after the spores reached the host, that concerned Mills and led him to develop
two minimum infection curves, one for ascospores and one for conidia.
