Agriculture Reference
In-Depth Information
flowers, and fruits, conidial spores form in those lesions and can cause late-
season, secondary infections.
For a crop to be infected by apple scab, temperatures must be above
freezing and there must be drops or films of liquid water (free moisture)
on susceptible tissues. Under continually dry conditions, apple scab creates
few or no problems, even on susceptible varieties. As temperatures rise, the
amount of wetting time required for infections to develop decreases.
Table
when to apply fungicides to control the disease.
Secondary infections caused by conidia from established lesions happen
quickly, generally in 6 to 8 hours from the beginning of wetting when tem-
peratures are above about 50°F (10°C). Unfortunately, prolonged rains make
fungicide applications difficult. Also, organic growers do not enjoy the lux-
ury of being able to use fungicides that are effective against pathogens once
they are established. For organic growers, prevention is the key to managing
apple scab.
Management.
Many cultural practices reduce scab problems. Selecting a site
with low humidity and good air movement helps minimize wetting periods
and primary infections. An open orchard plan that emphasizes air move-
ment and training systems that create open canopies are also important, as
is regular and effective pruning. Remove cull and dropped fruit as soon after
harvest as possible. Removing leaves from the orchard floor in autumn and
composting, burning, or burying them helps reduce inoculum in the orchard.
Collecting leaves is usually not practical in large orchards, however. An al-
ternative is to shred the leaves with a mower and apply nitrogen to speed up
decay of the leaves. In conventional orchards, urea is often used as a nitro-
gen source. In organic orchards, any liquid fertilizer such as fish emulsion or
compost tea can help speed up decomposition.
