HortFACT - Black Spot (Venturia inaequalis) in Apples and Pears
What is it ?
Black spot (Venturia inaequalis) is a fungal disease of apples, often referred to as apple scab outside of New Zealand. Apple black spot is a different fungus to pear black spot, and both are different to black spot on roses.
What does it look like ?
The initial infection in the spring shows up as dark green velvety patches on the leaves and developing fruit. Later the lesions on the leaves become black and slightly raised. By harvest, fruit infection spots are brown and cracked with a grey/black halo.
Black “pepper spots” are the results of secondary infections and are usually found in the calyx or close to the original infection.
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| Figure 1: Black spot symptoms on leaves | Figure 2 : Severe black spot infection on fruit |
What conditions are needed ?
Black spot is a wet weather disease. In general, the higher the temperature and the longer it rains, the more severe the infection period will be.
Where is it found ?
Apple black spot is found all over the world where-ever apples are grown. In New Zealand, black spot is an important problem in all regions.
Why is it a problem ?
Even the smallest black spot is unacceptable on an export apple. The disease can develop in coolstore as a result of infection which has occurred in the orchard before harvest.
Severe black spot on the leaves of young trees can retard tree growth.
What are the current chemical controls ?
Fungicides are classed as protectants or curatives. A range of protectants are available which can be applied throughout the growing season. The dithiocarbamate fungicides (eg. Polyram, Manzate, Mancozeb) give good fruit finish during the russet-sensitive period but overuse can disrupt integrated mite control programmes.
Curatives are usually from the DMI group of fungicides. There is a high possibility that the black spot pathogen will develop resistance to the DMI group, so they must be used sparingly.
Are there any other forms of control ?
The black spot inoculum overwinters in leaf litter. Therefore, removal or breakdown of the leaf litter will help to prevent the carryover of the disease. Urea is often applied in autumn to speed up the breakdown of the previous season’s leaves. Some growers graze sheep under the trees in autumn or winter which eat the fallen apple leaves.
Remove all the apples from the trees after harvest to reduce the risk of carryover infection.