Scab (or black spot) is one of the most serious diseases in apples and pears and is found in all NSW growing regions.
The fungus that infects apples (V. inaequalis) cannot infect pears, nor can the pear fungus (V. pirina) infect apples.
Disease identification
Scab infection results in small, olive green–brown irregularly shaped lesions on the leaves that become larger as they mature.
Symptoms on the fruit are similar to those on the leaves but tend to have well-defined margins. As fruit lesions mature, they become brown to black and appear dry.
Damage
Primary infections occur on leaves and fruit when overwintering spores (mostly from last season’s leaf litter) land on wet foliage and germinate.
Once mature, primary infections can lead to secondary infections when spores are splashed through the canopy during rainfall.
Growth around the lesions becomes distorted as the fruit expands, giving it an uneven shape. Fruit lesions are superficial and the fungus does not extend into the flesh, but scabbed fruit is considered unsuitable for wholesale and retail fresh markets.
Monitoring
For apple or pear scab to occur, the host plant must remain wet long enough for the fungal spores that cause the disease to germinate. Warm (17C to 20C), wet conditions are ideal for infection.
Disease models using rainfall, temperature and period of wetness to predict the likelihood of infection are available. Infection warning services operate in some regions. Alternatively, growers can establish their own weather station(s) and monitor infection risk in-house on their orchard.
Inspect foliage and fruit closely for infection when the primary infection period is over (around the first week of December in most districts).
Management
Cultural and physical
Good orchard sanitation, particularly during autumn and winter, helps to reduce scab carryover and disease pressure in the spring.
A postharvest foliar nitrogen spray (usually Lo-biuret urea at 500g/100L of spray) at early leaf fall will encourage microbial action to break down the fallen leaves. Any action that removes or helps break down the fallen leaves will be beneficial. This includes raking, removing or sweeping and slashing to chop leaves into small pieces which then break down more rapidly.
Biological
There are currently no biological control agents available for apple and pear scab.
Growing scab-resistant varieties can reduce or negate the need for intensive fungicide spray programs for this disease. Consult your fruit nursery supplier for information on resistant varieties.
Chemical
Chemical spray programs for scab should focus on achieving excellent control during the spring primary infection period (from green-tip to end of spring).
If controlled well then, the potential for secondary infections through the rest of the growing season is dramatically reduced.
Download the Orchard plant protection guide
See this article in Tree Fruit Sept 2022
