Apple scab and pear scab

Apple 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 (Venturia inaequalis) cannot infect pears, nor can the pear fungus (Venturia 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.

Blossoms

Black patches develop on the flower stalks. These patches extend, and soon the flowers and stalks blacken, shrivel and die.

Leaves

On the upper surface of the leaf, spots first appear as light green areas that soon turn brown, then black and velvety. There might be only a few scattered spots, or so numerous that they coalesce and cover almost all the leaf surface. Infected leaves might curve inwards and become blistered and distorted. On the lower surface of the leaf, the disease can appear as a diffuse dark discolouration, tending to extend along the veins. Late in the season, many small, rather angular lesions might be seen on the lower surface of the younger leaves.

Fruit

The spots on fruit are at first small, black and circular. They can be so numerous that, as they grow, they coalesce to form large scabbed areas. The central, older parts of the spots become brown and corky, and the growth of the fruit in these areas is greatly restricted. Fruit infected early in the season becomes badly distorted, and deep cracks can develop over the scabbed areas.

Spots formed on larger fruit late in the season cause much less distortion; these spots are often surrounded by a whitish band of loosened skin. Early-season infection of pears can result in the formation of large corky areas on the fruit. Infection later in the season causes the formation of dark sunken spots.

Infections which occur just before harvest can be invisible when fruit is picked, but will be apparent when fruit is removed from the cool store some months later.

Twigs

Infected twigs develop a flaky appearance, and the bark becomes blistered and ruptured in places. Twig infection is rare with apples, but more common with pears.

Dissemination

The fungus is spread by wind and rain, and is favoured by warm, wet weather conditions.

Life cycle

Throughout the growing season, the scab fungus lives and grows on the tree, producing vast numbers of spores that spread the disease.

The fungus can live over the winter in infected areas on twigs and bud scales, but it mainly survives the dormant season within the leaves that fall in autumn. During winter, it forms special structures (called perithecia) in the dead leaf tissues, and in these structures, spores called ascospores are formed. By late September, many perithecia are mature. When it rains, they discharge their ascospores, which are carried by air currents onto the trees. Most early or primary infection comes from this source.

If an ascospore lodges on a leaf or fruit, germinates, and penetrates into the tissue, a scab will appear 2–3 weeks later. The scab largely consists of a mass of dark-coloured spores called conidia. These conidia can infect other susceptible parts of the tree when carried there by rain and wind.

Infections caused by conidia are sometimes called secondary infections. They can occur from the time of the first primary lesions until the end of the season. The more spores emitted, the greater the chance of infection. Consequently, in a season favourable to scab infection, the disease is easier to control in a clean orchard than in a heavily infected one.

Estimation of infection periods

The following conditions are necessary for infection:

• mature ascospores must be in the orchard air, or conidia must be on lesions
• the leaves or fruit must remain wet long enough for the spores to germinate and for the resulting fungus growth to enter the plant tissue; this period of wetness is called the infection period.

Infection occurs most rapidly at temperatures between 17°C and 24°C. The trees must remain wet for longer for infection to take place at temperatures outside this range.

The wet period can be continuous, but intermittent wet periods can also induce infection if there are not more than 8 hours of dry weather between them. If the wet periods are intermittent, add their durations together until a period of about 8 hours of continuous tree dryness occurs. If it is sunny and drying is quick and thorough, you can assume the danger period has passed 8 hours after the trees have dried. Any further rain will then be taken as the starting point for the next danger period.

Slow-drying conditions with high relative humidity make it difficult to be sure the trees are really dry. In these cases, extend the 8-hour dry period needed to end an infection period by a ‘safety’ margin of a few hours, especially if the total wetting time is almost long enough to make up for an infection period. It is better to apply a curative spray that might not be needed than to risk losing a crop by not spraying.

Early in the season, before any conidia are present (before any scab lesions are visible), the infection period begins from 7 am, regardless of when the wet period began during the night. This is because the fungus needs light as well as moisture to trigger the release of ascospores, which are not in the orchard air at night, so rain falling after 6 pm cannot induce infection. If rain begins during the day, judge the infection period to start at the same time as the rain.

When conidia are on scab lesions, judge the infection period to begin at the same time as the rain, irrespective of the time. The table below (known as the Mills Table) details how long trees must remain wet at particular temperatures for infection to occur.

Note: The most essential item of equipment you need to make those calculations is a maximum and minimum thermometer. From these readings, you can estimate the average temperature for the wet period.

Adapted from Mills and Laplante 1954, Cornell Extension Bulletin No.711.

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 by rain.
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 (17–20 °C), 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 weather station(s) and monitor infection risk in their orchards.

Inspect foliage and fruit closely when the primary infection period is over (around the end of November to the start of December).

Management

Cultural and physical: good orchard hygiene, especially during autumn and winter, helps to reduce scab carry-over and disease pressure in spring. A postharvest foliar nitrogen spray (usually Lo-biuret urea at 500 g/100 L of spray) at early leaf fall will encourage microbial action to break down fallen leaves. Any action that removes or helps break down fallen leaves will be beneficial. This includes raking, removing or sweeping and slashing to chop leaves into small pieces.

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: spray programs for scab should focus on achieving thorough control during the spring primary infection period (from green-tip to the end of spring). If controlled well then, the potential for secondary infections through the rest of the growing season is dramatically reduced. Refer to the Orchard plant protection guide for chemical suggestions.

Fungicide resistance

The apple scab fungus in some orchards has developed tolerance to some newer fungicides, which have been used excessively. To prevent such tolerance arising in an orchard, a number of different fungicides should be used in a disease control program.