13/04/2019
scab
Apple scab or black spot is caused by the fungus Venturia inaequalis. It infects leaves, shoots, buds, blossoms and fruit. It occurs almost everywhere apples are grown and is the most serious and widespread disease of this crop, especially important in regions with high rainfall and relative humidity during the growing season.
The first symptoms usually appear as small spots on the underside of young leaves or as spots on either surface of older leaves. The spots (or lesions) are brown or olive-green and have an indefinite margin at first. The lesions on older leaves are more definite in outline, and become velvety-grey to sooty black. Sometimes the tissue below individual lesions becomes raised, giving such leaves a blistered, scabby appearance. The lesions merge to cover most of the leaf surface if infection is severe. Infected leaves are often twisted, distorted and stunted and tend to fall prematurely.
The fungus may infect developing flowers, but is more usually seen on reproductive parts after the fruit has set. Infection of fruit stalks usually causes the young fruit to fall.
Early infection of the fruit itself can cause distortion, because growth under the lesion stops while the surrounding tissue develops normally. Late infections on more mature fruit appear as dark, roughly-circular spots. The centres of large lesions become dry and corky and a greyish, raised cuticle may be evident along the border of the lesion. The corky areas may crack, allowing entry of fruit-rotting. Infection of almost-mature fruit results in small spots with little distortion of the fruit. Lesions may continue to develop in storage from symptomless infections present before harvest, resulting in storage scab.
Apple scab is present throughout Victoria, but is most serious in districts with high rainfall especially during spring. The disease can reduce the quality and size of fruit. It can also cause premature fruit drop, defoliation and poor development of fruit buds in the next season. Under favourable conditions and where control measures are not applied, total crop failure can occur.
At present, the potential threat of infection can be overcome only by a costly spraying program, requiring much labour, machinery and spray chemicals and involving up to 12 sprays each season.
The disease usually survives over winter under the trees, in the dead, infected leaves from the previous season. Occasionally twig infections, infected bud scales or infected leaves which remain on the tree over winter can provide an overwintering source of spores. Leaves infected with the apple scab fungus usually fall from trees in autumn or early winter. The fungus continues to live within the leaves during winter, forming small, flask-shaped bodies, in which spores (ascospores) develop. These ascospores mature in spring and are forcibly ejected during spring rains.
Most ascospores are released during daylight. Night discharge and any discharges into dew are less important. Wind currents carry ascospores to developing flowers, fruits and leaves, where they adhere quickly on contact, and resist removal by more rain.
Ascospores germinate and grow into the tissue if fruit or green leaves are wet when ascospores lodge, and if they remain wet long enough. The time needed for germination and infection varies with the temperature and is called the infection period.
The fungus grows under the cuticle of leaves and fruit and sends up stalks on which summer spores (conidia) are borne. This is when the typical scab lesions can be seen and happens after about 9-17 days.
Control measures are designed to interrupt the life cycle at various stages and to modify the orchard environment to reduce disease. When planning a new orchard:
Use scab-resistant varieties where practicalAvoid locations with high spring rainfallUse dwarfing rootstocks and training systems to maintain an open and easily sprayed tree canopyAvoid use of overhead irrigation, which could begin or prolong scab infection periodsPlan the orchard to allow for sanitation practices such as mulching of overwintering leaves.
At present the main methods of control are the use of protectant and post-infection sprays during spring, and sanitation practices to reduce scab carryover. Protectant spray programs remain the basis of effective apple scab control, especially where disease pressure is high. They may be supplemented by the use of post-infection sprays and autumn sanitation practices.
Protectant sprays of suitable fungicides are needed at green tip, pink bud and at regular intervals afterwards, to ensure that rapidly-developing leaves and fruits are covered with a fungicide residue that will prevent spores from germinating.
- sprays
Some fungicides have the capacity to kill or suppress established infections. The period of curative action is short. If curative fungicides are to be effective, they must be applied within a few days from the beginning of the wet period that started the infection period and not from when scab lesions are first visible.
Most fungicides will not cure an established apple scab infection after the lesion becomes visible to the naked eye, but some will suppress the formation and germination of spores on the established lesion as long as spray residue remains.
Both mechanical and electronic instruments that can record leaf wetness and temperature in the orchard are available. enable the grower to determine when scab infection conditions have occurred. This knowledge allows more precise timing of scab sprays. In orchards with a low risk of disease, such as those in irrigation districts where rainfall in spring is low, a control program based only on post-infection sprays can be followed. This may save several applications of spray when compared with a routine program of spraying protectants. Post-infection sprays, based on accurate knowledge of infection periods, can also supplement a program of protectant sprays.
