Apple scab or black spot is a disease that infects leaves, shoots, buds, blossoms and fruit caused by the fungus Venturia inaequalis. This fungus may infect developing flowers, but is frequently seen on reproductive parts after the fruit has set. Usually Apple scab occurs on apple, crab-apple and many other species in the genus Malus.
Infection of fruit stalks usually causes the young fruit to fall. This disease occurs almost in many parts of Pakistan where apples are grown and is most serious and widespread disease of this crop, particularly in regions with high rainfall and relative humidity during the growing season.
During early infection distortion is caused, due to which growth under the lesion stops while the surrounding tissue develops normally. Mature fruit during late infections appear as dark, roughly-circular spots and at the centers of large lesions appears as dry, corky and a greyish, along the border of the lesion raised cuticle may be evident. While the corky areas may crack, thus allowing entry of fruit-rotting. Thus in case of storage lesions may continue to develop from symptomless infections present before harvest, causing storage scab.
Apple scab is most severe in districts with high rainfall especially during spring. The disease can decrease the quality and also size of fruit resulting in premature fruit drop, defoliation and poor development of fruit buds in the next season. During these favorable conditions and 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.
Few Control measures can effectively reduce disease in apple orchard .So, in the start when you plan a new orchard:
- Use varieties which are scab-resistant.
- High spring rainfall locations must be avoided for the plantation of apple.
- Always use dwarfing rootstocks and training systems to maintain an open and easily sprayed tree canopy.
- Overhead irrigation must be avoided, which could commence or prolong scab infection periods
- Sanitation practices must be allowed while planning orchard such as mulching of overwintering leaves.
Currently the key methods of control are the proper use of protectant, post-infection sprays during spring, and sanitation practices to reduce Apple scab. Where disease pressure is high, protectant spray programs goes on the basis of effective apple scab control. They may be added by the use of post-infection sprays and autumn sanitation practices.
Protectant sprays of appropriate fungicides are required at green tip, pink bud and at consistent intervals afterwards, to confirm that rapidly-developing leaves and fruits are covered with a fungicide deposit that will inhibit spores from germinating.
Some fungicides have the ability to suppress or kill infections. So, the period of healing action is short. If we want these fungicides to be effective, they should be applied within a few days from the commencement of the wet period. Most of the fungicides will not treat an established apple scab infection once the lesion becomes visible, while some will suppress the development and germination of spores.
Other methods include sanitation to overcome scab carryover. Sanitation management practices can greatly reduce the risk of scab in spring season and improve the efficiency of scab control programs. Some practices of sanitation include:
- Leaves on the tree can be immediately treated with nitrogenous fertilizer before leaf fall.
- Mulch the leaf litter after leaf fall. Use a mechanical shredder and sweep leaves into rows, slasher or flail mower to chop leaves into small pieces which then break down more rapidly.
- Leaf mulching can be combined with a ground application of a nitrogenous fertilizer.
Following above recommendations will help in managing apple scab which is reducing production and quality of Apple in Pakistan and all over the world. Jazz Bakhabar Kissan is a service which provides complete information regarding Agriculture crops grown in Pakistan. Farmers can directly call on help line or through Photo Analysis for free recommendation by our agriculture specialists. We help them in controlling the diseases of fruits, vegetables and field crops thus improving crop productivity.