Apple scab, caused by the ascomycete fungus Venturia inaequalis, poses a troublesome challenge for apple growers as it infects both leaves and fruits. This fungal disease presents itself as dull black or grey-brown lesions on the surface of tree leaves, buds, or fruits. Additionally, lesions may occasionally appear on the woody tissues of the tree. Notably, the undersides of leaves and fruits are particularly vulnerable to infection.
While the disease rarely leads to the death of the apple tree, it can have a significant negative impact on both fruit yield and quality. The presence of black fungal lesions renders scabby fruits unsuitable for consumption and diminishes their market value.
The primary source of survival for apple scab lies in infected leaves that have fallen onto the ground and in the soil. During springtime, spores are released from the previous season’s fallen leaves, leading to fresh infections on young leaves and small fruitlets. As the season progresses, the leaf spots mature rapidly and generate new fungal spores, which then spread to other leaves, perpetuating new infections throughout the growth period.
The dispersal of fungal spores from primary lesions occurs through a combination of raindrops splashing and wind, affecting flowers, leaves, or fruits. These spores initiate further infections when the environmental conditions of temperature and leaf moisture enable them to germinate and establish themselves. These subsequent infections, known as secondary infections, typically occur within a tree or between neighboring trees, rather than over long distances.
Throughout the growing season, the secondary cycle of apple scab can occur multiple times, presenting a significant challenge for control, especially when the disease becomes established from primary infections. Managing apple scab becomes particularly difficult in the presence of frequent rainfall.
During periods of dampness or rain, newly emerging apple leaves are highly vulnerable to infection. The duration of leaf wetness directly influences the severity of the infection.
Wet and cool weather conditions create favorable conditions for the development and spread of the disease.
The initial appearance of disease symptoms is commonly observed on the undersides of leaves, and similar symptoms can be found on fruits as well.
At the onset, small, irregular spots or lesions emerge, displaying a light brown to olive green coloration. As the infection progresses, these lesions adopt a more circular shape, acquiring a velvety olive green texture, and eventually darken to a dark brown or black hue.
Infected tissue thickens, causing the upper surface to bulge upwards and the lower surface to develop a depression.
On mature leaves, the lesions typically exhibit raised characteristics, appearing as dark green to gray-brown patches with well-defined edges. This leads to a cupping effect on the underside of the leaf. Leaves heavily affected by scab may undergo curling, twisting, shriveling, and puckering, ultimately detaching prematurely from the tree.
If the leaf petioles become infected, early leaf drop occurs, while infection in the pedicels can lead to premature fruit drop.
In young fruits, scab infections initially appear as spots with an olive green to brown color. Over time, these lesions expand, becoming hardened and transforming into black, corky areas on the fruit that are unsuitable for consumption.
Severely infected fruits may exhibit deformities and are prone to cracking open.
Infections primarily take place under moist conditions, such as rain, dew, or constant irrigation. The severity of these infections is influenced by temperature variations.
To achieve comprehensive disease control, it is recommended to adhere to a fungicide spray schedule, especially for more susceptible apple varieties.
For effective control, it is crucial to thoroughly and uniformly cover all leaves and developing fruits with the fungicide. The use of a commercial spreader-sticker, such as INTEGRA, can ensure the foliage is adequately coated with the fungicide, promoting better coverage.
The following fungicides have been deemed suitable for combating apple scab:
- TRINITY GOLD 452WP: Mix 50g with 20 liters of water.
- COLONIZER 440WP: Mix 50g with 20 liters of water.
- ABSOLUTE 375SC: Mix 10ml with 20 liters of water.
- GREENCOP 500WP: Mix 50g with 20 liters of water.
- CHANCETYL ELITE 800WDG: Mix 50g with 20 liters of water.
- EXEMPO CURVE 250SC: Mix 15ml with 20 liters of water.
- GEARLOCK TURBO 250WP: Mix 25g with 20 liters of water.
- Implement proper sanitation practices.
- Promptly remove and dispose of infected leaves, flowers, and fruits.
- Opt for resistant varieties whenever feasible.
- Rake the area beneath the trees and eliminate infected leaves to minimize the availability of fungal spores that can initiate a new disease cycle.
- Water plants in the evening or early morning, avoiding overhead irrigation, to allow the leaves sufficient time to dry before the onset of potential infection.
- Practice crop rotations with non-host plants.
- It is essential to implement a fungicide resistance management strategy, as the fungus can quickly develop resistance to fungicides.
- Ensuring proper nutrition is crucial for enhancing the crop’s immunity.
- Repeat sprays should be conducted to achieve effective control.
- Timely management of the disease is of utmost importance.