Why is it a problem? The major economic loss through apple scab is the substantial reduction in fruit quality of scabbed apples. Considerable management, including the use of fungicide, is needed to prevent loss of fruit quality (“improve apple colour”) in susceptible areas. More severe infections cause defoliation and reduces tree vigour, restricting bud formation.
Where and when is it a problem? It is a widespread problem in Bumthang and Haa, and in high and humid areas of Paro and Thimphu Dzongkhags (such as Begana and Yusipang). Apple scab was one of the three diseases (the other two being apple rust and premature leaf fall) that reduced marketable quality and quantity in the early 1990s in Bhutan. This resulted in the adaptation of calendar spray of fungicides during those years. The disease severity has subsided since then but it is noted to be re-emerging.
Symptoms can occur on leaf blades, fruits, petioles, sepals, blossoms, young shoots and bud scales. On leaves, lesions may occur singly or be scattered over the entire leaf surface. Earliest symptoms appear on the underside of the leaves, extending along the veins and the midrib with irregular and poorly defined borders. On leaves the spots are at first a velvety olive-brown with a feathery margin. On the upper surface, lesions occur as a lighter shade of green compared with the healthy surface. The lesions are circular and increases in size. Later the inner portion of the lesions may become gray and some spots look nearly black in colour. Lesions on fruit are very small at first and are darker and more sharply bordered than leaf lesions. Fruits infected young become deformed. Scab lesions on fruits enlarge and cover a large area and the fruits sometimes crack due to the callus tissue not expanding as the fruit enlarges.
Confusion with other pests: Apple scab lesions are characteristic, especially on fruit.
Lifecycle: In spring, the new infection of leaves and fruits arises from the sexual spores of the fungus that have survived the winter season in the fallen infected leaves left from the previous year. The sexual spores are the primary source of infection and are important in causing apple scab in the orchards. These sexual spores are discharged into the air from early spring to about the fruitlet stage. The discharged spores become lodged in leaves and fruit, producing new infections if moisture and temperature is suitable. Young leaves are very susceptible to infection. The leaves and fruits become more resistant as they mature but are not immune. Further infection will spread from the infected young leaves and fruit through the production of asexual spores. The asexual spores are mainly spread by splashing of water within the tree. Aerial dispersal can occur but is insignificant compared to wet dispersal within a tree. Thus, if the tree has been protected from early infection it will not be infected by its neighbours.
Dispersal: Sexual spores present early in the season, including in fallen leaves, are wind-borne. Asexual spores produced during the season are mainly splash-dispersed within the canopy.
When can damage be expected? It usually appears in early to mid-spring and can be more prevalent during rainy weather.
Hosts: It is a globally important pest of apple, but has been reported from pears and other related species. In Bhutan, it has only been reported on apple.
In other countries, where resistant varieties are now widely grown, chemical spray against apple scab is said to be rare. In Bhutan, the popular varieties such as Red and Golden Delicious are susceptible to apple scab. Therefore, constant monitoring in addition to adoption of good cultural practices are required to manage the disease. Calendar spray of fungicide along with cultural practices is recommended in conducive areas with severe outbreak of the disease.
For apple scab, assessing scab infection is important in the autumn to prepare for the next season. Therefore, it is advisable to assess the level of foliar infection after harvest (before leaf fall). High levels of foliar infection at the end of the season warrants scheduled fungicide spraying during the next season. It is difficult for Bhutanese farmers’ (or mostly caretakers) to conduct actual assessment of infection level. Therefore, it is recommended to contact NPPC for technical help.
Effect of variety
Different apple varieties can differ in their susceptibility. Although varietal effect has not been evaluated in Bhutan, Red and Golden Delicious are known to be susceptible to scab.
- Destroy fallen leaves over the winter by burning or burying. These leaves serve as the primary source of fungal spores that infect trees in the following season. Destroying them will help break the cycle of infection within the orchard.
- Alternatively, spray leaves with 5% urea (5.4 % kg per 50 litres of water) after harvest (around November or leaf fall period). This includes spraying leaves which are still on the tree as well as on those which are on the ground. This will speed up decomposition.
- Trees should be well-pruned to enhance drying and improve spray coverage.
- Timing of chemical spray is critical to prevent wasted money and effort and avoid failure.
- The following spray schedule is recommended for susceptible areas where there are disease outbreaks or where severe infections were observed in the previous year. Sometimes calendar sprays will be needed for 2-3 years. Monitoring the orchard for infection levels in autumn is essential to decide whether sprays need to be continued in the following season.
- 1st spray: hexaconazole (100 ml per 100 litres of water) at silver to green tip.
- 2nd spray: mancozeb (200 gm per 100 litres of water) at pink bud stage to early bloom (roughly 10 days after the 1st spray).
- 3rd spray: carbendazim (50 gm per 100 litres) at petal fall (about 14 days after the 2nd spray).
- 4th spray: captan (200 gm per 100 litres) at fruit development (14 days after the 3rd spray).
- 5th spray: urea (5 kg per 100 litres) at pre-leaf fall stage.
Image acknowledgements: Bugwood.org