Alternaria mali or Gloeodes pomigena
Why is it a problem? As the name suggests, premature leaf fall (PLF) disease of apple results in defoliation of parts or entire trees while apples are still developing. On rare occasions, it can also infect apples which become black by harvest time. Severe problems can be avoided through good orchard practices.
Where and when is it a problem? PLF is not widespread. It can be serious around Paro and Thimphu, with some orchards becoming completely defoliated on occasion. In Bhutan, PLF problems were first observed in 1995.
In Bhutan, PLF can be caused by two unrelated pathogens, Alternaria mali (leaf spot on apple) and Gloeodes pomigena (sooty blotch). Confirming the identification of the responsible pathogen is difficult and requires a specialist. Recent studies in the USA found about thirty putative fungal species associated with apple leaf blotch and flyspeck.
Brownish leaf spots on the upper side of mature leaves appear in late May. When they are numerous they connect. The surrounding tissue turns yellowish and defoliation occurs. The infection can affect a few branches of the tree but also the whole tree. Affected branches are very conspicuous since they are already yellow by June or July. In severe cases, only small apples can be seen hanging on trees with no leaves or a few yellow leaves. Fruits are generally not affected, but in severe cases of leaf attack, brown spots also appear on the surface of the apple. Affected apples are almost black at harvest time.
Confusion with other pests: Leaf blotch due to soil moisture fluctuation at the end of summer or early autumn may be confused with leaf blotching due to PLF. Phytophthora trunk and root rot may cause similar symptoms of yellowing leaves.
Lifecycle: Infected leaves of the previous season serve as the primary source of inoculum for the following season. Mature spores are produced just before the bloom stage of bud development. Discharging of all spores takes 3-4 weeks. Rain is needed for the release of spores. The first brownish leaf spots appear on the upper side of mature leaves towards the end of May. Infection can affect a few branches but also the whole tree. Sometimes it also infects the surface of apples.
Dispersal: Spores are dispersed by wind and rain splashes.
When can damage be expected? PLF is more prominent in June, especially with increased rainfall and warmer days.
Hosts: Fungi associated with leaf blotch/PLF are found on wide range of plants including cultivated and wild apples, black berries and willows. In Bhutan, they have only been associated with apples.
Management of PLF is achieved through orchard sanitation, pruning and fungicide spray.
Look for symptoms as part of regular orchard inspections, especially yellowing of foliage on branches by June/July. Monitoring is critical for determining whether spraying of fungicides is necessary.
Effect of variety
- A spray of 5 % urea (5 kg per 100 litre) before leaf fall helps in reducing the inoculum of the fungus by speeding up decomposition process of the leaves. Leaves on the ground must also be sprayed.
- Good pruning allows adequate air circulation and helps to reduce disease development.
- Biological control: In America sprays with an antagonist fungus (Chaetomium globosum) are used to control blotch fungus and could be explored as a control method for Bhutan.
- If the orchard has had no history of previous PLF attacks, then fungicides should be used as the first symptoms are noticed:
- Spray mancozeb (2 gm per litre of water), carbendazim (0.5 gm per litre), copper oxychloride (2 gm per litre) or Captan (2 gm per litre).
- Give a second spray after two weeks using a fungicide different from the one used for the first spray.
If PLF also occurred in the previous season, the following spray schedule is recommended:
- 1st spray: Apply a mixture of mancozeb (200 gm in 100 litres of water) and carbendazim (50 gm in 100 litres of water) at petal fall stage.
- 2nd spray: apply mancozeb 14 days after the first spray.
- 3rd spray: apply mixture of mancozeb and carbendazim 14 days after the second spray.
Image acknowledgements: NPPC.