Why is it a problem? This fungus is mostly a problem in nurseries and young orchards where it prevents growth, although new growth in older orchards can also be affected. Severe infections kill branches. It can also attack young fruitlets causing distortion and premature fruit drop.
Where and when is it a problem? Powdery mildew occurs in almost all the citrus growing regions of Bhutan, especially in Samtse, Pemagatshel, Chukha, Tsirang and Wangdue Phodrang Dzongkhags. It is more severe during spring and late autumn in the humid subtropical zones of southern Bhutan and is particularly prevalent in nurseries, shady orchards and orchards in low valleys where aeration is poor. Depending on orchard location, powdery mildew can be a significant problem in recently pruned trees, infecting all the new re-growth.
All aerial parts of the plant are susceptible to infection, but the fungus mainly attacks young, growing leaves and twigs. Initial symptoms develop on young shoots on the lower half of the tree. The disease can easily be recognised by the appearance of white powdery patches of mycelium on twigs and both surfaces of the leaves. However, the white mycelia are not visible during heavy rainfall as they are washed off the trees. Infected young leaves become deformed, shrivel and fall prematurely. In severely affected twigs dieback is common. In that case shoots die progressively, beginning at the tips. The fungus can also attack young fruits, causing them to drop prematurely.
Confusion with other pests: Powdery mildew is not easily mistaken with other diseases, especially when the white mycelia are present on the plants. However, there may be more than one species of powdery mildew in Bhutan, and species-level identifications would need to be confirmed by a specialist. Nonetheless, species identification will not affect management. Tree dieback due to powdery mildew may be mistaken for HLB symptom.
Lifecycle: Powdery mildew overwinters as mycelium in buds. Warm and moist conditions during spring and autumn favours germination. The fungus can survive on both leaves and twigs throughout the year. The young leaves are infected during warm humid conditions and the fungus spreads rapidly down the young shoots.
Dispersal: The spores (conidia) are easily dispersed by wind, clothing, equipment (pruning saws, secateurs) and vehicles.
When can damage be expected? It is more severe during spring and late autumn in the humid subtropical zones of southern Bhutan. New infections are not common during periods of heavy rain. However, prolonged periods with heavy morning dew or mist in combination with a few hours of sunshine create ideal conditions for the spread and growth of this fungus.
Hosts: In Bhutan it has been found on mandarin and some Citrus hybrids being grown at research centres.
Powdery mildew should not normally be a problem in well managed and located orchards. Regular monitoring and timely management should prevent this disease from becoming a serious problem.
Inspection for powdery mildew should be part of regular orchard inspections. Greater vigilance is needed in nurseries and in orchards with a history of problems.
Effect of variety
It can attack all forms of citrus. However, cultivars like mandarins, sweet oranges and tangerines are highly susceptible, while varieties like lemon, lime and citron are only moderately susceptible.
The disease can be controlled through a combination of measures:
- Select sunny orchard sites, and ensure proper planting distances as clustered trees will create more humid conditions.
- If only a few plants are affected then prune off the affected plant parts to prevent further spread. Pruned plant parts should be collected and burned to prevent disease spread by wind. Pruning of severely diseased plant parts should be conducted in spring and summer after the first and second flush.
Chemical treatment is feasible only in nurseries and young orchards. Two sprays of wettable sulphur at 14-day interval gives good control. Contact NPPC for more details on chemical management.
Image acknowledgements: NPPC