Temprate Fruits (1+1)

SPECIAL PROBLEMS: Premature leaf fall

Premature leaf fall

• Apple plantations in Himachal Pradesh have been ravaged by a unique phenomenon of leaf shedding in mid-summer for the last few years.
• The problem starts in the month of June-July and by mid August majority of orchards affected and severe cases only fruits are seen hanging on the defoliated branches near maturation.
• The disease which was first noticed in 1995 in some orchards, which now spread to all apple growing districts of the State.
• All the commercial Delicious cultivars are susceptible. Premature leaf shedding has also been reported recently from Kashmir, neighboring Uttaranchal and Bhutan.

Symptoms

• Disease symptoms first appear as dark green circular patches on upper surface on the mature leaves giving rise to 5-10 mm size brown leaf spots especially in the months of June and July, which turn dark brown in the due course.
• When lesions are numerous they coalesce to from larger dark brown blotches and the surrounding areas turn yellow.
• Severe leaf shedding follows these symptoms in the following weeks quite a head of natural leaf fall in autumn.
• In affected orchards, fruits nearing maturity are commonly seen hanging on the defoliated branches.
• Symptoms also appear on the fruit as clear brown spots, which are initially circular (3-5 mm in diameter) and become oval, depressed and dark brown later.
• Numerous small black colour pinhead specks, the acervuli are visible in the affected tissues.

Casual organism and disease development

• This disease is caused by Marssonina coronaria,but alternaria leaf blight is also involved in this malady.
• This fungus is reported to perennate in the fallen leaf litter on the orchard floor. The seeds of the fungus are mature by the time of blooming in the spring and they are liberated in the orchard for a quite long period.
• In Himachal Pradesh, the perfect stage of this fungus is not frequently intercepted.
• This fungus was found to perennate in the infected leaf litter in the form of acervuli which produce fresh conidia in early summer to start primary infections.
• Frequent rains are helpful for disease development. Infections first appear on mature leaves turning yellow and abscise prematurely.
• Countless conidia and microconidia are formed on diseased leaves which cause secondary infections leading to epiphytitic development in favorable humid conditions.

Management

• This disease can be controlled effectively integrating different technologies like field sanitation, proper pruning and judicious use of fungicides as follows:
• The orchardists are advised to collect and destroy the fallen leaves from the orchard floor in winter. Urea (5%) spray on the leaf litter is also be helpful in reducing the primary inoculums by enhanced decomposition of leaves.
• Proper pruning allows adequate air circulation in the tree canopy thereby modifying the microclimate and reducing disease development.
• Protective 3-4 sprays of fungicides like mancozeb (0.3%), carbendazim (0.05%), thiophanate methyl (0.05%), benomyl (0.05%), propineb (0.3%), dodine (0.075%), ziram (0.3%), dithianon (0.05%) and zineb (0.3%) are effective in controlling the disease.