A management strategy has been developed to tackle a devastating disease of apples that has become established during the past 10 years in Bilpin, NSW, and in the Granite Belt regions of Queensland.

The disease, Alternaria leaf and fruit spot, is caused by the fungal pathogen Alternaria mali, which infects high-value varieties of apples like Gala and Cripps pink.

The disease is characterised by large red-brown lesions with purplish borders on leaves, which first appear in December, but rapidly expand.

If an infected orchard receives heavy rain in February, trees can lose all of their leaves within a week.

This weakens trees, fruit is sunburned, and the reduction in the reserves produced by photosynthesis leads to reduced fruit quality next season.

Equally serious is a fruit spot produced by this disease, which can reduce fruit production by more than five per cent.

NSW Department of Primary Industries (DPI) temperate fruits industry leader, Dr Shane Hetherington, said the majority of fungicide applications in NSW orchards were intended to control the disease black spot, also known as apple scab.

"The apple industry has conscientiously reduced their fungicide applications and now tends to only apply management in response to weather conducive to black spot development," he said.

"Ironically, this reduction in fungicide application has led to the emergence of diseases which had not previously been a problem.

"This may be the case for Alternaria."

He said orchardists’ strategies for black spot seemed to have no effect on this new disease.

"Management strategies from overseas were trialled and failed over a series of seasons," he said.

"Orchardists don’t like applying extra sprays because they cost money and aren’t in line with what apple consumers want."

A collaborative project between NSW DPI and Queensland Department of Primary Industries and Fisheries was funded to test whether an effective black spot management strategy could be developed which also resulted in a lowering of Alternaria leaf and fruit spot disease severity.

Dr Hetherington said trials in Queensland highlighted three fungicides - mancozeb, metiram and dithianon, which, when applied late in the season for black spot control, led to lower Alternaria severity.

Research in NSW followed to determine if the Queensland results could be improved to give an even better result for orchardists.

"In addition to being an extremely effective early season fungicide for black spot, Trifloxystrobin was thought to also inhibit Alternaria mali," Dr Hetherington said.

"However, where this fungicide was applied without the three fungicides highlighted by the Queensland research, it had only a very small effect."

He said the surprise came when Trifloxystrobin was applied early in the season and metiram was applied late in the season.

"Trifloxystrobin applications make metiram applications more effective," he said.

Where no fungicides were applied, nearly 100pc of leaves were infected by harvest.

"Only 30pc of leaves were infected where both Trifloxstrobin and metiram were applied," he said.

"This reduction is likely to result in less diseased material being available to start the disease next season."

He said the challenge now was to see if the management strategy was repeated over a series of seasons, whether the disease incidence would be decreased even further.

Contact Dr Shane Hetherington, (02) 6391 3860, shane.hetherington@dpi.nsw.gov.au