An emerging disease in Australian apple orchards—affecting apple leaves and fruit mid to late season in high spring/summer rainfall areas— Alternaria can cause significant yield and fruit loss, with the disease progressing from leaf infection and premature defoliation to fruit spotting.
Rated alongside apple scab as the most significant apple diseases in Queensland, Alternaria has also been a problem in NSW orchards around the Sydney basin and Orange, and in Western Australia when warm, wet weather favours the disease.
There have been reports of Alternaria-like symptoms in both South Australia and Victoria.
Research to control Alternaria
The increasing incidence of Alternaria—and limited registered fungicides for its control—led Horticulture Australia and the apple and pear growers organisation, APAL, to fund research in 2001 into the disease and its potential control with existing orchard fungicides.
Research by Qld Dept of Agriculture, Fisheries & Forestry plant pathologist, Christine Horlock and colleagues, reported that the Alternaria species impacting on Qld, NSW and WA orchards were also widespread in all major Australian apple-production areas.
The research confirmed that Alternaria leaf blotch caused significant premature leaf defoliation as early as January, with the majority of Alternaria fruit spots appearing two to four weeks prior to harvest.
A Qld-gov/Uni Qld/HAL–backed PhD study by Dalphy Harteveld, confirmed symptoms of leaf blotch developing from November to February, with fruit spot peaking in January and ongoing up to harvest in February.
Dragon 700WG for management of Alternaria
Data from QDAFF and private research-company field trials on susceptible varieties demonstrated that the multi-site protectant fungicide Dragon 700WG would be effective in reducing the number of infected leaves later in the season, preventing premature defoliation and reducing the number of fruit spots.
On the basis of this research, Crop Care applied to the APVMA for Alternaria to be added to the label of Dragon 700WG—which was already used by Australian orchardists against black spot/apple scab and bitter rot.
Crop Care regulatory affairs manager Bronwyn Vorpagel said the application had been successful, and that Australian growers could use Dragon 700WG for Alternaria management this season at the same registered rate as for apple scab and bitter rot: 18g/100L with a WHP of 21 days.
“It is the only dithianon (Group M9) fungicide registered in Australia for Alternaria.”
Combined with the critical strategy of good orchard hygiene to reduce accumulation of Alternaria-infected leaf litter, Ms Vorpagel said researchers had concluded that:
The primary infection occurred in spring, with the number of infections increasing rapidly in warm, wet weather.
A disease-management strategy could save the apple industry between 15 to 25 per cent in lost production of high-value varieties.
An effective, registered fungicide remained the most important factor in symptom reduction.
Timing fungicide applications to reduce the progression of Alternaria leaf blotch can prevent extensive defoliation and remove the source of fruit infection.
The best timing for applying fungicide to control Alternaria leaf blotch and fruit spot in apples was during flowering and early fruit development stages, before any symptoms appeared.
Preventative measures
Ms Vorpagel encouraged Qld and NSW apple growers with significant Alternaria infection last season to use preventative measures this season.
“Using a broad-spectrum fungicide like Dragon 700WG several times during early-mid growing season for apple scab/black spot will also reduce Alternaria in the orchard.
“After the black spot season, growers are advised to make further approved fungicide applications to manage Alternaria fruit spot in Gala, Pink Lady™ and Red Delicious varieties—especially from eight weeks prior to harvest up to the recommended WHP.”
In WA, Alternaria outbreaks have been associated with higher than average spring/summer rainfall, so she advised WA growers to make fungicide applications on the basis of weather.
“Even with average rainfall, WA orchards with a history of Alternaria infection should also be treated with Dragon 700 WG in a protectant program.”
Similarly, she advised growers in other states to monitor for leaf blotch if spring/summer rainfall occurred, and to begin a preventative fungicide program at the earliest sign of leaf infection to protect fruit.
Contact Bronwyn Vorpagel, Regulatory Affairs Manager, Crop Care Australasia
phone 07 3909 2017
cropcare.com.au
See this article in Tree Fruit June 2015
