Angle Vale leaf tatter and defoliation in almonds (AL05003)
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What was it all about?
The Angle Vale Leaf Tatter and Defoliation disorder (LTD) causes economic losses in almonds on the northern Adelaide Plains (NAP). No other production district has reported similar losses or symptoms. The losses result from defoliation and loss of photosynthetic capacity, bud and twig dieback, yield losses as a direct result of bud death and indirectly as a result of affected trees having higher levels of sticktight nuts at harvest.
The failure of LTD to develop on feral trees, during any season to-date, suggests chemicals or a chemical-biological interaction may be involved in the LTD disorder. An investigation of this was the focus of initial field trials.
In 2005/06 non-pareils were confirmed as the variety earliest and most severely affected. Symptoms were widespread on most non-pareil trees by mid-October 2005. The first symptoms were yellow, translucent lesions randomly spread across the leaf blade. Some of these lesions were preceded by greasy, reflective spotting. Leaves in the outer canopy appear worse-affected, although it has been noted that symptoms are usually widespread by the time of first detection within any one season, in susceptible varieties. The onset of symptoms is often sudden. The leaves become tattered and shotholed as the lesions develop necrotic centres. Affected leaves fall while still green.
Although the cause of LTD remains unknown, progress was made in 2005/06, on its management. A fungicide with two active constituents of differing chemistry, successfully delayed the establishment of the disorder in the 2005/06 season. Trees treated with this product (BAS 51604F) on two occasions during the season did not defoliate, and remained of healthy appearance until six weeks before harvest. All other fungicidal treatment programs successfully controlled the known fungal diseases of almonds but were ineffective on LTD.
In the 2006/07 season, BAS 51604F and its components were trialled on a commercial scale and chemical combinations with similar efficacy range, were applied as superimposed treatments. Other blocks were maintained untreated. Some were ‘chemical free’ while others received no foliar nutrients. The trial orchard had an extended and consistent history of LTD, however LTD did not develop in 2006/07.
Only one orchard on the NAP was observed to have LTD in 2006/07. The extended dry period from winter through February, resulted in few chemical crop protectants being applied in any orchards, negligible development of common almond diseases and lower orchard humidity generally. It is considered likely that these conditions and the lack of applied chemicals had an effect on LTD development.
Laboratory and greenhouse work confirmed that neither of the fungi consistently isolated from LTD lesions are primary almond leaf pathogens, nor likely causes of LTD. The scanning electron microscopy (SEM) work revealed that chemically-induced LTD-like lesions have distinct, erumpent and vertically-severed margins. ‘Field’ LTD lesions have a similar appearance at the leaf surface. Under SEM, LTD lesions appear to have neither diffuse margins nor a halo, as may be expected around a pathogen-caused lesion.
Consideration of risk factors associated with LTD, revealed neither planting material nor harvested products to be likely mechanisms for LTD introduction or spread. The disorder has an economic impact associated with yield decline and potentially, input costs. With the cause of
LTD remaining undefined, it has not been possible to assign risk ratings to the identified risk
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This project was funded by Hort Innovation (then Horticulture Australia Limited).
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