This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

The Australian almond industry observed in spring 2008 a widespread bud growth disorder in the pollinator, Carmel. Carmel was the only cultivar seriously affected in Australia. The disorder presented as extensive areas of bare wood, tufted terminal growth on some shoots, sparse canopies, poor leaf out and in some cases reduced flowering and low nut set. This project was initiated to determine the potential cause of this disorder and its likelihood of spreading. The outputs included several reports and a fact sheet and literature review on bud development and factors that affect it.

This 18-month investigation focussed on bud dissections from ‘affected’ and ‘non-affected’ trees. The Carmel trees were from orchards across three production districts that had experienced recent heatwaves. Bud dissections from affected trees, revealed that many buds that appeared to be normal externally, had internal necrosis. Damaged buds did not recover and central buds sustained more damage generally than outside buds, at each node.

Damaged buds were visible by early summer, but the incidence and extent of damage increased as the season progressed. Predictions of bud emergence problems in spring could be made from bud dissections in the previous autumn, with some accuracy. Budsticks with high levels of central bud damage had low leaf out. Necrotic outside buds generally had a low percentage and/or delayed transformation to flower buds. Field visits and dissections have provided evidence that once bud failure was seen, an increasing proportion of the tree’s canopy became unproductive each season.

In general, trees planted in 2005 and later appear to have suffered more bud failure than mature trees in the same districts. There was evidence that during the almond planting boom, which peaked in 2006-07, budwood was sourced from locations outside the industry’s Monash budwood repository. Our testing of Monash buds demonstrated their superior health status and size and it was recommended that future Carmel budwood only be sourced from this location. Specifically, the low budsticks closest to the original tree bud were more likely to support buds with low bud failure potential. Similarly, trees from the original buds/budstick introductions of Carmel into Australia, were likely to have low bud failure potential and their locations other than at Monash, and their orchard performances during recent heatwaves, were determined in further investigations of bud failure.

The contribution of bud genetics to non-infectious bud failure in Carmel in California had been well researched. It had been found that bud genetics determine the bud failure potential, but high temperatures at critical bud development periods, and annual growth rates, influence the onset of bud failure. It appears that Australia’s Carmel bud failure was not readily distinguishable from non-infectious bud failure in California.

A review of temperatures during recent growing seasons in the Riverland (Renmark, SA), Sunraysia (Mildura, Vic) and the MIA/Riverina (Griffith, NSW) revealed that the heatwaves of March 2008, January 2009 and November 2009 had likely contributed to the widespread expression of bud failure, especially in young trees. Young trees have extensive annual growth, and limited reproductive growth. The November 2009 heatwave appears correlated with the widely-reported poor leaf out in Carmel this spring (September 2010). Bud failure in previously non-affected trees had been seen this spring, as was predicted through the autumn bud dissections. Some affected trees showed evidence of three years of abnormal growth and 90 per cent canopy bareness, yet none had died.

Bud failure, once triggered cannot be eliminated or controlled. Although our cursory attempt at top-working was not successful, top-working was a recognised management option for non-infectious bud failure in California. It relies however on the capability to identify good buds (low bud failure potential). Orchard management requires consideration of the economic viability of affected trees. Our recommendation was to remove young (fourth leaf or younger), affected Carmel trees if more than 30% of the canopy was bare.