An integrated disease management program for the Australian almond industry (AL16005)
What’s it all about?
Complementing the work of the industry’s integrated pest management program, funded through An integrated pest management program for the Australian almond industry (AL16009), this program is tasked with helping growers improve the on-farm management of key almond diseases. It will culminate in the development of integrated disease management (IDM) guidelines for use in almond production.
Since the last project update, the team performed the third year of fruit abscission trials aimed at understanding the physiological basis of fruit drop, along with second year trials to evaluate the role of tree carbohydrate status on flowering and fruit set.
Understanding the physiological basis of fruit drop
Results indicated that carbohydrate metabolism and signalling are key physiological drivers of fruit abscission. They also showed that in well managed orchards nitrogen and essential elements do not play a role in fruit abscission, however a relationship between aluminium and fruit abscission was discovered. Further studies will be undertaken to better understand and expand on these findings.
Evaluating the role of tree carbohydrate status on flowering and fruit set
The team found that while tree carbohydrates influence the level of flowering and fruit set, increasing avocado yield will require new management tools to decrease fruit abscission and manipulate fruit set.
Field trials planned for the 2020-21 season
Field trials for 2020-2021 will address the physiology of fruit abscission and the potential for practical applications for managing fruit drop.
The trials include:
- Drought stress-fruit abscission trial with the aim of linking changes in tree carbohydrate status with fruit abscission
- Fruit growth trial utilising fruit dendrometers to characterise changes in fruit growth rates with abscission and determine if they can predict fruit abscission events
- Analysis to determine if fungal infections are involved in avocado fruit abscission and, if so, evaluate which pathogens are involved and whether fungicide treatments can reduce impact.
Since the last project update, the research team continued with the following activities.
- Industry-wide surveys conducted during the 2018-19 and 2019-2020 seasons provided an understanding of the national distribution of diseases in almonds. More than 2,000 trees were tagged, and it was found that Shot hole and lower limb dieback (LLD) were the most common, followed by hull rot and trunk diseases, with other ailments very minor.
- Research activity focused on management strategies for hull rot and the trunk disease complex for Australian almonds. Field trials investigated the effects of mummy density, spatial variation, varietal differences and nitrogen and water restriction. No relationship was observed between mummy density and hull rot disease outcome, however a nitrogen/water restriction trial demonstrated that limiting water reduces hull rot severity. The relationship between high tree water status and increased disease continues to be examined.
- In vitro fungicide testing commenced on Rhizopus stolonifer. Although incomplete, early results for several fungicides and a biological control agent are promising, with tests continuing.
- Five pathogenic fungal species related to trunk diseases were tested in vitro for inhibition of mycelial growth. More than 16 products were evaluated in the first round, with more products to be added, with all experiments to then be repeated.
- The almond breeding program has several trial plots which include varieties Nonpareil, Carmel and Price plus 21 new breeding lines. A plot planted in 2013 was assessed for hull rot severity, mummy numbers and mummy infection over the 2018-19 and 2019-20 seasons. Hull rot severity varied considerably between varieties.
- Phytophthora disease was diagnosed in at least 15 orchards in the Riverland and Adelaide Plains regions during visits and after surveys in the 2019/20 season. Nine Phytophthora spp. isolates were collected and identified. Future trials will be conducted to gain a better understanding of this disease.
- In October 2019, SARDI scientists conducted an intensive study in the Riverland with the aim to develop an easy-to-use visual assessment scale for LLD and better understand the syndrome. This study will be repeated in other orchards during spring and summer 2020.
The research team has been progressing on the following activities:
Industry-wide disease surveys
The second year of surveys have been completed showing that shothole and lower limb dieback were the most prevalent diseases, followed by hull rot and trunk diseases. However, whilst shothole was the most frequently recorded disease, it was always observed at a low severity and is not currently a disease of economic concern. Regional differences were observed and it was found that disease influence was significantly influenced by certain agronomic factors (e.g. water, tree age, variety, tree density and fungicide use), varying according to the disease. The third year of surveys began in October 2019.
You can read more about the disease surveys in the following two fact sheets:
Hull rot field trials
Field trials were assessed for hull rot, investigating the effects of mummy density, spatial variation, nitrogen and water restriction, and variety differences. Across all trials, increased numbers of mummies on trees over winter, both pre-season and post-season, correlated weakly with hull rot severity at harvest. In general, however, many mummies had no sign of infection, indicating that other factors also contribute to mummy formation. Restricted nitrogen resulted in fewer mummies retained on the trees, and fewer infected mummies. Restricted water resulted in less hull rot in the variety Carmel. Infected nuts had high levels of fumaric aid, the toxic metabolite produced by the hull rot pathogen, and this was at significantly lower levels in infected nuts taken from the restricted water treatments. There was considerable variation in hull rot severity across 24 varieties from the almond breeding program, with Nonpareil being the worst affected.
Risk factors for hull rot were identified as rain events 7 to 20 days prior to harvest, which were significantly correlated with hull rot development. There was a strong correlation between the amount of rain and the amount of disease. The optimum temperature range for Rhizopus growth was determined to be 20 – 35 C, with growth suppressed but not killed at 40 C and temperatures below 15 C. Several risk factors for LLD were identified. The most influential risk factors are shading, water management, presence of trunk disease pathogens, and varietal susceptibility. In addition, risk factors for trunk disease include rootstock and varietal susceptibility, irrigation practices, tree age and the use of “alternative” products.
In vitro fungicide testing
This activity is underway with a list of candidate products finalised, including both chemical fungicides and biological alternatives. The candidates were selected based on evidence of efficacy on trunk pathogens, registration status, commitment by supplier to registration process and advice from key stakeholders such as the Almond Board of Australia, chemical resellers and Hort Innovation.
The research team has carried out four trials to examine the effect of sanitation (in collaboration with An integrated pest management program for the Australian almond industry (AL16009)), as well as water, nitrogen and variety (with projects Identifying factors that influence spur productivity in almond (AL14005) and Australian almond variety evaluation and commercialisation program (AL12015)) on hull rot incidence and severity in almond.
Overall, the team found only a weak association between increased numbers of mummies on trees over winter with high severity of hull rot at harvest. They found fewer mummies and fewer infected mummies under low nitrogen conditions, and less disease severity under low water conditions. There was considerable variation in hull rot severity across 24 varieties from the almond breeding program, with Nonpareil being the worst affected. Trees will continue to be assessed and researchers are interested to find out whether the severity of hull rot influences the number of mummies left on the trees after harvest.
According to an industry-wide survey conducted in 2018, the most common disease reported by growers is hull rot, followed by lower limb dieback, rust, anthracnose, phytophthora and shothole, with all of these, except rust and shothole, perceived as having an impact on yield. These findings were used to guide disease surveys in the second year of the project.
The disease survey was carried out in spring (October to December 2018) and in summer (January to February 2019). A total of 2,112 trees were assessed in 1,169 ha of almond plantings within 10,179 ha of orchards across NSW, SA, Victoria and WA.
The survey confirmed that lower limb dieback and shot hole are prominent, with hull rot and trunk diseases also occurring, though less frequently than anticipated. Rust and anthracnose were recorded at very low levels except for WA, which also had very low levels of hull rot.
Samples from trees with hull rot strike symptoms during the surveys have shown that Rhizopus stolonifer is almost exclusively the cause of this disease in Australia.
Samples from trees with trunk disease symptoms frequently had fungal species present, including Diplodia seriata, Eutypa lata, Cytospora spp. and Collophora spp. Other commonly isolated organisms include Alternaria spp. and Fusarium spp, as well as several unidentified organisms.
Seven fungal species, including those found on trees with trunk disease symptoms, are being tested in a preliminary trial to find out which causes lower limb dieback syndrome and trunk disease. The trial, set up by SARDI in Adelaide late in 2018, involves infecting branches of trees with the various species. In May 2019, branches that have been incubating for 6 months will be harvested and assessed.
In order to develop IDM guidelines for growers, the program is undertaking disease surveys across Australia’s major almond-producing regions to look at the prevalence and impact of endemic diseases including, but not limited to, hull rot and lower-limb dieback. The surveys will take place over three years of the investment and be timed to coincide with symptom emergence of symptoms of the major almond diseases. The surveys will also look at factors contributing to disease increase, and the effectiveness of current management practices.
In recent months, the project team asked growers to take part in a quick and anonymous online survey to provide essential baseline information for planning the disease surveys. At the time of last reporting to Hort Innovation, field visits had also taken place in Victoria, South Australia and New South Wales, allowing the researchers to view production practices, collect pathogen samples for use in future work, and visit potential industry collaborators and trial sites.
As information is gathered, the program will progress trials to better understand the major diseases and to identify and tailor effective management practices for them, including cultural, biological and chemical options.
This project is a strategic levy investment in the Hort Innovation Almond Fund