What was it all about?

Phytophthora root rot is the most significant threat to Shepard avocado orchards in north Queensland. It attacks the fine feeder roots of avocado trees and reduces productivity by lowering yields, reducing fruit size, quality and shelf life. Severe infection can kill trees. Growers need to manage the disease in orchards using cultural and chemical management techniques to prevent widespread tree decline or deaths.

The main chemical control option for root rot is potassium phosphonate applied to the tree as a trunk injection or foliar sprays. Tree injections are costly so some growers are known to have moved to applying potassium phosphonate treatments only once during the year.

Phosphonate levels need to be raised in tree roots to be effective but once applied, the chemical moves to the part of the plant that is growing most actively. Existing data suggested that foliar sprays did not increase phosphonate levels in roots enough to control root rot over the critical summer period.

This project, which ran from May 2012 to May 2013, investigated the effects of timing of phosphonate application in five Shepard avocado orchards in far north Queensland. Trees continued to receive the farmer’s standard phosphonate trunk injection program (either one or two injections per annum) and three orchards were supplemented with an additional foliar phosphonate treatment.

Researchers monitored trees and sampled roots to determine seasonal variation in root phosphonate levels over the growing season.

Key findings included:

• Foliar phosphonate applications were found to be effective when applied at, or shortly before, the periods of peak root flushing
• Two key time periods to apply phosphonate were identified: March to June and mid-November to early December
• Applying phosphonate only once a year is likely to be inadequate for disease control.

The researchers recommend applying phosphonate at the time of root growth flushes, monitoring root levels during critical times and applying the correct amount and concentration, either as a high-volume spray or through injections. This should be done in conjunction with mulching and optimised nutrition and irrigation.

Hort Innovation reminds that all chemical use should adhere to permit and label instructions.