RESEARCH & EVENTS
Phytophthora diseases of almonds
By Brittany Oswald 1 , 2 and Mark
Sosnowski 1 , 3
¹ South Australian Research and Development Institute ( SARDI ), Plant Research Centre , Urrbrae , SA .
2
Centre for Horticultural Science , The University of Queensland , Queensland Alliance for Agriculture and Food Innovation , Ecosciences Precinct , Brisbane , QLD .
3
School of Agriculture , Food and Wine , Waite Research Institute , The University of Adelaide ,
SA .
Background Phytophthora spp . are destructive pathogens impacting various crops , including almonds . These pathogens , present in Australian almonds since the 1980s , have become widespread , especially in the Riverina , in mature trees and newly established orchards . They cause root rot , crown rot , and aerial cankers .
Symptoms Infected root hairs rot , impeding water and nutrient uptake , leading to gradual decline or sudden death when the tree is under stress . Crown rot originates at the base of the infected tree . Sapwood becomes discoloured and will sometimes eventuate in gumming on the bark ( Figure 1A ). If severe , the disease can kill the host . Aerial Cankers are characterised by gumming in the trunk and scaffolds ( Figure 1B ). These cankers spread rapidly , causing discolouration in the sapwood ( Figure 1C ). In mature trees , cankers often start at the tree crotch , while in young trees , amber gum appears around the middle of the trunk ( Figure 1D ).
Management Agronomic Practices New orchard sites should be selected carefully with well-drained soils to prevent water-logging . Ideally , sites that are not prone to flooding would be selected . In high-rainfall regions with clay soils , or in areas where water cannot drain freely , prepare the site by creating mounds or ridges to prevent from water pooling around trunks . Although almond scions can be infected directly , the rootstock is
A B C D
Figure 1 . Crown rot and aerial trunk canker symptoms on almond trees . A . Gumming on the crown of a tree affected with crown rot . B . Gummosis on the upper trunk and scaffolds of a mature tree . C . Discolouration of the sapwood on a mature tree with aerial Phytophthora canker . D . A young tree with a Phytophthora infection on the mid-trunk .
often infected first , making rootstock selection a critical component of managing Phytophthora diseases . Rootstocks with plum parentage are typically more resistant to colonisation by Phytophthora spp ., while peach / almond hybrid rootstocks are often susceptible . Finding the right rootstock for the orchard is important , and online tools can assist with making educated decisions . When planting , ensure the graft union is above soil level to protect the scion . When irrigating , avoid over wetting soil and pooling and ensure sprinklers do not wet the trunk . As the tree matures , move drip lines away from the base of the tree .
Chemical Applications Phosphorous ( or phosphonic ) acid products ( Crop Doc ® 600 , SprayPhos ® 600 , Phosic ® 600 , and Agri-fos ® 600 ) and Metalaxyl ( Ridomil Gold ® 480 SL ) are registered for use in almonds for the management of Phytophthora spp .
Diagnostics Phytophthora diseases require different management than other trunk diseases . Accurate diagnosis is essential , involving soil , root , and wood tissue samples if tree decline , gummosis or cankers are present . Samples should be taken before applying chemical treatments to ensure an accurate diagnosis by a commercial diagnostic service .
Conclusion Phytophthora diseases can
significantly impact almond production , especially in high-rainfall areas or heavy soils . Effective management starts with preventive measures like site selection , soil preparation , avoiding water pooling , wetting the trunk , and selecting resistant rootstocks . Monitoring and diagnostics are important for early intervention . In addition to cultural practices , chemical applications of phosphorous acid or metalaxyl are vital in managing outbreaks . Ongoing research may offer new treatments , providing alternative control options in the future . As part of Hort Innovation funded AL22002 An Integrated Disease Management program for the Australian almond industry , commercial almond orchards are being surveyed by SARDI researchers . These surveys aim to investigate Phytophthora populations , their regional variations , and symptom differences among species . Rootstocks and scions will be evaluated for susceptibility to common Phytophthora spp . along with the effect of water stress and temperature on disease development . Understanding these factors will assist in improving management recommendations for Phytophthora disease . This article is a summary of a more detailed factsheet . More detailed information regarding symptoms , disease management , sample collection and diagnostics is available here .
34 In A Nutshell - Winter 2024 Vol 25 Issue 2