INDUSTRY NEWS
Are your almond trees virus free?
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SINCE the early 1990s the Almond Board of Australia has committed to managing a dedicated virus-tested budwood program for growers. Mother trees were first planted in Monash, South Australia, in the early 1990s with an article published in the“ Australian Almond News” March 1993 explaining that“ the majority of nursery trees supplied to almond growers to date have been infected with pollen-borne virus”. The ABA budwood program provided the solution for growers to access virus-free material and has supported the rapid expansion of the industry. Viruses have not been a big issue since the program was implemented in the early 1990s. However, viruses can easily reestablish and spread within and between almond orchards, so we need to be on our guard. health of the tree when infected.
It’ s not always easy to detect Younger trees may struggle to viruses. Some varieties don’ t display establish while older orchards may symptoms until trees are two or three gradually decline in growth and years old or until stress triggers a production over time. Some viruses response. have less impact.
Also, symptoms in large plantings
The ABA budwood program which
with the same varieties may be easily has been supplying nurseries with misdiagnosed as a disease, fertiliser virus-free material for over 30 years response or caused by‘ something in manages mother trees to promote the soil’ as they can all look similar. vegetative growth, using sterilising But when young trees with viruses equipment between trees and are planted side-by-side with clean removing all flowers to prevent virus material the contrast is clear. spread via pollen. Extensive testing
Virus-free trees will grow bigger is undertaken each spring and no and have stronger scaffolds and budwood material is cut tree until the tend to come into production earlier. laboratory results show its virus-free. Estimates in the literature suggest The following viruses are known to the range in yield difference between infect almonds and spread naturally clean and virus-infected planting via a vector and pollen and are material can be 15-60 %. screened against annually through
The most efficient way to manage the ABA budwood program. the disease is through preventative measures and avoid planting
Viruses known to infect almond and contaminated material. occur in Australia
When ordering trees growers should Prunus Necrotic Ringspot Ilarvirus ask their nursery for ABA budwood( PNRSV) also known as almond and rootstock material which has calico, has worldwide distribution and been tested annually to be virus-free a wide host range including almonds, providing trees with the best chance to apricots, peach and plum as well as establish and develop strong growth. many Rosa sp. It can result in yield The ABA Nursery tree supply reduction which may be higher when agreement template and ABA Nursery affected with other viruses such as tree classification standards can be PDV. used when ordering trees which • Symptoms have been associated includes virus testing requirements. with necrotic shock, bud failure, Once a tree is infected with a virus calico and chlorotic mottling. For there is nothing that can be done in some cultivars trees may‘ recover’ the field to control it. and become symptomless after The level of impact will depend on the initial“ shock” period when the virus, almond variety and general symptoms are first expressed in
In A Nutshell- Spring 2025 Vol 26 Issue 3
Figure 1. Calico leaf symptoms of Prunus Necrotic Ringspot virus.
trees. However, yield losses of up to 60 % have been reported when trees are infected with both PNRSV and PDV.
• Transmission can occur via seed and pollen. Thrips feeding on infected pollen may transmit the virus. There is some evidence for spread by vectors including mite, nematode and thrips. Transmission can occur using infected propagation material.
Prune Dwarf Ilarvirus( PDV) is associated with almond mosaic disease and can be found worldwide. It affects several prunus species including almonds, apricot, sweet and sour cherry, peach and plum. It can result in similar yield losses as PNRSV which may be exacerbated when infected with other viruses.
• Symptoms may be similar to those caused by other Ilarvirus species( e. g. PNRSV and APMV) and may vary year to year depending on the growing conditions. Some cultivars may be symptomless. Susceptible varieties may show leaf and flower bud failure, leaf chlorosis, mosaic symptoms, chlorotic spots, shot hole, and stunting. In combination with PNRSV rosetting of the shoots may be observed.
• Transmission occurs through seed and pollen( wind or insect borne) and in propagation material or using contaminated pruning tools and hedging equipment. There is some