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United States Department of Agriculture

Agricultural Research Service

Related Topics

Research Project: Identification of Almond Rootstocks with Resistance to Armillaria Root Disease

Location: Crops Pathology and Genetics Research

2013 Annual Report


1a.Objectives (from AD-416):
Identify Armillaria-resistant rootstocks for almond, by first screening a set of commercially-available Prunus rootstocks, and determine the relationship between the results of our infection assay in the lab and field observations.


1b.Approach (from AD-416):
Armillaria root disease is a serious problem in almond, as it cannot be solved by conventional methods. To address the need for resistant rootstocks, we developed an infection assay that is rapid and reliable. The funds we request are to use this assay to screen almond rootstocks. We have developed a target list of rootstocks, based on: published literature; input from all almond farm advisors in California; input from almond breeders; input from plant pathologists; input from nurseries.


3.Progress Report:

This agreement was established in support of parent project objective #1 which is to characterize the etiology, biology, and ecology of key phytopathogenic agents and their interactions with economically important tree and grapevine species.The goal of this project is to identify Armillaria resistant rootstocks for almond.

Armillaria root disease affects all almond regions of California. The causal fungus, Armillaria mellea, colonizes and kills the roots, and then decomposes the root wood as its source of nutrition. Such destruction to the roots significantly reduces crop yield and growth, inhibits nutrient and water uptake from the soil, and eventually kills infected trees. Fumigants like methyl bromide are only effective to the limited extent that they reach and penetrate residual roots. Systemic fungicides are also ineffective, as has been demonstrated in almond, in part, because fungal decomposition of the root crown disrupts systemic movement of fungicides through the vascular tissue. Instead of re-tooling these same, ineffective tactics, we propose to identify resistant rootstocks as an effective, long-term solution. Our 1st objective is to identify Armillaria-resistant rootstocks for almond, by first screening a set of commercially-available Prunus rootstocks. Afterwards, we will determine the relationship between the results of our infection assays in the lab and field observations. We grow plants in tissue-culture medium, which supports both the plant and the pathogen. With this infection assay, we have overcome the major barriers of inoculating plants in the greenhouse, namely by eliminating ‘escapes’ and bringing about repeatable levels of mortality. Currently, we are screening the following rootstocks: Empyrean 1 (Barrier 1), Lovell, Nemaguard, Bright 5, Hansen 536, Krymsk 1 (VVA 1), Krymsk 86 (Kuban 86), and Marianna 2624. We have completed one replication of the experiment initiated in 2012, and the second replication is in progress.


Last Modified: 9/10/2014
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