Location: Crops Pathology and Genetics Research
2013 Annual Report
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.