Inpact in Walnut Production
Armillaria root disease is caused by the fungus Armillaria mellea (Fig. 1), which decomposes the roots. Such destruction to the root system inhibits water and nutrient uptake from the soil, reduces crop growth and yield, and eventually kills infected trees.
Symptoms include stunted shoots, wilting, dwarfed foliage, premature defoliation, and dwarfed fruit (Fig. 2). A landscape characteristic is disease centers -- localized areas of dead and dying trees (Fig. 3).
Preplant soil funigtion is minimally effective in controlling Armillaria root disease.
No post-plant treatments are currently available to manage Armillaria root disease.
Armillaria screening is notoriously difficult in field and greenhouse experiments. Greenhouse trials require 2-3 years, infection is hit-and-miss, and infected plants rarely develop symptoms. Meanwhile there has never been a field test of walnut rootstocks with controlled inoculations.
We developed an in vitro assay to identify resistance to Armillariain walnut germplasm. In preliminary trials with the 3 commercially available rootstock clones, we found RX1 to be the most resistant and VX211 the most susceptible.
Our rapid screening procedure relies on rooted cuttings in tissue culture (Fig. 5). The vegetative stage of the fungus (mycelium, Fig. 6), is grown in liquid culture. (Fig. 7). The culture is then homogenized into many small fragments, which are adjusted to the same concentration across all strains used in the experiment. The pathogen is inoculated onto the tissue culture medium and grows to colonize the entire medium. Plants are then incubated, during which time dead plants are tallied. Mortality is our measure of resistance; rootstocks with the highest % mortality are the most susceptible.