The 2010 ASHS Annual Conference

Root rot caused by Armillaria mellea persists in soil for decades and causes loss of replanted trees in citrus, subtropical and deciduous orchards throughout the United States.  Aside from soil fumigation, there are no known fungicidal controls for the disease.  In 2004, bare root Prunus persica with ‘Nemagard’ rootstocks grafted to ‘Saturn’ scions were planted in an infested avocado orchard.  Four planting treatments were applied an effort to deteriorate Armillaria inoculum and were compared to a standard-sized planting hole in a randomized complete block design with nine replications: large planting hole (2m³); large planting hole with 0.33m³ fresh yardwaste amendment (not composted) mixed with the planting backfill; large hole with yardwaste and Trichoderma inoculant; and large hole with Trichoderma only.   Each year, we measured trunk diameter and height and noted tree mortality.   Trees with fresh yardwaste amended in the planting hole were initially stunted during the first two years but increased in size so that there were no treatment growth differences by 2010.  Likelihood of survival was greatest with a large planting hole (70% chance of survival) and least (6% chance of survival) with the standard planting hole.  Neither the application of organic matter nor Trichoderma impacted tree survival. Soil disturbance during preparation of large planting holes may disrupt Armillaria inoculum surviving on old root pieces.