Abstract
Phymatotrichum Root Rot, caused by Phymatotrichum omnivorum (Shear) Duggar, imposes severe losses upon dicotyledonous horticultural crops in the southwestern United States and northern Mexico. Rootstock resistance could benefit pecan (Carya illinoensis) production in affected growing areas; however, erratic growth habit of this pathogen and site variability prevents effective field screening. We have developed a containerized screening method for horticultural crops, using a commercial soilless growing medium. In sterile cultures, 2.5 × 60 cm glass tubes containing Metro Mix 500 yielded more grams of P. omnivorum sclerotia than cultures grown in Houston Black Clay, a traditional medium for cotton research. Preliminary screening with Okra (Abelmoschus esculentus) in non-sterile Metro-Mix 500 resulted in 75% mortality of inoculated plants in 30-35 days. Preliminary screening with 12-week-old, open-pollinated `Apache' and `Moore' pecan rootstocks has resulted in 25% mortality of inoculated plants in 150 days. Pecan seedlings with visible taproot infection appear to delay mortality by adventitious root formation.
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