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Rootstock resistance to soil-borne phytopathogenic fungi, such as Phymatotrichum omnivorum (Shear) Duggar, is an important factor in disease control. Measurement of natural rootstock resistance is often based on plant survival/mortality percentage, and /or growth data. Fungal colonization of host roots in disease screening experiments may not be uniform for many reasons, causing variability in host response. Quantification of fungal colonization is needed in order to better understand rootstock performance. Ergosterol, a structural sterol in cell membranes of fungi, is not found in higher plants, and can thus be a measure of fungal colonization. Ergosterol was extracted from roots of pecan seedlings artificially inoculated with P. omnivorum and grown in an environmental growth chamber. Analysis of extracts with HPLC revealed that seedlings which were killed in screening, or had low root performance ratings, had increased levels of ergosterol. Non-inoculated controls also contained Ergosterol. indicating contamination and possible competition by other fungi.
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.