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Thomas M. Gradziel and Dechun Wang

Colonization and sporulation of aflatoxigenic Aspergillus flavus Link on intact and injured seed was evaluated for a selection of almond [Prunus dulcis (Mill.) D.A. Webb] cultivars. Barriers to fungal development were identified at the intact seedcoat and at the seed cotyledon tissue. The seedcoat barrier was expressed as a delay in fungal colonization for up to 3 days following the inoculation of intact seed. Seedcoat resistance was uniformly high for all cultivars tested. Cotyledon resistance, which was expressed as a lower rate of disease development was identified only in the cultivars Ne Plus Ultra, Ruby, and Carrion.

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T.M. Gradziel and Dechun Wang

Rate of brown rot lesion development following inoculation with Monilinia fructicola (Wint.) honey varied within clingstone peach (Prunus persica (L.) Batsch) germplasm evaluated in 1990 and 1991. High levels of resistance were identified in selections derived from the Brazilian clingstone peach cultivar Bolinha. Resistance appeared to be limited to the epidermal tissue. No relation was detected between brown rot resistance and concentration of phenolic compounds or polyphenol oxidase activity in the susceptible California germplasm. An inverse relation was observed between disease severity and rating for phenolic-related discoloration when `Bolinha' derived selections were analyzed. A moderate positive correlation was observed for all germplasm tested between genotype means for phenolic content and enzymatic browning. Any causal relationship, if it exists, between phenolic content and brown rot resistance is obscured by an array of physical and chemical changes in the maturing fruit.