Treatments of sporangia and zoospores of Phytophthora infestans race 1.2.3.4 with methionine or riboflavin for durations of up to 8 h under fluorescent light did not affect its colonization of rye-seed agar. In contrast, exposure of sporangia and zoospores to methionine-riboflavin mixture for 2 h or more resulted in the failure of race 1.2.3.4 to grow when transferred to rye-seed agar medium. Hyphal growth of races 1.2.3.4 and 0, when incubated in liquid synthetic medium, was inhibited by free riboflavin. Potato plants, Solanum tuberosum cv. Kennebec, when pretreated with a methionine–riboflavin mixture and then spray inoculated with race 1.2.3.4 developed fewer diseased leaves per plant than plants pretreated with water, methionine, or riboflavin. Key words: Solanum tuberosum, hydroxyl radical, superoxide anion.
Root apices of Apium graveolens L. resistant and susceptible to race 2 of Fusarium oxysporum f.sp. apii (R. Nels. & Sherb.) were studied at various times after inoculation, using light and electron microscopy to determine structural response(s) of the hosts during penetration and colonization by the pathogen. Penetration was intercellular and intracellular and involved mechanical and enzymatic mechanisms. At the onset of penetration, the host cell walls manifested fluorescence, induced with either aniline blue or sirofluor, at the point of penetration. The fluorescent area was more intense and larger in the resistant host. Fluorescence disappeared with time. After incubation with β-1,3 glucanase fluorescence disappeared, indicating β-1,3 polysaccharide (probably callose) presence. Callose deposits were 2 and 3 times greater in the epidermis and 4 and 9 times greater in the cortex of the resistant than in two susceptible hosts, respectively. Hyphal counts in the cortex of the resistant host were 50% fewer than in the susceptible hosts. Increased callose deposition on host cell walls was associated with reduced colonization. Callose formed in vascular tissue as the fungus colonized it. Callose detection with sirofluor was more sensitive; background fluorescence common with aniline blue without periodic acid – Schiff's reagent pretreatment was absent.
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