Production, Storage, Germination, and Infectivity of Uredospores ofUredo eichhorniaeandUromyces pontederiae

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Abstract
U. eichhorniae and U. pontederiae are pathogens of aquatic plant genera in the family Pontederiaceae. Usually < 20% of freshly harvested uredospores of these fungi germinated on water at suitable temperatures, but > 70% germinated when also exposed to optimal concentrations of .beta.-ionone, 2-hexanone, 2-heptanone, .alpha.-ionone, 5-methyl-2-hexanone, 1-octanol, retinol, retinal, 1-nonanol, or n-normal (in order of decreasing efficacy). The spores could be stimulated to germinate on water, water agar or leaf surfaces at temperatures from 10-30.degree. C (optimum 20.degree. C). The uredospores lost germinability within 4 wk when stored in ambient air at 20, 5, -5 or -12.degree. C; at 20.degree. C in vacuum; or with anhydrous calcium sulfate, silica gel or phosphorus pentoxide. At 5.degree. C, relative humidity (RH) of 35, 52 or 61% in storage was more conducive to spore germinability than 0, 82 or 100% RH. Spores stored in liquid N remained germinable for > 18 mo. and did not require heat shock to activate germination. Consistent host infection occurred and uredia were formed only when newly inoculated plants were incubated with 500 .mu.l of gaseous 2-heptanone, 5-methyl-2-hexanone, or 2-hexanone per ml of air in a dew chamber. Spraying leaves with water agar-based inoculum containing 1-100 .mu.l of .alpha.-ionone/ml was less effective in promoting infection. Uredospores of U. pontederiae infected Pontederia lanceolata leaves at 15, 20 and 25.degree. C, but uredosori were formed only at 20 and 25.degree. C. Spores germinated but did not form appressoria on the host at 30.degree. C; they failed to germinate at 35.degree. C. No cross-infection occurred with the 2 rusts on P. lanceolata and Eichhornia crassipes.