Quantitative Inoculation of Eastern Cottonwood Leaf Tissue withMelampsora medusaeUnder Controlled Conditions

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Abstract
A procedure for quantitative inoculation of excised leaf tissue of eastern cottonwood [Populus deltoides] with urediospores of M. medusae is described. The procedure does not require large numbers of spores or elaborate apparatus. Ten-.mu.l samples of a urediospore suspension in 0.1% water agar were placed on the underside of leaf disks (17 mm diameter) supported on 1% agar in petri dish incubation chambers. Favorable incubation conditions were a 14 h photoperiod (10,000 lx) at 18.degree. C. Uredia usually appeared within 6 days after inoculation and most were erumpent within 8 days. Disks from leaf quadrants that had been washed briefly with 52% ethanol prior to inoculation had significantly greater infection than unwashed or water washed disks from quadrants of the same leaf. A linear relationship was obtained by plotting .**GRAPHIC**. transformations, in which x = infections/disk, against log2 of the initial inoculum density. The range of most efficient inoculum density was 1250-5000 urediospores/disk, which indicated that 25-29 urediospores could cause a single infection. Leaves of about the same size and position on the stem from the same clone sometimes differed significantly in susceptibility to rust. This intraclonal variation can be accommodated statistically by distributing disks from the same leaf among experimental treatments.

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