Phakopsora pachyrhizi: uredial development, urediospore production and factors affecting teliospore formation on soybeans

Abstract

PI 230971 and TK 5 soybean (Glycine max) plants were inoculated with uredospores of P. pachyrhizi in a growth room programmed for 12-h photoperiods (2060 lux), 60-100% relative humidity and diurnal temperatures of 24 ¦ 1¦C (day) and 15 ¦ 1¦C (night). Mature uredia were observed 8 days after inoculation and new uredia continued to form for 30 days, reaching 6.2-6.3 uredia/lesion. TK 5 had maximum uredospore production at day 15, and PI 230971 at day 25 after inoculation. Between 9 and 45 days after inoculation, TK 5 produced 2386 and 12 646 and PI 230971 produced 1823 and 9396 uredospores per uredium and per lesion respectively. More telia were recorded on TK 5 than on PI 230971. The number of telia/cm2 decreased from the lower to upper leaves on the plant. Source of inoculum from either PI 230971 or TK 5 did not influence telia production. With each increase in inoculum concentration, there was a concomitant increase in telia production. Two-week-old soybean plants could be used for telia induction. Telia were produced in soybean leaves after 2 weeks at 10 and 15¦C and 3 weeks at 20¦C, but not at 25¦C. Telia production on soybean leaves in the field was common when the daily temperature remained below 20¦C for more than 15 h and the average temperature did not exceed 25¦C for at least 15 days.