Laboratory Method for Assessing Field Tolerance of Soybean Seedlings toPhytophthora megaspermavar.sojae

Abstract

Two-to four-day-old seedlings of soybean cultivars with known levels of field tolerance to Phytophthora root rot were planted in slit Styrofoam cups filled with vermiculite. The cups were placed in plastic trays, 17 X 25 X 6 cm, containing 100 g of steamed greenhouse potting mix and 200-250 ml water. Inoculum usually consisted of 4,000 zoospores obtained by flooding lima bean agar cultures of Phytophthora megasperma var. sojae. Plants were grown 15-20 days in a growth chamber at 28 C and a 12-hr photoperiod (17,000 lux). Disease was assessed by measuring dry weights and overall lengths of individual plants. With race 3, lengths of the cultivars Hark, Williams, and Agripro 26 were 36%, 43%, and 55%, respectively, of the uninoculated control plants. Dry weight measurements gave similar results. Results were comparable with race 7 and with a natural soil infested with race 4. The results agree with relative field tolerances reported previously. The response of the field tolerant cultivar Agripro 26 was maintained against eight races of the pathogen. The cultivars Woodworth, Agripro 25, SRF 307 P, and Wayne, also known to have field tolerance, showed less reduction in growth than did varieties without field tolerance.

Variation in the Resistance of Japanese Soybean Cultivars to Phytophthora Root and Stem Rot during the Early Plant Growth Stages and the Effects of a Fungicide Seed Treatment

“…The resistance of these two cultivars may not be expressed as quickly as that of ‘Conrad’. Although different inoculation and evaluation protocols were used, disease resistance increased as the age of plants at inoculation increased for all of the tested Japanese soybean cultivars, which is consistent with the results of previous studies ( Dorrance and McClure, 2001 ; Jimenez and Lockwood, 1980 ; Lazarovits et al, 1981 ; Paxton and Chamberlain, 1969 ). The plant ages associated with increases in resistance varied among the cultivars, but the timing of the expression of resistance in the Japanese cultivars is likely consistent with that of partial resistance ( Schmitthenner and Dorrance, 2015 ).…”

“…Several plant species exhibit a disease resistance at various developmental stages, with varying levels depending on plant age or tissue maturity ( Develey-Rivière and Galiana, 2007 ). The relationship between plant age/maturity and disease resistance has been studied in a soybean– P. sojae pathosystem ( Dorrance and McClure, 2001 ; Jimenez and Lockwood, 1980 ; Lazarovits et al, 1981 ; Paxton and Chamberlain, 1969 ). Consistent with the results described herein, soybean plants reportedly become more resistant to P. sojae as they mature.…”

“…Soybean cultivar ‘Conrad’, which does not carry Rps genes and expresses a high level of partial resistance ( Fehr et al, 1989 ), is highly susceptible to P. sojae infections at 0–5 days after seeding, with a seedling emergence rate significantly lower than that of plants inoculated later ( Dorrance and McClure, 2001 ). Jimenez and Lockwood (1980) observed that even ‘Hark’, which is a susceptible cultivar with little field tolerance, exhibits increased resistance to P. sojae when plants are inoculated at 16 days after seeding or later. In the current study, the rates of dead plants due to a P. sojae infection for two susceptible cultivars, ‘Himeshirazu’ and ‘Nattosyoryu’, were significantly lower for plants inoculated at 25 and 26 days after seeding or later than for plants inoculated at 16–21 days and 16–18 days after seeding, respectively.…”

“…The resulting primary symptom is decreased root colonization ( Dorrance et al, 2007 ). Additionally, ‘Hark’ plants (i.e., susceptible cultivar) inoculated at 16 days after seeding are more resistant to Phytophthora root and stem rot than younger plants ( Jimenez and Lockwood, 1980 ). Meanwhile, one of the best-studied cultivars regarding this type of resistance, ‘Conrad’, which lacks Rps genes, is initially susceptible, but then becomes highly tolerant to P. sojae ( Fehr et al, 1989 ), with significantly less seedling emergence when plants are inoculated up to 5 days after seeding compared with inoculations at 7, 10, and 14 days ( Dorrance and McClure, 2001 ).…”