Phytophthora Results Have Been Added to 2011 Performance Trial

Resistance to Phytophthora sojae is probably one of the biggest success stories in soybean breeding and development, at one time many lines were very highly susceptible.  For varieties with no resistance there will be 100% loss on the heavy Phytophthora ground in much of the region, which occurred during the late 60’s and again in the late 70’s.  Management has been accomplished by utilizing both single gene mediated resistance as well as partial resistance (also called tolerance in some companies).  A single gene that is effective will give total control, and no root rot loss.  While for partial resistance, some disease does develop.  Partial resistance (also called tolerance or field resistance in the industry) has a range of levels (see Figure 1), from moderately susceptible (scores in this performance trial of 6 to 7); moderately resistant (scores of 4 to 5) and high levels of partial resistance (3).  Note, every company has a different scoring system so it is necessary to get out the magnifying glass and read the very small fine print from every seed company.

Only 124 entries were tested for resistance to Phytophthora sojae, and the results are posted at http://oardc.osu.edu/soy2011.  This test requires seed that has not been treated, and only these entries had extra seed shipped in to evaluate for resistance.  Lines that were not tested have a dash (--) in the cell.  The first test was to assess what Rps gene is present.  This was accomplished through a series of inoculations with different isolates (races) of Phytophthora sojae.  Each of these has a different pathotype or virulence formulae.  The first inoculations are with an isolate designated race 1.  All of the known Rps genes are effective against this isolate.  If a plant is susceptible, then it has no Rps genes, and these were designated as none.

The next isolates we used were race 3 (recognizes 1a); race 4 (recognizes 1a and 1c); race 25 (recognizes 1a, 1b, 1c, 1k); race 7 (1a, 3a, 6, 7); OH1.S.1.1 (1a, 1b, 1k, 3a, 3c, 4, 5, 6, 7).  Using these we can assess what the genes are in each line.  For example, if an entry is resistant to race 3, but susceptible to race 4 and race 25, it has the Rps1c gene.  If an entry was susceptible to all of the isolates, then it has the Rps1a gene.  For the 2011 trials more than 13% of the entries evaluated have a 2 or novel gene stack.  Approximately half of these have the Rps1k & Rps3a combination.  Having 2-genes in a variety gives the plant more of a chance, as not all isolates within a field will be able to cause infection on both genes.    These 2-gene packages combined with partial resistance are the highest levels of resistance now available on the market.  Almost 20% of the entries had no Rps gene or the Rps1a.  Rps1a has not been effective in Ohio for a long time, this pathogen is unusual in some respects as it does not forget or lose virulence in field populations.  However, among this group of entries, almost all have high and moderate levels of partial resistance.  Those with the highest levels of partial resistance will perform very effectively in our soils.  Those who farm the heavy ground should avoid those lines with none or Rps1a, and very low levels of partial resistance (Table 1).

We were able to test 95 entries for their levels of partial resistance.  We are still testing those with the 2 gene combinations, and those experiments will be completed in January.  In this year’s analysis of the entries for partial resistance, the news is very encouraging.  More than 20% of the lines we evaluated had the highest levels of partial resistance.  This level of resistance is equal to Conrad, which this last year, under heavy disease pressure, yielded as much as the super gene stack in several locations.  There are now more choices for those producers that deal with Phytophthora root rot on an annual basis. (Figure 2).

Table 1.  Summary of resistance gene and partial resistance levels for entries in the 2011 Ohio Soybean Performance trials.

 ‘nd’ indicates that experiments are still in progress