Sorghum Tips

Sugarcane Aphid “Tolerant” Grain Sorghum Hybrids: Part I – What does ‘Tolerance’ mean?

Statewide

Here I will describe what is meant by the designation of some grain sorghum hybrids as tolerant to the sugarcane aphid (SCA). Two companies (with six hybrids, see the Dec. 10, 2014 edition of Texas Sorghum Insider) have announced they have grain sorghum hybrids that tested as tolerant in independent testing conducted by USDA-ARS entomologist Dr. Scott Armstrong, Stillwater, OK. (One additional hybrid has been announced by a third company as having some level of tolerance, but it was not to our knowledge part of the USDA tests.) Test criteria include known tolerant checks including ‘Tx2783’, which is in the background of some commercial hybrids (though not necessarily the hybrids designated as tolerant). These tests were initiated by introducing a small number of SCA on 2-3 leaf seedlings and evaluating after ~15 days. In general—but not with certainty—the results of tests like these are believed to translate with some level of confidence to plant response to aphids on older plants. But keep in mind there is no substitute for evaluating sorghum hybrid response to sugarcane aphid in field conditions—other than field conditions! The field introduces factors that don’t exist in a growth chamber/greenhouse where these USDA tests were conducted. Test measurements include:

  • Foliar chlorosis/plant death ratings from 1 (normal) to 9 (dead) {1-3 is resistant/tolerant; 3-6 is moderately resistant/tolerant—Dr. Armstrong notes that even hybrids of ratings of 5 to 6 may confer helpful tolerance to SCA; 6-9 is susceptible}.
  • Height differential between normal plants and those infested with SCA (the greater the height difference between [normal – infested], the more susceptible).
  • In some tests, aphid numbers per plant may be reported.

First, even ‘tolerance’ does not mean the plants can’t be affected by the aphid. Tolerance certainly does not mean ‘resistance.’ Field observations as well as lab studies suggest that SCA-tolerant grain sorghum hybrids may have as many aphids per leaf as susceptible hybrids, but the damage to the plant (leaf death and ultimately yield) is less than that on susceptible hybrids. Tolerant hybrids may be just as subject to honeydew and gummy harvest conditions as susceptible hybrids. Thus as the Dekalb and Sorghum Partners news releases note, tolerant grain sorghum hybrids are part of an integrated pest management (IPM) approach addressing SCA in Texas. The current list of company designated tolerant grain sorghum hybrids identified in USDA screening tests—as previously reported in Texas Sorghum Insider—is noted below.

Hybrid Maturity Grain Color Head Type Notable Characteristics

Sorghum Partners

KS310 Med-early Bronze Semi-open Best suited for double crop, late
NK5418 Medium Bronze Semi-compact May tiller more under favorable conditions
K73-J6 Med-long Bronze Semi-open Staygreen
SP6929 Med-long Bronze Semi-open Targeted for South Texas

Dekalb

Pulsar† Med-early Bronze Semi-open Greenbug Biotype I resistant (main current biotype)
DKS 37-07 Med-early Bronze Semi-open Greenbug Biotype I intermediate resistance
†This hybrid was Asgrow Pulsar for many years and marketed prior to about 2002 as an early maturity hybrid, but is indeed an ‘early’ medium-early in Texas A&M AgriLife testing and is now labeled accordingly. They hybrid appears to have been discontinued about 2010, but now reappears in Dekalb literature.

 

SCA Testing Results from Other Companies Apart from Reported USDA Tests?

Dr. Armstrong’s testing agreement with companies likely requires their permission to report results. He continues additional testing of more commercial hybrids. If a company you are interested in has not announced any SCA-tolerant hybrids you should not necessarily assume that they have no SCA-tolerant hybrids; they may not have arranged for or completed independent testing yet. Producers may ask their grain sorghum hybrid seed representatives if their company has tested for SCA tolerance, and if so what are the results. One additional company designated SCA-tolerant hybrid: Pioneer recently announced medium-long 83P56 has defensive traits against SCA, but the method (aphid screening, field evaluation) or source of results (internal or external) were not reported.

SCA-Tolerant Hybrids and Yields

In our next Sorghum Tip Texas A&M AgriLife will report what yield data we have on the above seven hybrids relative to other hybrids in our Crop Testing Program extending as far back as 2005. If an SCA-tolerant hybrid has lower yield relative to trial averages or hybrids you like, then you may elect to continue to plant your preferred hybrid. Remember, though, if you are concerned about SCA then planting at least some SCA-tolerant hybrids on your farm—even if 20% of total grain sorghum acreage—can still contribute to your IPM approach on at least some of your acres.   If you wish to take an early look at CTP hybrid trial results for these hybrids, consult http://varietytesting.tamu.edu/grainsorghum/index.htm and also note the link to archived grain sorghum hybrid trial reports on the right (back to 2005).