Asian soybean rust continues to travel through Louisiana and Texas. The week of July 16 found Louisiana with its first ASR in commercial soybeans this season. Of even more concern, however, is the fungal disease has nearly reached Dallas.
“It looks like ASR has flared in the Upper Coast region of Texas — the area between Victoria and Houston,” says Tom Isakeit, Texas A&M plant pathologist. “It’s really picked up around Victoria and appears to have been in place several weeks.
“Some fields are heavily infected. Other fields have leaves completely covered with rust, and plants are defoliating. Most, if not all, soybean fields in that area seem to have detectable levels of ASR.”
Traveling north checking soybean plants, Isakeit found more ASR at lower levels. “There was only a trace amount of ASR in Austin County fields. Closer to College Station, in Burleson County, there was also a trace amount in one field.”
Most surprising, however, was the July 19 find just south of Dallas. In one field corner Isakeit found rust was “quite extensive. There was rust on 100 percent of the R-6 plants in that corner. Most fields in that area are at R-5, or younger.
“In other (Dallas-area) fields I’ve found trace amounts of ASR confined to the lower canopy. The crop is in position to receive and benefit from a timely fungicide application.”
In Louisiana, alerted to a possible ASR find by a consultant near Cheneyville July 17, an ASR scout team from the LSU AgCenter checked Avoyelles Parish and Rapides Parish.
“We searched 17 soybean fields and 10, or so, were positive for ASR,” says David Lanclos, LSU AgCenter soybean specialist. “I believe there were seven positive fields in Rapides Parish and three in Avoyelles Parish. ASR is moving.”
Recent rain in Louisiana “is probably inhibiting the spread of ASR. That’s certainly a twist on things. To get ASR sporulation, we probably need more sunshine.”
Lanclos points to Texas as a “major worry — ASR appears to be popping over there. That’s obviously a concern for Louisiana, Mississippi and Arkansas.”
Most fields — all with R-5/R-6 soybeans — where the LSU team found ASR have been sprayed with a fungicide.
“Some of the fields had ASR to an extent it was easy to locate. In other fields, with four scouts, it took 10 or 15 minutes to find positive signs. So, it isn’t just blowing out the top yet. But even at a less detectable level it’s time to make educated decisions on fungicides.
“The earliest growth stage we found ASR in was R-4 in an Acadia Parish sentinel plot. That was a day earlier than the commercial fields.”
Are wet conditions hampering fungicide applications? “Ground-rig applications aren’t an option. A lot of producers are trying to use airplanes. Reports are that, in many situations, runways are too slick for planes to work safely. We need a couple of days of drying out. But the weatherman is saying more rain will hit (the week of July 23).”
Back in Texas, Isakeit says several differences between ASR in the south and north are showing up. In south Texas, there aren’t many foliar soybean diseases, so ASR is easy to find.
“In fields farther north I’ve found what I thought was rust but, under magnification, there are no spores. That makes me believe it’s probably something like bacterial pustule.
“In south Texas, a visual diagnosis for ASR — even without a hand lens — is possible. In north Texas, though, the little specks on leaves are much more deceptive. You’d better have a hand lens and know what to look for.
“If soybean plants in infected areas are at R-1 to R-5, a fungicide is warranted. And if the crop is at R-5, growers applying a triazole should consider the pre-harvest interval. Will the beans be mature prior to the 30-day pre-harvest interval? In some areas, farmers will go with a triazole over a strobilurin because of the curative properties.
“Some leaves may be infected but not yet showing symptoms. In those situations, triazoles would be best.”
Weather is driving ASR, says Isakeit. From south Texas northward, “conditions have been very wet. South Texas is soggy. When I was there earlier this week, every day there was a hard rain with blowing winds. Typically, this time of year it’s hot and dry.”
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