Alternate Crops

Comes Calling Earlier Each Year

Asian soybean rust (ASR) was first detected in the U.S. in 2004 in Louisiana, and it is believed that the disease’s windborne spores were carried along by Hurricane Ivan.

In 2007, experts confirmed Louisiana’s first ASR infestations in Kudzu on May 11 — 53 days earlier than the first infestation was noted in 2006 — and in soybean fields in late July.

“That was the first time we’d seen a significant infestation in a commercial field with the surrounding areas also infested,” said David Boethel, vice chancellor for research in the LSU AgCenter. “The good news, however, is that our scientists were on top of the situation — watching sentinel fields, communicating with farmers and consultants, conducting research and much more to combat this problem.”

ASR colonizes soybean plants, leaves turn yellow and premature defoliation occurs.

Environmental conditions affect the incidence and severity of ASR. Temperatures between 59°F-86°F, and humidity of 75%-80%, are required for spore germination. Signs of soybean rust begin on lower leaves as small lesions that increase in size on the undersides of leaves. Infection spreads to middle and upper leaves shortly after pod set. Spores can also spread on clothing of individuals walking through infested fields.
Late-planted soybeans double cropped behind wheat may be particularly susceptible to ASR.

Premature defoliation results in decreased pod production, seed fill, and/or smaller seed size. ASR can progress very rapidly and defoliate a soybean plant within four to six weeks after the initial infection.

“What the findings in these fields and the sentinel plots really give us and the farmers is a warning to be looking at commercial fields very carefully,” said LSU AgCenter soybean specialist David Lanclos. “The whole point is to really get out and scout for signs of disease.”

Growers should look carefully at plants and be sure to examine areas well within the canopies of the plants for signs of disease, rather than looking at just the tops. They
should also look carefully around tree lines where shade may keep plants cooler and allow moisture to stay on them a little longer.

Fungicides for ASR are broken down into two classes — triazoles and strobilurins. Strobilurins are preventatives; triazoles are both preventatives and curatives.

One of the newest triazoles is Topguard from Cheminova, which received a Section 18 in September of 2007.

“With the granting of Section 18 emergency exemptions in most soybean-producing states, producers will be able to use this effective tool against soybean rust when the need arises,” said Jim Barrentine, technical services director for Cheminova.

“Flutriafol, the active ingredient in Topguard, has been used extensively and successfully in other countries where soybean rust is a problem,” Barrentine added. “Because Topguard has a history of proven performance against soybean rust, we believe U.S. growers will find Topguard to be one of the most effective fungicides available for treatment whenever soybean rust hits in states with Section 18 emergency use exemptions.”

ASR Infestation Ti me Line

• 1902 — Discovered in Japan
• 1934 — Other Asian countries and Australia
• 1951 — India
• 1994 — Hawaii (the first U.S. State to become affected)
• 1996 — Kenya, Rwanda and Uganda
• 1998 — Zambia and Zimbabwe
• 2000 — Mozambique
• 2001 — Paraguay
• 2002 — Parts of Brazil and northern Argentina
• 2003 — Other provinces in Brazil
• 2004 — Spread to Louisiana via Hurricane Ivan.
• May 11, 2007 — Observed in kudzu, 53 days earlier than in 2006.
• Late July, 2007 — Observed in commercial soybean fields.