There are marked similarities between the climates in the Southern U.S. and Brazil. But it will be the “small differences between the two areas that will determine whether Asian soybean rust is a minor or major disease in the United States,” said Boyd Padgett.
With the incoming growing season, “we're going to learn a lot,” said the Louisiana Extension plant pathologist, who spoke to a packed house at the recent annual Tri-State Soybean Forum in Dumas, Ark. “Rust will lead the dance for the first year or two. But we're going to find out how to manage it at the same time.
“What I want to talk about specifically is how to identify the disease. This is very important because the disease starts in the lower canopy and will be hard to detect. But if you have a bit of knowledge about what the symptoms look like and what kind of conditions make the disease spread, it might give you some insight into whether you do have a problem. And if you do believe you have a problem with this, do not hesitate to contact your local county Extension agent.”
According to Padgett, here's the worst case scenario: no symptoms on day one of infection, but six days later Asian soybean rust is at the plant top. Thirteen days after infection, defoliation begins, and 27 days after, defoliation is complete.
“If conditions are favorable, this is possible,” he said. “However, if the weather isn't right, the disease won't move this fast.”
While not downplaying yield losses suffered in other countries due to the rust, Padgett said not enough attention is placed on how those losses were calculated.
“(Brazil has) experienced up to 80 percent yield losses. But the range of their losses is from 10 percent to 80 percent. And these numbers represent situations where the growers did nothing. So, doing nothing in some situations meant losing 10 percent of yield. We lose 10 percent some years in our crop.”
In the short period of time it's been in Brazil, farmers have found that rust severity fluctuates from year to year. Padgett said this can eventually play into management strategies.
“If we can learn to identify what years the rust… will be more severe, it'll give us a better handle on managing the disease. It would also give us an opportunity when conditions aren't favorable for the disease. In those years, we can save on inputs used to manage the rust. So I believe such a range of yield losses is actually encouraging.”
Soybean rust is caused by a fungus — phakopsora pachyrhizi. “If someone tries to sell you something that kills bacteria — or something other than a fungus — it won't work.”
Asian soybean rust usually starts out as tan or brown spot — or lesion — on a leaf. That spot can be as large as a quarter inch in diameter. However, spots Padgett recently saw in south Louisiana were about an eighth inch in diameter.
Typically, the lesions will show up on bottom leaves and begin working upward. They can, however, be present on the petioles or pods.
“Most of the time, you'll see these symptoms on the lower canopy when the plant goes into a reproductive stage. I'm not saying it can't infect seedlings, but normally it doesn't.
“The lesions usually begin as small water-soaked lesions very similar to bacterial pustules. They can turn gray, brown or tan, depending on the variety. They are usually restricted by leaf veins and are most abundant on the bottom of leaves. That's very characteristic of most rust diseases.”
There are two types of lesions.
One is called a “tan” reaction, also classified as a “compatible” reaction. “That means the fungus is more aggressive in those particular lesions, resulting in more spore production.
“The second type is a ‘red brown’ lesion that is a ‘semi-compatible.’ These lesions do have spore production, but they aren't as abundant.”
The Asian rust organism is an obligate parasite — it must have a living host to survive. Unfortunately, it needs no help to become infectious.
“It doesn't need a wound for entrance; it doesn't need an insect or the wind to break a plant over. If the warm temperatures and adequate moisture are available, it will penetrate directly into a leaf. It has a life cycle between five and 10 days — that's pretty quick.
“To put this in perspective, we already have leaf rust and stripe rust here. Asian soybean rust moves very similarly to those two.”
Asian rust spores tolerate temperatures between 59 and 77 degrees — but prefer 68 to 77 degrees. And they really like moisture.
“From what I've read, the fungus needs about six hours of leaf wetness in order to (begin growing). Disease epidemics dependent on leaf moisture will move more at night because that's when dew arrives. For that reason, we really need to keep a close eye on nighttime temperatures.”
The disease can produce pustules — spore factories — on the underside of leaves for almost two months. “That's a long time. You've got one small spot that can pump out spores for weeks. For a pustule, it takes about 10 days from birth to spore production. Under the most favorable conditions, it will be ready in five days. It will produce spores for three weeks. So, new factories are being produced for two months. Each factory is, in turn, producing spores for three weeks.”
Scouts, get ready for more aching backs and eye strain.
“I'm not going to lie: it's going to be difficult to scout for Asian rust, because it will be in the lower part of the plant. Bacterial pustules usually initiate on the upper canopy. If you see pustules at the plant top, it likely won't be Asian rust.
“If you're in your field and think you have rust, you should pull the leaflets and carry them back to your house. Call your county agent or consultant and then moisten a paper towel, put the leaves on it and then (bag everything). Put the bag in an area that is about 75 degrees.”
The more samples collected, the better, said Padgett. However, knowing producers have more to do than walk bean fields, Padgett suggests several areas to check if pressed for time. “Sample sites in the field that are shaded in the morning. Check any place in the field where moisture will remain in the canopy longer.
“I can't emphasize enough that early detection is critical. I'm not sure exactly when a crop is beyond help with a fungicide. I've been told if the rust gets into mid-canopy, a fungicide won't do any good. That's scary. I'm not sure if that's accurate, but hearing it certainly got my attention. Again, that may be the case in Brazil but not the Southern U.S.”
Another tip: if you can't see the pustules or lesions, but suspect they might exist, hold the leaf up to the sun. That can help illuminate any new problem a bit better.
It's also a good idea to use a hand lens to check for pustules. Padgett recommends using a lens with at least a 20X power.
One thing that will aid in finding a soybean variety resistant to Asian rust is researchers now have two growing seasons to work with. “Last year, we couldn't screen all soybean varieties in the United States because we didn't have Asian rust. Now, we can take screened varieties from Brazil and bring them in to screen them further. That should expedite the process for finding resistant varieties.”
As far as cultural practices, planting dates or row spacing, “they've found little that has an effect on the disease,” said Padgett. “However, anything you do that encourages the canopy to be moist will be to your detriment. Am I saying to quit irrigating your beans? By no means. David Lanclos (Louisiana Extension soybean specialist) has told producers — and he's right — to not do anything to their bean crop that is agronomically unsound. Grow beans like you always have. Don't plant super early — knowingly cutting your yields — to try and avoid the disease.”
Typically, the first fungicide spray will be made at R-1. But don't let that be a guide, warns Padgett. “Until we get a feel for how this disease will spread, I'll be looking at my soybeans all the time. As far as when to quit spraying, that's hard to answer — perhaps at R-6.”
Currently, since there's no genetic resistance to Asian rust. Padgett sees little advantage to growing a Group IV over a Group V. “One argument that could be made, I suppose, is a Group IV is in the field less time, and that means less exposure to the rust. But if conditions are favorable for rust, a Group IV will be hit, too.”
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