Many Mid-South producers may be “missing the big picture” regarding nematode-related crop losses, says Tom Allen, assistant Extension and research professor at the Delta Research and Extension Center, Stoneville, Miss.
“We’ve got a lot of nematodes in our soybean fields,” he said at the annual meeting of the Mississippi Agricultural Consultants Association. “And unlike the Midwest, which has predominantly soybean cyst nematodes, we have several species in Mississippi that can cause yield reductions.”
Those causing the most crop losses, he says, are primarily reniform and root knot, while soybean cyst nematode is not near as prevalent throughout the state.
“While reniform nematodes get more attention in cotton, they can also be a soybean problem,” he says. “They favor a fairly specific set of soil characteristics — they like soils with a greater silt content, whereas root knot nematodes prefer sandier soils.”
Pull a plant up in a reniform-infested field and you may not see characteristics such as those from root knot nematodes, which typically produce galls on the roots, he says. “You need a microscope to see reniform nematodes.”
The key to detecting and determining the extent of the problem, Allen says, is soil sampling.
“It’s the only way to tell what you have in the soil. I know it’s not cheap — but in the long run sending a sample to a diagnostic laboratory will help you a lot. There are a number of seed treatment nematicide products available that are outstanding on reniform nematodes in the right situation. If you have a low to moderate reniform problem, they can be quite effective, but if you have 50,000 nematodes per pint of soil, it’s going to be like shooting a BB at an M-1 tank — you’re not going to get much protection.”
The soybean cyst nematode is a problem only in soybeans or related leguminous hosts, Allen notes.
“With large SCN populations, yield losses can be extensive,” he says. “In some cases, you can pull up a soybean plant, look at the root, and not think that nematodes are the culprit causing shorter soybean plants. It can be difficult to determine that there is a SCN problem; for one thing, you need a really good hand lens to see the nematodes on the root itself. Even so, it is still best to send off a soil sample for analysis.”
Root knot nematodes can cause extensive losses in cotton, Allen says, depending on the level of infestation. There are currently no varieties with resistance to the pest, although some are less susceptible and can produce good yields even with fairly high levels of root knot nematodes.
“When you encompass all these nematode issues, one of the best management strategies is rotation to a non-host crop,” he says. “However, in some cases you need to know which particular nematode is present before you can make a rotational choice.”
Southern nematode survey
He is participating in a United Soybean Board-funded project across the southern U.S. to determine the important nematode species and infestation levels. In 2011, each state submitted samples for analysis to determine the most significant nematode problems in the respective state. Mississippi submitted 77 samples from 32 counties.
As expected, the analyses indicated that reniform was the predominant nematode pest throughout Mississippi. Other nematodes included lesion, spiral, root knot, and to a lesser extent, soybean cyst nematode.
“A lot of cotton fields that have been moved into soybean production in recent years have a tremendous reniform problem,” Allen says. “Four of the 77 samples were above the threshold that we would expect yield loss. Two fields were above the threshold where we would expect a yield loss. However, another 11 fields had moderate reniform levels. One field was above the threshold for root knot nematodes; however two fields had extreme root galling present on soybean plants in the field.”
Samples from four fields in the northwestern corner of the state had some soybean cyst nematodes present, he says. “In one field, a non-host crop had been grown since 2007 and the grower still had detectable SCN.”
If a SCN-infested field also has sudden death syndrome, the combination “can cause even more severe yield losses,” he says. “If you have SDS, please take a soil sample and send it in so we determine if there is also a SCN problem.”
In the area of soybean diseases, 2011 saw a significant aerial web blight problem in eastern and northeastern Mississippi.
“Aerial blight can decimate an entire soybean field if you’re not paying attention,” Allen says. “It will knock off blooms and pods, as well as leaves. You can put a combine in the field at the end of the season and cut nothing.”
High temperatures in 2010 and 2011 combined with sporadic rainfall and high humidity did “a really good job of cooking up aerial web blight. Couple those environmental conditions with the type of fields in the hills region that have trees bordering them and limited air flow, and it only adds to the problem. For scouting purposes, you have to get into the field and part the canopy back to find aerial blight. It can be sporadic across a field where some localized areas have more disease than others.”
Allen said he and retired Extension plant pathologist Billy Moore have conducted a number of fungicide trials, and “the take-home message is: If you have an aerial web blight problem, choose a strobilurin-based fungicide and apply it.
“Economically speaking, one application of a strobilurin-based fungicide made in the presence of the disease at R-5 or a bit earlier, will pay for itself. Use the labeled rate of the fungicide — don’t mess with this disease.”
In research plots, Allen says, “We easily saw a significant difference between treated and untreated areas.”
Aerial application is not advisable, he says. “You need to apply the fungicide with a ground rig, using as much water pressure and as much water volume as possible. I would prefer 60 psi or more and 20 gallons of water, with a full rate of the labeled fungicide.”
Wheat, cotton diseases
Stripe rust was a problem in some isolated Mississippi wheat fields last year, Allen says, “so be aware of the potential for it to occur in 2012. Stripe rust doesn’t look like leaf rust — the pustules produce spores that look more yellow than leaf rust. The main delineating characteristic is that the pustules have a defined pattern to them, as a line or stripe.”
Generally speaking, as plants get older, they become more tolerant of the fungus, he notes. “If you find it on young plants (tillering stages), hold tight and watch the progression of the disease, because in a few weeks it may be less of a concern. Typically, you don’t need to be making fungicide applications to young, vegetative wheat. But with that in mind, the environment could dictate the need for a fungicide if ‘hot spots’ of stripe rust are present in a wheat field.”
A cotton disease, previously known as alternaria leaf spot, is now being called corynespora leaf spot, Allen says. “They’re trying to get the name of the disease changed in cotton to target leaf spot to make it a little easier.
“We’re seeing a lot more of this disease. A few years ago, we thought maybe the disease was more prevalent in fields with a continuous history of cotton, but now it seems to be in most cotton fields, typically in the lower canopy. It’s the main foliar disease of cotton that Georgia’s fighting now, and we seem to have identified more of the disease over the past few seasons. It’s the same fungus that causes target spot of soybeans. Data suggest the presence of the disease isn’t related to nutrient deficiency.”
In Louisiana, Allen says, instances are being documented where sheath blight and aerial web blight, both caused by Rhizoctonia solani, have developed resistance to azoxystrobin, the main active ingredient in Quilt and Quadris.
“Numerous fields in several parishes were sampled, mainly in fields having a history of a rice/sobean rotation,” he says. “In some situations, it wouldn’t be difficult to suggest that there might be some resistance to additional strobilurin products as well. If you have fields this year with aerial blight, we’d like samples so we can monitor for the development of resistance to these materials in Mississippi.”
Two additional states, Louisiana and Missouri, have detected strobilurin resistance to Cercospora sojini, which causes frogeye, Allen says.
“If you have a field with this disease and you have either made in the past, or will make a fungicide application for management purposes, we’d like to have a sample, please.”