By drastically narrowing diversity in fields, agriculture has largely created its own disease problems. “We plant genetically uniform material over large acreage,” said Don Groth, LSU AgCenter professor and research coordinator, at the recent Rice Research Station field day in Crowley, La. “That draws a bulls-eye on the crops for some pathogens.”
A pathogen is usually specific to a particular plant. It wants to survive and so a competitive advantage is its ability to reproduce through spores.
“A fungus that can produce 100 spores isn’t as competitive as one that can produce 1,000 spores. One hundred spores might produce one lesion while 1,000 spores produces 10 lesions. A pathogen that can produce more spores basically takes over the population.”
With that background, “take the pathogen out of the natural environment with a lot of diversity and very little infection efficiency and put it in a rice field with a susceptible variety. Those 1,000 spores might produce 500 lesions instead of 10. That’s because all the plants are susceptible instead of just a fraction. That’s how a disease can become so explosive.”
In recent studies, Groth and Brooks Blanche, LSU AgCenter rice breeder, are trying to see what might happen if diversity is reintroduced into cropping systems “by mixing both resistant and susceptible plants in different ratio and variety combinations and. An example would be to use Bengal (a rice variety that is susceptible to sheath blight and blast) and Jupiter (a variety moderately resistant to sheath blight and resistant to blast).”
The research, now in its second year, is looking at plots inoculated with sheath blight and also those where sheath blight and blast have been allowed to occur naturally.
“We’ve had some success and some failures. With sheath blight, we’re consistently seeing more disease in the blends than we anticipated.
“The best analogy I can provide is the susceptible plant between the resistant plants acts as a ladder for the sheath blight. The disease grows up on the susceptible plants and then moves to the resistant plants (which aren’t entirely resistant, but postpones the disease) higher and more often.”
Interestingly, the blended plot yields are still better than if the rice lines are kept pure.
Blast is a different case. “We saw some positive effects from blending. With the resistant plants between the susceptible, we’re able to block spores and break the disease cycle. In some combinations, we’ve seen between 15 to 30 percent reduction of the disease. That has resulted in several hundred pounds of yield increase. We’ll continue this research for several more years before we even provide any specific recommendations.”
This year, Groth has been fielding three major questions from growers.
Will cercospora be a major problem in 2008 like it was in 2006?
“You may remember that in 2006, cercospora — normally a secondary disease that doesn’t cause much damage — took advantage of a lot of overwintering rice. That allowed the fungus an early start and a lot heavier innoculum load. We had a very wet year (it rained most of July) and we had a high percentage of susceptible varieties.
“Another contributing factor was we were using mainly Quadris fungicide to treat our rice. It doesn’t have as much activity against cercospora as the propiconazole-containing fungicides. As a result, we had fields that looked as though they were sprayed with a desiccant. They dried up prematurely and growers lost yield and milling.”
In 2007, Louisiana didn’t have as much overwintering rice.
“We also knew we needed propiconzoles in our spray program and which varieties were susceptible and need to be sprayed. Approximately 70 to 80 percent of the rice acreage was treated with a propiconazole-containing fungicide. As a result, we didn’t see much damage and, for the most part, had a very good crop.”
Unfortunately, that didn’t carry over to the second crop. It turned wet in the fall and there was a lot of damage in the second crop, “which was typical for this disease.”
This year, Groth isn’t seeing the disease spreading from overwintering material. “In susceptible varieties in fungicide trials, I’m seeing the least amount of cercospora in several years. The disease pressure seems to be very light in commercial rice fields and I’m not anticipating the damage we saw in 2006.”
When should a fungicide be applied?
“It depends on the disease. For sheath blight and cercospora, anytime from 2-inch to 4-inch panicle to boot stage is best. That’s when the fungi establish and move up the plant, but before they do major damage.”
For blast, growers should wait for a later application. “Spray at 50 to 70 percent heading. Emerging heads are tender and succulent and that’s when the fungus can attack the rice head.”
Sheath blight timing isn’t as critical as blast. If spraying occurs plus or minus a week, “you’ll likely get very good control with an early- to late-boot application. But the 50 to 70 percent heading is critical. You need to apply your sheath blight and blast materials by then because after heading, the benefits of the fungicide application drops drastically. Also, because of label restrictions, cercospora-active fungicides must be applied before heading.”
I’ve got a really good crop and not much disease. Do I need to spray a fungicide?
Groth says this is the toughest question to answer. “We had a fairly dry year early and have not seen much disease. Sheath blight was very low in the canopy, near the waterline and not developing in most fields.”
Now that Louisiana has gotten some rains, “sheath blight is moving up the plants rapidly. The problem is sheath blight is present in every rice field and starts developing as soon as moisture levels increase, especially after the heading stage, when fungicides can’t be applied.”
On very susceptible varieties such as CL161 or Trenasse, “if you have good yield potential, you probably will want to use a fungicide prior to heading just as insurance against late-developing disease. For moderately susceptible/moderately resistant varieties, if the disease doesn’t show up early and at higher levels you probably will not need a fungicide.”