A plant disease once thought under control has re-emerged in South Texas grain sorghum fields, displaying resistance to seed treatments and creating concern among farmers and scientists over management possibilities.
“Sorghum downy mildew (SDM), caused by the fungus Peronosclerospora sorghi, appeared in several sorghum fields in Wharton and Jackson counties during the spring of 2001 and again in 2002,” says Thomas Isakeit, Extension plant pathologist.
He says SDM's prominent reappearance was “surprising and disturbing. Evidence from one field trial in Wharton County in 2002 suggests that the pathogen is resistant to Apron XL or Allegiance seed treatments.”
Isakeit says symptoms include seedlings with pale yellow leaves or light-colored leaf streaking or mottling, often accompanied by a white fuzzy growth on the underside of leaves.
It's a systemic infection from soil borne spores (oospores) of P. sorghi that infect young seedlings. Germinating sorghum seed is more prone to infection early in the season, when soil temperatures are cooler.
Infections that emerge later show white parallel stripes of green and white tissue.
Isakeit says late symptoms may mimic those of iron chlorosis, which results in a pale color in-between veins. “The white stripes of SDM are not limited to veins and vary in width. Later in the season, striped areas die, turn brown, and disintegrate, resulting in a shredded appearance of the leaf.
“Fungus oospores are produced in this tissue and eventually end up in the soil.”
That white, fuzzy growth on systemically infected plants poses other problems. “These are short-lived spores, known as conidia, which are produced in cool, humid or wet weather,” Isakeit says. “Conidia become airborne and infect leaves of other plants, causing a local lesion phase of SDM. Local lesions are brown and somewhat rectangular.
“Local lesion infections can become widespread throughout a field and may be dramatically noticeable but cause no yield loss. These infections are usually short-lived because temperature increases prevent spread to later-emerging leaves. Most importantly, local lesions do not produce oospores.”
He said under cool, wet conditions conidia may infect young seedlings, which could result in systemic infections.
Systemically infected plants produce no grain and act as weeds by competing for water and nutrients with healthy plants. Competition is particularly noticeable when infection occurs in patches or clumps.
Isakeit says SDM was a serious sorghum disease in the Coastal Bend and Upper Coast areas of South Texas more than 20 years ago, but resistant hybrids provided primary control until new races or pathotypes occurred.
“Many hybrids grown in South Texas during the early 1980s were susceptible to pathotype 3, which allowed it to became a problem across the region, especially along the Upper Coast. However, widespread use of metalaxyl fungicide as a seed treatment effectively controlled systemic SDM caused by all pathotypes.”
Isakeit says a number of factors interacting over many years contributed to the recent outbreak of SDM. Factors may include: low seed treatment rates of metalaxyl; continued planting of pathotype 3-susceptible hybrids; increased grain sorghum monoculture; and seed treatments with Concep III herbicide seed safeners.
“The most critical factor,” Isakeit says, “is the low seed treatment rate of metalaxyl, which is ineffective or less effective in controlling SDM. These low rates allow other factors to increase both the yearly incidence of SDM and the soil borne populations of oospores.
“The low rates of metalaxyl can also aid selection for resistant populations, while sorghum monoculture allows such resistant populations to build rapidly in the soil.”
Isakeit says resistance to metalaxyl has occurred in at least one field in Wharton County. “Resistance has probably developed independently in other fields as well.”
Last year, Isakeit observed several sorghum fields in Wharton County with high incidences of SDM. “These fields were planted to pathotype 3-susceptible hybrids, and at least some were treated with the recommended rate of metalaxyl. Repeated sorghum monoculture will encourage continued build up of SDM oospores in soil, with the increased potential for yield loss.”
Some hybrids remain resistant to pathotype 3, Isakeit says.
“Treating these resistant hybrids with metalaxyl prevented establishment of new pathotypes, which might overcome resistance of those hybrids. When metalaxyl is no longer effective, chances of a new pathotype developing increase, particularly under a sorghum monoculture.
“Using SDM-resistant hybrids should not be a replacement for metalaxyl seed treatment. To do so while continuing a monoculture increases the risk of losing hybrid resistance as a control method,” he says.
So, how do farmers deal with the problem? “Ideally, SDM control should integrate several methods: fungicide seed treatment, resistant hybrids, and crop rotation, which reduces oospore populations over several years,” Isakeit says.
“An integrated approach will maintain the longevity of all control methods.
“Currently, metalaxyl resistance does not appear to be widespread, even within Wharton County. So metalaxyl can and should be used as a seed treatment, even in fields with a resistance problem. New pathotypes that would overcome resistance do not develop because they are sensitive to metalaxyl.
“The likelihood of selecting a new strain of sorghum downy mildew that simultaneously overcomes host plant resistance and is resistant to metalaxyl is very small.”
Isakeit says in fields with known or suspected metalaxyl resistance, farmers should concentrate on reducing soil pathogen populations with crop rotation.
“Two years out of sorghum would be a good start; three years would be better. Corn is the only other crop susceptible to SDM, but it should be safe to plant in these fields, provided growers plant at typical early planting dates.
“Even the most susceptible corn hybrids get little or no SDM in the cool soils present at normal planting dates, but warmer soils can result in increased infection. Infected corn produces little or no oospores.”
Johnsongrass is also susceptible to SDM, but is not a major contributor to disease problems because infection rates are generally low and the plants are usually controlled within a field.
After a period of rotation, Isakeit recommends planting fields with hybrids resistant to pathotype 3.
And don't abandon metalaxyl. “In fields with no SDM, use metalaxyl as a preventive seed treatment as follows:
Varieties susceptible to pathotype 3: Use a metalaxyl rate equivalent to 1 ounce (dry weight) active ingredient/100 pounds of seed for SDM control with susceptible varieties. Commercial formulations of metalaxyl labeled for this rate may not be available.
The corresponding rate for mefenoxam (also known as metalaxyl-m), the active isomer of metalaxyl, is 0.5 ounces (dry weight) active ingredient/100 pounds seed. For example, the SDM control rate of Apron XL 7 LS, a commercial formulation of mefenoxam, is 1.28 fluid ounces of formulation/100 pounds seed.
“This rate is critical to prevent fungicide resistance.”
Varieties resistant to pathotype 3: Use a rate of metalaxyl labeled for SDM control in resistant varieties, which ranges from 0.25 to 0.5 ounces (dry weight) active ingredient/100 pounds of seed. For example, with Allegiance-FL, use the formulation rate of 0.75 fluid ounces/100 pounds of seed.
The SDM control rate of Apron XL 7 LS is 0.32 to 0.64 fluid ounces of formulation/100 pounds of seed.
“Refer to the fungicide product label for additional information, particularly for allowable rates, as well as precautions,” Isakeit says.