Nematodes attacking soybeans

Fred Warner
MSU Diagnostic Services

Soybean is host to many types of plant-parasitic nematodes. Obviously, the nematode that demands the most attention is the soybean cyst (SCN). SCN is a major limiting factor in the production of soybeans worldwide and is estimated to cost U.S. soybean producers over 1 billion dollars annually. However, because of its importance, other nematodes are often ignored.

Northern root-knot, lesion, lance, dagger, stunt, pin and spiral nematodes are found in soil samples collected from soybean fields in Michigan. Of these, only root-knot nematodes are regarded as serious pathogens of soybean worldwide, ranking in the top 10 of most important soybean pathogens. In our state, however, the northern root-knot nematode is infrequently (less than 25% of samples) detected in soil and root samples. This is probably due to the fact that soybean is often rotated with field corn or small grains and these plants are non-hosts for the northern root-knot nematode. Virtually all other types of plants will host this nematode, and it is a serious problem in vegetable, fruit and landscape plant production. The damage threshold for the northern root-knot nematode is unknown for Michigan but is estimated at 500 second-stage juveniles (J_2s) per root and soil sample.

Lesion nematodes are common (25 to 75% of samples) in samples collected from soybean fields. These nematodes are not regarded as serious pathogens of soybean, but anecdotal evidence suggests their feeding will result in the production of symptoms including yield loss in Michigan. They are typically found in high population densities on soybean when soybean cyst nematode is absent. As SCN numbers increase, lesion nematode population densities often decrease. Lesion nematodes are a concern because they feed on virtually all species of cultivated plants. In addition, they can predispose plants to invasion by other soil plant pathogens that inhabit the soil. The damage threshold for lesion nematode in Michigan is estimated at 300 per root and soil sample although lower numbers have been reported to damage soybeans in the southern United States. As mentioned, lesion nematodes can reduce yields of other field (row) crops, so they should be closely monitored and managed.

Lance nematodes are infrequently (less than 25% of samples) found in soybean samples collected from Michigan. They are rather large nematodes and are considered fairly serious pathogens of soybeans in the southern United States where the damage threshold is reported to be 4-100 per 100 cm³ soil. In Michigan, we estimate this threshold at 125 per root and soil sample. Lance nematode is also an important pathogen of corn.

Dagger nematodes are not considered important pathogens of soybean in Michigan. They are often found in high numbers on corn, and damage has been observed at population densities above 250 per 100 cm³ soil. Many grass species appear to host dagger nematodes. These nematodes are important in fruit production because they vector some very important plant viruses.

Stunt nematodes are similar to daggers in that they are largely insignificant on soybean but can reduce corn yields. They all do very well on many grass hosts. They do not vector plant viruses.

Pin and spiral nematodes are common in soil samples collected from fields in Michigan where field crops have been grown. Neither is regarded as a pathogen of soybean. Anecdotal evidence suggests that pin nematodes can retard growth of sugar beet in our state. High population densities of spiral nematodes are, on occasion, associated with stunt corn plants.

When root and soil samples are collected from fields and submitted for nematode analyses to Diagnostic Services at MSU, all genera (cyst, root-knot, lesion, etc. are common names of nematode genera) of plant-parasitic nematodes are identified and counted. For samples submitted using the MSPC-sponsored SCN program or sentinel plots, risk ratings will be assigned for all nematodes on soybean. Nematode risk ratings are as follows: 0 = none; 1 = low; 2 = moderate and 3 = high. Management recommendations will also be included.

Additional information regarding nematodes on field crops can be found in MSU Extension Publication E-1582, Insect, Nematode and Disease Control in Michigan Field Crops or any of the MSU publications on field crops ecology. For questions, please call me at 517-432-1333, Dr. George Bird at 517-353-3890 or Angela Tenney at 517-353-8563.

Stewart’s disease reported in field corn in Michigan

Willie Kirk, Diane Brown-Rytlewski and Jan Byrne; Plant Pathology
Beth Bishop, and Chris DiFonzo; Entomology

Stewart’s disease of corn has been reported in several field corn fields in Saginaw and Allegan counties so far this summer. The disease has also been confirmed in Clinton and Gratiot. So far we do not how extensive this disease is in Michigan.

Stewart’s disease is a disease of corn, caused by the bacterium Pantoea (Erwinia) stewartii and is sometimes referred to as bacterial wilt or bacterial leaf blight. The disease is common in North America, but is economically important only during periodic outbreaks (In a literature survey we found the disease appeared about every 30 years in Michigan in sweet corn.). Stewart’s disease is generally associated with sweet corn, but some hybrids and inbred lines of field corn are very susceptible. It is transmitted to corn seedlings during feeding by the corn flea beetle (CFB), Chaetocnema pulicaria (Photo 1). Bacteria invade the vascular tissue of the plant (Photo 2); disrupt nutrient and water flow, and cause leaf lesions (Photo 3), leaf wilting, barren stalks, or plant death.

Stewart’s disease has two phases; the wilt phase and the leaf blight phase. The symptoms initially appear as leaf lesions originating from corn flea beetle feeding scars (Figure 1 and 4). Disease severity depends on the variety and on plant age at the time of infection. Early plantings of highly susceptible seed corn inbreds and sweet corn hybrids are likely to have the most severe symptoms. The bacterium is carried in the digestive tract of the corn flea beetle. Stewart’s disease severity is thought to be dependent on the overwintering survival of infected corn flea beetles, which in turn is dependent on winter temperatures.

Earlier last century, a risk index was developed, defined as the sum of the monthly average temperatures (°F) during December, January and February. Indices less than 90 predict no risk for Stewart’s wilt, and greater than 100 predict severe risk. The overwintering habitat of the vector is not well understood, but is thought to be in the upper soil profile and in the root zone of grasses; however, the predictive index was based on ambient air temperature. The temperature profiles of the upper soil, as well as the duration of temperature below a critical threshold, may result in a more accurate prediction of corn flea beetle survival. High risk indices were recorded in winters of 1997-1998 (greater than 100), and low risk indices in 1998-99 (less than 90) the latter forecasting poor survival of corn flea beetle, yet sweet corn growers experienced an average of 44% crop loss (susceptible varieties) from the disease in both 1998 and 1999. A survey of Michigan sweet corn growers indicated that growers aware of the index did not use it as they considered it unreliable.

Previous monitoring. (Discontinued in 2003 after Stewart’s disease no longer reported in Michigan)

The temperature of soil (6-inch depth), soil surface and air (36 inches above the soil) was monitored at various locations within fields (tree or ditch line, margin of grass beyond the headland, at the edge of the field and 30 feet into the field) in different Michigan counties from 1997-2003. In all years, corn flea beetle have survived the winter and emerged during the spring to early summer from 1998 to 2003. However, since 1999, the incidence of Stewart’s Disease has declined markedly. The results for 2002 through 2003 are shown in Table 1. The soil temperature at each location within the fields remained on average close to 32°F at both Clinton and Macomb sites. The soil temperature average predicts possible to high survival potential especially at the ditch margin (where corn flea beetle are thought to hibernate). Soil surface and air temperatures were generally more variable than soil temperature (see standard errors in Table 1). The least variable location within both fields was at the margin of the ditch. However, it is not yet known how many hours of exposure below a fatal temperature are required to kill corn flea beetle. As in previous years, traps will be set to evaluate the survival of the beetles over the winter, and from these numbers, it may be possible to determine which of the temperature recording sites within the fields best predicts survival.

In additional studies conducted in Monroe and Macomb, overwintering survival of corn flea beetles in different sites was monitored by yellow sticky traps. Traps were placed in these sites in the fall of 2001. Traps were collected and replaced weekly. Corn flea beetles captured on these traps were counted. Traps were also placed in these same sites in the spring of 2002. Flea beetles overwintering in different sites clearly had different survival rates (as determined by comparing the number caught on traps in fall 2001 vs. spring 2002). As expected, beetles overwintering in the protected ditch bank survived in higher proportions than beetles overwintering next to the relatively unprotected farm road. From late August through late October 2002, corn flea beetle were captured in Macomb and Monroe counties. The percentage of corn flea beetle carrying the Stewart’s disease bacterium (E. stewartii) was calculated using a technique developed at MSU. Each corn flea beetle was crushed and plated on Erwinia-selective culture medium. The yellow colonies were further subcultured and their DNA extracted. The bacterium Erwinia stewartii has a unique pattern of DNA in a specific area of its DNA plastid which can be identified using a molecular technique called PCR. If the DNA plastid contains one or both of two unique bands, it shows up on an agarose gel as a bright band. The percentage of corn flea beetle carrying E. stewartii was then calculated (see Table 2). The percentage (and number of captured corn flea beetle) declined through late 2002, but it was clear that a very small proportion of corn flea beetle were carrying the bacterium.

Although it is not possible yet to determine how temperature survival prediction potential will interact with the bacterium carrying potential of corn flea beetle, it is clear that both factors play a crucial role in determining Stewart’s disease risk potential. Many growers have determined they will use an insecticidal seed treatment as an insurance against Stewart’s disease for two reasons; the temperature predictor is unreliable, and seed is usually treated during the winter months prior to the end of February (the end of the prediction period). The use of the molecular prediction system may help growers determine potential risk in the future once we have determined the critical inoculum potential that may initiate and epidemic the following year.

The thermal indicators for both Allegan and Saginaw (Table 3) for 2005-06 indicated that the survival potential for corn flea beetle was very high (using soil temperature as an indicator), but that if air temperature was used as an indicator, survival potential was low. Having symptoms and signs (bacterial streaming) of Stewart’s disease at this stage of the season should not cause concern for this year’s crop. However, the presence of corn flea beetle plus Stewart’s disease affected plants indicates a potential for carryover to 2007. For a problem to occur in 2007, the thermal survival potential for corn flea beetle has also to be taken into account. As noted in the previous paragraph, growers may have to decide early whether or not to use an insecticidal seed treatment such as Gaucho in 2007; alternatively growers could rely on a foliar insecticide (if corn flea beetle survive and are reported in the spring of 2007) to avoid the cost of a seed treatment. Additional information about varietal susceptibility to Stewart’s disease (if known) should be requested when selecting varieties for 2007.

A video is available at http://www.greeen.msu.edu/newspage.htm on Stewart’s disease management in sweet corn with a brief footage of corn flea beetle (live).

Table 1. Average temperature from December 1, 2002, to February 28, 2003, at four locations (field ditch base, margin of field ditch, edge of field and 50 ft into the field) within two fields at Manchester (Clinton County) and Romeo (Macomb County) at 6" soil depth, soil surface and 36" above the soil surface.

	soil			soil surface				air
	Average temperat-ure ^oF	standard error of estimate	Likeli-hood of CFB survival	Average temperat-ure ^oF	standard error of estimate	Likeli-hood of CFB survival	Average temperat-ure ^oF	standard error of estimate	Likeli-hood of CFB survival
Manchester
ditch base	29.6	3.8	ns	26.8	5.3	ns	22.1	11.3	ns
ditch margin	33.4	2.0	hs	29.9	2.5	ns	21.8	11.7	ns
edge	32.6	2.9	ps	29.6	3.1	ns	21.8	11.6	ns
field	34.5	3.4	hs	28.2	3.9	ns	21.8	11.3	ns

Macomb	soil			surface				air
ditch base	30.5	3.4	ps	28.8	5.0	ns	23.3	11.0	ns
ditch margin	32.2	4.7	ps	28.8	5.6	ps	23.2	11.0	ns
edge	32.4	3.0	ps	27.0	7.5	ns	23.3	10.9	ns
field	29.0	4.3	ns	29.0	4.3	ns	23.4	10.9	ns

Scale: hs - high survival potential (>33.3); ps - possible survival potential (30 to 33.3); ns - no survival (<30)

Table 2. Number of corn flea beetles trapped during late August, mid-September and mid-October in 2002 at five sites in Macomb and Monroe counties and the percentage testing positive for presence of Erwinia stewartii. The absence of data in September and October means that no beetles were trapped.

County	Site	CFB #	% positive
Aug 2002
Macomb	1	3	0.0
Macomb	2	17	0.0
Macomb	3	18	5.6
Macomb	4	15	0.0
Macomb	5	48	2.1
Monroe	1	44	2.3
Monroe	2	59	1.7
Monroe	3	59	11.9
Monroe	4	65	15.4
Monroe	5	102	5.9
Sept 2002
Monroe	3	200	0.0
Oct 2002
Monroe	3	200	1.0

Table 3. Average temperature from December 1, 2005, to February 28, 2006, at 2 and 4" soil depth, soil surface and 48" above the soil surface.

	soil 2" depth			soil 4" depth				air
	Average temperat-ure ^oF	standard error of estimate	Likeli-hood of CFB survival	Average temperat-ure ^oF	standard error of estimate	Likeli-hood of CFB survival	Average temperat-ure ^oF	standard error of estimate	Likeli-hood of CFB survival

Allegan	35.1	2.02	hs	35.4	1.91	hs	28.9	8.53	ns
Saginaw	36.3	2.45	hs	39.6	1.79	hs	27.1	8.80	ns

hs - high survival potential (>33.3); ps - possible survival potential (30 to 33.3); ns - no survival (<30)

Regional reports

1 – Southeast

Ned Birkey

Weather

Weather has been unsettled with scattered and locally heavy rain and even hail. Temperatures are moderate and rains are generally welcome, though rains are past the point of helping most crops.

Commodity reports

Alfalfa has potato leafhoppers over threshold. The third cutting is continuing, though unsettled weather is not helping with a timely harvest.

Corn is mostly dented and some is mature. Silage harvest began about two weeks ago. Some corn will be harvested in September. Some on-farm grain storage is being constructed. European corn borer and corn earworm traps are still catching moths.

Soybeans are turning in some fields with a few fields losing leaves. Late August rains will help the yield of some green soybean fields. Japanese beetles are still prevalent. Bean leaf beetles and corn rootworm beetles can be found, though very few soybean aphids. The MSU Soybean Variety Trial location in Lenawee County will be signed today so farmers can observe the varieties. It is located on Holloway Road, west of Bucholtz Highway and north of Deerfield. It will be interesting to learn if tropical storm Ernesto has brought any soybean rust spores to Michigan. At a field day yesterday, a Conklin sales rep asked if any farmers had Asian rust and a Washtenaw County farmer and a Monroe County farmer raised their hands. I seriously doubt that any of them knew what they were talking about.

Wheat acreage will remain high this fall, depending upon the fall soybean harvest