Farm News, IA
05-10-07
Wet conditions likely to cause more diseases
By Kristin Danley-Greiner, Farm News staff
With the seemingly endless rainshowers that have deluged the state, the excess
water likely will cause problems for producers and their crops.
Alison Robertson, Iowa State University Extension field crops pathologist,
said that producers may see more gray leaf spot, northern leaf blight, seedling
blights, Diplodia stalk and ear rot, and Fusarium diseases. Palle Pedersen,
agronomist at Iowa State, said that less than 1 percent of the soybeans have
been planted so far, so it’s too early to determine exactly what diseases will crop
up.
But producers definitely will have SCN and SDS, which are the two most
devastating diseases in soybeans.
“Other soilborne pathogens like BSR will probably also be present,” Pedersen
said. ‘‘Because of the wet conditions that we are currently faced with, in addition
to the reasonably warm seedbed, there is a potential that we will get a higher
incidence of Phytophthora root rot in our fields this year.”
“The wet conditions that many farmers in central Iowa are going to deal with this
year will influence our stand. Over the last four years, with support from the
checkoff and the Iowa Soybean Association, we have investigated seeding rate
recommendations from Iowa. Our data are very consistent and shows that final
stand of 100,000 plants per acre is enough to maximize your yield and return,” he
continued. “The challenge is what kind of a seeding rate that it takes to get a final
stand of 100,000 plants per acre.”
Pedersen noted that based upon past years’ research on saturated soils and wet
seedbed conditions like this year, producers can expect an average plant
mortality (the difference between seeding rate and final stand at harvest in
percent) of 30 to 40 percent compared to 20 to 25 percent under more perfect
seedbed conditions.
‘‘My recommendations for growers using a planter in Iowa, regardless of row
spacing, is using a seeding rate of 125,000 to 140,000 seeds per acre to get to a
final stand of 100,000 plants per acre. The lower end of the scale (125,000 seeds
per acre) is under ‘perfect’ seedbed conditions and then the high end of the scale
(140,000 seeds per acre) is for the areas in central Iowa this year where it is far
from optimum,’’ he said.
‘‘A fungicide seed treatment could also be a wise suggestion this year, especially
one that will protect you against Pythium and Phytophthora root rot. The wet
conditions that we currently are facing will favor seedling diseases that will
reduce our stand. Especially Phytophthora root rot could be a common problem
this year with the very wet and potential for warm seedbed conditions,’’ he
added.
Robertson said that the increase in disease pressure this year also stems from
the increase in corn-on-corn acres.
‘‘One of the most fundamental disease management practices is rotation to a
non-host crop. Obviously in corn following corn acres, this is not being done,’’
she said.
To help combat the increase in disease pressure, Robertson said that producers
should have planted or will be planting hybrids with good disease resistance,
good emergence and standability.
‘‘Then scout fields for disease and consider applying a fungicide if foliar diseases
are present or schedule an early harvest if stalk and/or ear rot is prevalent,’’ she
said.
Producers need to be checking their fields for lesions of gray leaf spot/northern
leaf blight and rust on leaves, which indicates a foliar disease, as well as moldy
ears indicative of ear rots and stalks that pinch easily, which demonstrates stalk
rot, Robertson said.
Producers also need to be scouting for the sporadic yet troublesome black
cutworm moth. Iowa State entomologist Marlin Rice reported that ‘‘significant’’
numbers of black cutworm adult moths were captured in pheromone traps during
April across Iowa.
Iowa State University entomologist Rich Pope said that many factors will
contribute to whether or not this pest has an economic impact this year, such as
the rain, as well as the intensity of egg laying, survival of eggs and larvae
between hatching and cutting size, plus the growth of the corn plants as food
sources.
‘‘Intense rainfall events could dislodge or drown larvae; however, there is no
good way to estimate how effective that is in an area. Areas that are ponded for a
few days might have the larvae killed, but adjacent field areas might have
surviving populations of insects,’’ Pope said. ‘‘Our advice is for producers to start
scouting based on the degree day models, and to make certain that they
correctly identify the larvae (dingy cutworm has a similar-appearing larva that can
feed lightly on corn leaves, yet is not an economically significant problem).
‘‘Then, if larvae are found feeding, follow recommended economic thresholds to
base potential rescue treatments on. I note that south-facing slopes that might
gain a bit more heat than the average area and also weedy fields at the time of
the flights are likely areas to scout first,’’ he added.
Pope said that the southern third of the state should be scouting this week for
early flight, then May 17-18 for the later flight. The middle third of the state should
scout May 18 and then May 19 in central Iowa. The top third of the state should
scout May 21, then May 20 in northwestern Iowa.
Pope and Rice said that if producers find leaf feeding and only black cutworms,
then mark off 100 plants in a row with stakes or flags and scout these same
plants for cutting for a period of several days at several locations across the field.
Then they can monitor the cutworm activity and determine whether the pests are
cutting plants and the percent cut plants.
‘‘The economic threshold is when cutworms average less than 3/4 inch in length.
An insecticide should be considered if 2 or 3 percent of the plants are wilted or
cut, or if cutworms are longer than 1 inch, treatment should be applied if 5
percent of the plants are cut,’’ they said. ‘‘If the field has a poor plant population,
20,000 or less, these thresholds should be lowered.’’
Stop scouting when the field is sprayed or when plants have five fully developed
leaves (stage V5). Cutworms have difficulty in cutting plants in the V5 stage
because of the larger stalk diameter, but occasionally they chew into the side of
the stalk and kill a larger plant, they said.
One ray of hope is that 75 percent of the bean leaf beetle population in Iowa is
predicted to have died in central Iowa and 92 percent is predicted to have died in
north-central Iowa.
More should die if no soybeans are available as a feeding source for the next two
or so weeks, said John Holmes, Iowa State Extension field specialist. But
producers are advised to begin scouting any soybeans that will be emerging
within the next week to 10 days.