VegNet Vol. 12,
No. 6.
On the WEB at: http://vegnet.osu.edu
If experiencing problems receiving this
fax, Call 614-292-3857
In This Issue
1. Early Sweet Corn and the Cold (2 articles)
2. Growers Guide to Understanding the DMI or SBI Fungicides
3. Crop Reports
Early
Sweet Corn and The Cold
The record low temperatures have played havoc with early sweet
corn from NJ to southern IL. The delightful mid April weather enticed many into
the field in order to avoid the extremely wet and cold conditions experienced
in May and June during the three previous springs. In OH, some early sweet corn
fields got frosted out as well as some early planted field corn. Along the OH river, several plantings had severe yellowing but made it OK
and will return to normal with good weather. Some bare ground corn in the 2
leaf stage escaped 27 degree temperatures with just some burnt leaf edges. More
mature plastic corn got fried but in several cases it is still alive and will
recover. Other plastic corn, with water for frost protection, had burn leaf
edges and turned extremely yellow but should return to normal rapidly with
warmer temperatures. Se corn, planted around Apr, 20, the beginning of the cold
spell, is taking anywhere from 2-3 weeks for emergence. In several cases, the
seed seems to be OK but there are some locations where the seed has
rotted. Soil temperatures in South
Charleston, OH went from 67 degrees down to 50-52 degrees in as little as 4 or
5 days in late April.
Growers should remember that sweet corn is safe from a complete
kill by frost until the 5th or sixth leaf stage. It is at this stage when the
growing point is at or above ground level and is susceptible to a killing
frost.
Now that sh2 planting season is here, there is an excellent
warning tag that some seed companies staple to their
seed bags. It says, “ Plant sh2 varieties in soil that
will maintain 65 degrees or higher to help assure adequate germination”. This
advice correlates well with 3 years of thermogradient
table test results on the germination ability of high eating quality sh2
varieties.
Below is an excellent article on sweet corn and cold weather from
the NJ Plant and Pest Advisory Newsletter
State of
the Union in the Sweet Corn Fields by Raymond
J. Samulis, Burlington County Agricultural Agent,
Rutgers Cooperative Extension. From: Plant and Pest Advisory,
This year, as in most years, weather conditions have been less
than cooperative for early sweet corn growth. Some fields that are planted
under plastic mulch are growing fairly well, although they exhibit some leaf
burn and wind damage. Fortunately, despite the unseasonably cold weather, the
growing point of sweet corn is still below the soil line and somewhat protected.
Sweet corn does have the ability to survive cold weather as was evidenced in
some of my field plots about 15 years ago. After a 1 to 2 inch snow, the emerged
plants not only survived, they went on to produce a crop. The problem for the
most part is that although growth of the plants was slow to nonexistent for 2
to 3 weeks, the actual days to maturity remains the same. This resulted in
shorter plants with smaller ears because the biological clock of the plant was
still ticking, and slow growth days are still calculated into the estimated
days to harvest. Fortunately, some varieties like Ice Queen and Sweet Ice to
name a few, can grow under less than ideal conditions particularly when soil temperature
hover around 50 to 55 degrees F.
Slower growth also creates other issues besides simply smaller
ears and stalks. Weakened plants are definitely more susceptible to seedling
diseases such as Fusarium, and although currently
there are some very effective fungicides applied by the seed companies, under
severe disease conditions they are never 100% effective. I think this scenario
was evident last season in the case of phytopthora on
pumpkins and tomatoes.
Slower growth predisposes the emerging seedling to particular
insects like seed corn maggot and wireworms. Many fields I look at have skips
that are often written off. Early season sweet corn under
clear plastic as a planter problem or poor seed quality when the real cause is
insects below ground. Problems are even more exaggerated for newer,
super sweet varieties because the seedling strength is weaker due to the
limited food reserves in the shriveled seed.
Two other problem areas I often see are low soil pH and not using
the proper ratio fertilizers. Some growers have used a calendar basis for
applying lime. Unfortunately, this is not adequate because of the use of
acidifying fertilizers and the natural weather related drop in pH. Fields that
test in the mid 5’s or below will never realize their full yield potentials
regardless of how much nitrogen is added. Remember that pH and the addition of
lime is the overall regulator of nutrient availability in the soil. Soil tests
are needed to evaluate the need for additional phosphorous after banded
fertilizer at planting. Many soil tests show high P levels and thus require
only marginal or no additions of phosphorous. The contrary is true with potassium
or potash. The sandy nature of our soils can result in medium to low potassium,
and thus, the usage of standard 1-1-1 ratio fertilizers do not address this
shortage. Soil testing is the only way to determine the need for both of these
elements.
Despite the trials and tribulations of insects, diseases, soil
fertility and the weather, I still expect to see good sweet corn supplies due
to the diligence and dedication of growers throughout
Growers
Guide to Understanding the DMI or SBI Fungicides
Andy Wyenandt, Ph.D., Specialist in
Vegetable Pathology,
The DMI (DeMethylation
Inhibitors) or Sterol Biosynthesis Inhibit ing (SBI’s) fungicides belong to FRAC group 3 which include the triazoles and imidazoles. Some of
these fungicides are commonly known as Tilt (propiconazole),
Nova (myclobutanil) and Procure (triflumizole).
SBI’s work by inhibiting the biosynthesis of ergosterol which is a major component of the plasma
membrane of certain fungi and needed for
fungal growth.
Resistance by fungi to the SBI
fungicides has been characterized and is generally known to be controlled by
the accumulation of several independent mutations, or what is known as
‘continuous selection’ or ‘shifting’, in the fungus. Such that in any given
field population the sensitivity to the SBI fungicide by the fungus may range
from extremely high (highly sensitive, i.e. will be controlled by fungicide) to
moderate (partially sensitive) or low (mostly resistant to fungicide). This
type of resistance is also known as quantitative resistance. With quantitative resistance
there are different levels of resistance to the fungicide due to independent
mutations, which is unlike the target mutations that occur in qualitative
resistance associated with the QoI fungicides (Group
11) (Plant & Pest Advisory 5/4/05). Because different levels of resistance
to the SBI fungicide may exist in the field, the fungal population may behave
differently to different rates of the SBI fungicide being applied. Hence, it is
suggested that using a higher rate of a SBI fungicide may improve control when
lower rates have failed.
For example, let’s say that a
Powdery mildew population on pumpkin has 25% high, 50% moderate, and 25% low
sensitivity to a SBI fungicide. If fungicide is applied at the low rate, only
25% of the population (highly sensitive) may be controlled. Whereas,
if the high rate was used, 75% of population may have been controlled.
The main point here is that if low rates of SBI fungicides have been used and
control seems to be weakening, bumping to a higher rate may improve control.
Unfortunately, it is difficult
to determine what proportion of the powdery mildew population is sensitive or
not sensitive by looking at the field until you have begun spraying. The best
advise, if you are using low rates and think those rates are not working like
you feel they should, move up to the high rate the next time the fungicide is
sprayed, and if the high rate doesn’t work it may be safe to assume the fungal
population has grown mostly resistant. Importantly, if the high rate fails,
whether you bumped up to a high rate or started with one, and control does not seem
adequate do not continue to use the fungicide. Recognizing if and when
fungicide chemistries are failing and when fungicide resistance is developing
is critical to producing successful crops and why scouting on a regular basis,
at least before and after each fungicide application, is important. Regular
scouting can help reduce unwarranted and ineffective fungicide costs. Remember
to always tank mix SBI fungicides with protectant (M)
fungicides (i.e. chlorothalonil) to help reduce the
chances for fungicide resistance developing and never apply SBI fungicides in consecutive
applications and always be aware of the fungicide rates you are applying.
Crop
Reports South east
Fog from the
Plasticulture
Strawberry harvest has begun on early maturing varieties such as Sweet Charlie,
Bish and