USDA’s mid-September crop report¹ predicted record level corn yields for Kentucky of 155 bushels per acre.  Coupled with increased acreage, the state’s production could top 175 million bushels, which is also a record.  However, a potential ‘fly in the ointment’ with this year’s crop is the delayed harvest coupled with damp weather which has led to stalk, ear and kernel rots.  As noted in previous news stories, potential problems with field fungi (Diplodia, Gibberella, Fusarium, etc.) have lead to concerns about subsequent storage.  While not all fungi produce mycotoxins, mold-damaged kernels are more susceptible to those that do.  So it is best to err on the side of caution and check corn lots with field mold for mycotoxins before feeding to livestock.

When harvesting mold-damaged corn, adjust combines to minimize mechanical damage so that sound kernels are protected and to maximize

cleaning, so that lightweight kernels are removed.  Harvest, handle and store damaged corn separately when feasible and market early to reduce demands on storage management.

Grain moistures above 18-20% favor the growth of field fungi and the longer corn remains in the field the greater the chance of mycotoxin production.  Thus, damaged corn should not be allowed to dry in the field to avoid drying costs.  Corn with light damage should be dried to 15% within 24 hours after harvest and cooled to 40 degrees as soon as weather permits, in order to control mold growth during storage.  This will create a storage environment within the grain mass that is below 65% humidity, which is dry enough to control mold growth and development (see values in the equilibrium moisture table).  Corn with heavy to moderate damage should be dried to 13 to 14%, respectively, cooled as quickly as possible and moved before March.

The table on the next page presents the equilibrium moisture contents for shelled yellow corn at different temperature and relative humidity conditions.  Example: Corn that is 40 degrees and 13.7% moisture will create a relative humidity of 55% within the grain mass, which is safe for storage.

If mycotoxin problems are suspected, check with crop insurance providers to see if adjustments may be needed and how to account for the areas that are impacted.  Insurance adjustments generally need to be made on standing corn at or before harvest. 

The following publications provide more information on vomitoxin, aflatoxin and grain testing labs:
http://www.ca.uky.edu/agc/pubs/id/id121/id121.pdf
http://www.ca.uky.edu/agc/pubs/id/id59/id59.pdf
http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-MISC-1.pdf

Considering the cool, wet, and late year we have just experienced, it should not come as a surprise to anyone that certain late-season soybean fungal diseases are more extensive than usual. In a more typical year, crops that mature in late summer, especially early maturing varieties planted early, tend to experience the most intense foliar, stem and pod fungal disease pressure. This is because those crops are filling pods and maturing at a time when conditions tend to favor disease development (hot and wet). Normally, doublecrop and other late-planted crops are less susceptible to late-season fungal diseases because they mature in September-mid-October when conditions tend to be dry. Not this year!

Every year we have copious amounts of pod and stem blight and anthracnose (Figure 1). However, these diseases usually come in late and tend to be superficial. As a result, they often look more damaging than they really are, as long as affected crops are harvested in a timely manner. If crop harvest is significantly delayed, however, the diseases can impact seed quality (especially pod and stem blight).  Just looking around, I do not get the sense that either anthracnose or pod and stem blight are especially problematic despite the season. But a wet October will delay harvest in many fields and this could result in a range of problems, including reduced grain/seed quality.

The disease that seems to have been impacted the most this season is Cercospora leaf blight (CLB). CLB, caused by the fungus Cercospora kikuchii, is usually first noticed by producers when the upper leaves in the canopy begin to turn yellow, often with a bronze tint (Figure 2). A close inspection of affected leaves reveals very small, dark lesions that are frequently on or near major leaf veins and on petioles (Figure 3). “Bronzing” is the result of a multitude of lesions that have coalesced. When symptoms are severe, the upper surface of affected leaves has a puckered, leathery appearance (Figure 4). Severely affected leaves are blighted and eventually drop off the plant. Blighting of upper canopy leaves was rapid in some fields this year due to the unusually wet late-season conditions.  Some producers suggested that the visual impact was akin to frost injury or “sunburn”.

Pods are also commonly infected and both pods and seed can have purplish discoloration (Figure 5). The purplish discoloration of seed is known as “purple seed stain”. Extensive purple seed stain can reduce grain marketability, and planting severely infected seed can result in stand reductions in subsequent soybean crops

The pathogen produces a light-activated plant toxin called cercosporin. Cercosporin is red, which accounts for the tendency of diseased tissue to develop a purplish discoloration. The toxin causes plant cells to rupture and die.  This is what causes most the symptoms we see.

The impact of CLB is highly variable and can range from no significant impact to substantial yield and grain/seed quality reductions.  The foliar phase of the disease contributes the most to yield loss when the disease is extensive. The time of disease onset relative to crop growth stage, and the speed with which the disease develops, are the key factors that determine crop impact. If blighting occurs while pods are filling, then significant yield loss can be anticipated. However, if pod fill is mostly complete prior to the onset of blighting, than damage will be minimal.

In most years, in most fields, CLB is a minor problem. Past experience and observations suggest that most commercially available soybean varieties have at least some resistance to CLB in spite of limited breeding efforts targeting the disease. However, no varieties are immune. Fungicides can reduce CLB, but no fungicides are highly effective. In general, strobilurin fungicides do a slightly better job than triazoles or thiophanate-methyl. Also, single applications applied at the R5 (beginning seed) growth stage tend to perform better than applications made at the R3 (beginning pod) stage. Of course, multiple applications perform better than single applications, but even then, results are marginal.


Figure 1. Typical symptoms of late-season pod and stem blight and anthracnose.


Figure 2. Cercospora leaf blight (CLB).

Figure 3. CLB close-up.

Figure 4. Advanced CLB, just prior to defoliation.

Figure 5. Pod purpling and purple seed strain (Ohio State University photo).

The 2009 Kentucky growing season, cooler and wetter than normal, was exceptionally favorable for apple scab disease (Figure 6), caused by the fungus Venturia inaequalis.  Some apple orchards were left unprotected during wet periods suitable for infection when fungicides could not be applied due to wet conditions or when fungicides were washed off the foliage by rain.  Thus, despite well-intentioned disease control efforts (Figure 7), some apple orchards had significant scab on leaves and fruit (Figure 8) by season’s end.  Scab left on leaves this autumn will overwinter and produce primary inoculum of the fungus to start next year’s disease cycle.

Growers with scab in the orchard now will want to assess the damage and take actions that will reduce scab disease pressure for next year.  The scab fungus overwinters in infected fallen leaves left from the previous season.  In Kentucky commercial orchards, most of the spores that can start an apple scab epidemic come from within the orchard, however, unsprayed apple and flowering crabapple trees growing nearby can also be a source of inoculum.  Because scab spores don’t travel very far, the risk of scab infection early in the season can be greatly decreased by reducing or eliminating any old infections in apple leaves on the orchard floor.

Research done in many apple growing regions has shown that either flail-mower chopping of fallen leaves or application of urea will significantly reduce apple scab spore production the next year.  Doing both leaf chopping and urea application will reduce scab inoculum even more.  After a moist 2009 growing season, Kentucky apple growers could benefit from reducing scab inoculum harbored in apple leaf litter.  Leaf shredding and urea applications are relatively inexpensive and reliable sanitation methods that will decrease the risk of apple scab in 2010.

Chopping and shredding leaves.  Shredding all leaves on the orchard floor after they have fallen in November is thought to reduce the number of scab spores by more than 50%.  Scab spore reduction is less if the flail mower cannot reach into the strip between the trees.  The smaller the leaf pieces that are left behind, the more easily they will become decomposed and the more likely they will be consumed by earthworms.  Even leaf shredding done in March or April can have some beneficial effect.  In springtime, the scab fungus has begun to grow and prepare to release spores into the air at about the apple green tip stage.   If shredding is done in early spring, some of the leaves and leaf pieces will be overturned and the spores formed on overturned leaves won’t be easily released into the air.

Urea treatments.  A 5% solution of urea (46-0-0 spray urea or greenhouse grade) in water (40 lb urea per 100 gallons of water) may be applied to apple trees in the coming weeks just before leaves begin to fall.  This should be done as late as possible, within a week of leaf fall, so that translocation of urea into the tree can be avoided, but early enough to have most of the leaves still on the tree.  Urea inhibits the development of apple scab fruiting bodies on the fallen leaves and also hastens leaf decomposition.  Trees sprayed with urea will tend to defoliate more quickly than unsprayed trees.  Urea can also be applied to the leaves on the ground, after all the leaves have dropped.  For good coverage, apply about 100 gallons per acre.  An air blast sprayer with only the lower nozzles turned on can be used, but field boom-type sprayers will likely provide better coverage.  The ground spray can also be done in the spring, two to four weeks before bud break.  Feed grade urea can be substituted for urea fertilizer; it is more expensive, but will dissolve more easily in the spray tank.  Be aware that urea treatments can supply about 20 lb actual nitrogen per acre, so seasonal fertilizer rates will need to be adjusted appropriately.  Leaf chopping and urea application combined will reduce scab inoculum more than either alone. 

Reducing scab inoculum now will make apple scab disease easier to manage next spring and summer.  Reduced disease pressure next year will make it more difficult for the fungus to develop resistance to fungicides.

Figure 6. Apple leaves heavily infected causing leaf scab and yellowing symptoms.

Figure 7. Apple orchard air blast sprayer used for applying fungicides for scab control.

Figure 8. Immature fruit and leaf with apple scab disease symptoms.