Maryland Field Crop Diseases

Soybean Rust Update
 
Soybean rust has recently been found in Suffolk, VA and in 3 locations in NC. In the last 3 weeks the disease has been spreading very rapidly in Mississippi and Arkansas and from those sources north up the Mississippi valley into KY, TN and IN. Meanwhile it has made less dramatic but equally rapid progress through GA, SC and now into NC & VA.
 
I have just recently returned from Florida where the soybean rust group was meeting and the consensus is that first finds like the ones just declared for NC and VA are typically 30-45 days after the spore shower brings spores to a susceptible (post-flowering) crop. As I traced back the spore shower model that is part of the national forecasting system, there were events from 20 Aug through 1 Sept that probably brought spores to those regions that resulted in these finds. The important thing to note is those same events include us. It is equally likely that soybean rust is in Maryland from those same spore shower events as it is likely that we were on the outer reaches so that spore densities were so low that transport to our region wasn't successful.  We continue to scout but are looking for a needle in the haystack. Virginia found only 1 leaflet with 12 pustules on it and it was after incubating the sample in the lab for 11 days. This means it is likely that infected plants are rare and only now are beginning to show symptoms in the field. If we also have infected sites then symptoms may not yet be apparent or are extremely rare.
 
The only risk to our soybean crop is to very late-planted fields that are somehow at least a month away from completing grain fill. It would take at least another 30 days after pustules start to develop to get enough spread and development within the canopy to begin to be damaging. There is little or no benefit from fungicide spraying once the crop reaches full seed in the upper nodes (R6). If you take that into consideration there is maybe 1% of the crop that is at any risk of low-level yield loss. And that is if we continue to get spore showers so that disease development is not totally dependent on spores developing from within the field in secondary infection cycles. The bottom line is there is no need to spray fungicides for soybean rust in Maryland. If we find the disease any really late planted field at risk can be evaluated on an individual basis and yield potential needs to be seriously taken into consideration.
 
We are still interested in finding soybean rust if it is here. Even though the commercial crop is essentially out of danger this season, every bit of data we can extract from every season is helpful. Although many growers in our area are no longer concerned with this disease, the real potential of soybean rust in the US is not yet fully understood.  An observation made by Dr. Jim Marois in Florida is that in the five years that soybean rust has been in the US, the number of days from planting soybeans to disease has been declining every year. This is in the Florida panhandle where rust overwinters every year.  One interpretation is that it is still getting established and the real potential for damage and spread is still on the horizon. We can't get complacent.
 
Corn Harvest

This has been a cool and relatively wet season. As a result the foliar disease mix has been somewhat different this year with Northern corn leaf blight, Southern corn leaf blight and Gray leaf spot all being involved to some extent on susceptible hybrids.  Perhaps of greater importance at this time of the year is the appearance of some ear rots or infected kernels. In cooler seasons like this we tend to see more Fusarium kernel infections as well as other ear diseases. The main point that needs to be made to producers at this time of year is that drying will be important to preserving the harvested corn. Seed needs to be dried to 15.5% moisture as soon as it is practical in order to slow down and stop kernel-to-kernel spread of the diseases in the bin. There are several different species of Fusarium that infect corn but most of them are capable of producing toxins in the grain.  This is a similar story to vomitoxin developing in scab infected small grains. It is in all our best interest to encourage drying harvested corn to keep toxins in this crop to a minimum.