SOYBEAN DISEASE UPDATE: SDS AND FROGEYE LEAFSPOT ON THE MOVE
By Don Hershman

Early last week we received several soybean sudden death syndrome (SDS) samples from Daviess, McCracken, McLean, Webster Counties. It is likely that new incidences will be reported this week. It is too early to be able to predict how extensive and damaging SDS will be in Kentucky this season. There were numerous reports of very early-planted crops this spring and this is known to encourage SDS. Time will tell if early planting has, in fact, "set- up" some crops for more extensive SDS than others. SDS has also been associated through the years with soybean cyst nematode (SCN) infestations. Thus, serious SDS may indicate an underlying SCN problem. Finally, SDS is frequently first evident in poorly drained and/or compacted areas of fields. This is most likely related to the soil moisture requirements necessary needed for root infection be the SDS pathogen, Fusarium solani. f.sp. glycines.

SDS is evident in plant foliage as random yellow blotches between the veins that progress within a week or so to dead tissue between the veins, with the veins remaining green. Diseased leaves on the upper part of the plant tend to roll up and may eventually fall off the plant. Many times the leaflets fall off, but the petiole remains attached to the stem. Usually, it is easy to find leaves on plants with the whole spectrum of SDS symptom expression. Foliar symptoms are caused by one or more plant toxins produced by the causal fungus, which is restricted to root tissue. Infrequently, virus-like foliar symptoms may precede the formation of leaf spots. This symptom may be the result toxin activity on expanding leaf cells.

SDS is primarily a root rot disease. Roots are infected in the late seedling and early vegetative stages. Roots then rot throughout the vegetative period and into flower and pod production. A milky, gray-brown stem discoloration is also diagnostic of SDS. Foliar symptoms usually become evident in the early to mid-reproductive stages. Usually roots are extensively deteriorated by the time foliar symptoms become evident. At this stage of the disease it is common to see pin-point areas of bright blue on rooted root tissue. These areas are masses of f. solani f.sp. glycines spores and this is another diagnostic feature of SDS.

Thus far, most of the affected plants we have seen have been at early pod formation to mid-pod fill. The amount of yield loss caused by SDS in a plant is heavily influenced by the stage of pod development when foliar symptoms become severe. If symptoms never progress past the mild stage (which is often the case), then little yield loss will be experienced. Severe yield losses are almost always associated with defoliation and pod abortion during early pod development (i.e., R3 stage or before). It is my experience that more times than not, SDS comes in too late to cause heavy yield damage. However, this is not always the case and I have seen almost total yield loss due to SDS.

The bottom line is this: If foliar symptoms are widespread and severe in a field prior to the mid- pod fill stage of development, then the likelihood that serious yield damage will occur is very high. In contrast, if the same level of disease exists in a field after mid-pod fill, then expect direct damage to be minimal. I say "direct loss" because even late SDS symptom expression will result in some indirect losses. Specifically, plants that are prematurely killed by SDS are prone to infection by pod and stem blight fungi and shattering. These situations have to do with the fact that prematurely-killed plants cannot be harvested until the rest of the field is ready to be harvested. As a result, prematurely- killed plants are left standing in fields longer than is desirable and, thus, experience problems normally associated with delayed harvest.

The key to yield loss due to SDS is timing, incidence and severity of foliar symptom expression. I have seen fields that began to express severe and widespread SDS foliar symptoms at full-pod, and some of those fields produced near-record crops. But, in most of those cases, the fields looked very, very rough and a casual observation would have not produced thoughts of record crop yields. If SDS is severe at this time, the only management decision that will have a positive impact on yield loss will be to harvest the affected field as soon as possible. This recommendation has to do with minimizing the effects of indirect losses due to SDS. More significant SDS management issues must be implemented at planting or before. I will discuss SDS avoidance strategies in a future KPN article.

The other disease situation that has recently been brought to my attention is that of frogeye leaf spot caused by the fungus, Cercospora sojina. Apparently, this foliar disease is severely impacting some fields in the southern tier counties, especially Simpson County.

Frogeye is evident on foliage as small, roughly circular leaf spots with a buff center and a purplish margin. A large number of leaf spot will give the foliage a blighted appearance. Over time, the centers of many of the leaf spots will drop out and the result will be a "shot-hole" symptom, which gives the affected crop a very tattered appearance. Stems, pods and seed can also be attacked by F. sojina, but the primary symptom is leaf spotting. The literature suggests that most soybean varieties commonly grown in the mid-south probably possess some resistance to frogeye leaf spot. This is probably why we do not see this disease more frequently. Occasionally, a highly susceptible variety is grown and, when the weather cooperates (i.e., a period of warm, wet weather which supports fungus spore production, and infection of soybean tissue), severe disease ensues.

The University of Tennessee at Jackson as done an extensive amount of research on frogeye leaf spot over the years. One thing that becomes very evident to me when looking over past year's research data, is how much frogeye soybean can take before serious yield loss becomes evident. For example, in a 1992 study, plots sprayed with two applications of the fungicide Topsin-M were rated as having 3-4% leaf surface area diseased and a yield of 60.8 bu/A. In contrast, the non-sprayed check plots had 40-50% of its leaf surface area diseased, but the yield was still a respectable 51.2 bu. That is a 9.2 bu difference, which is a very significant yield loss; nonetheless, I would have expected an even greater yield loss based on the amount of disease present in the check plots. Apparently, soybean has a tremendous capacity to compensate for loss of leaf tissue. As with SDS, I am certain that the soybean stage at the time symptoms become severe has a great influence on the amount of yield loss due to frogeye leaf spot. In the above example, the check plots had 11-25% diseased leaves on September 4. The reports that I have received from Simpson County this year indicate that diseased plants may already be at the 50% leaf surface area diseased or even greater. Considering that we are only in the first week or August, this might indicate a greater-than-normal possibility of serious yield loss in some Kentucky soybean fields. I hope this extent of frogeye is rare.

In any event, it is much too late in heavily diseased soybean fields to do any good by treating crops with a foliar fungicide. Several fungicides are labeled for use in managing frogeye, but those fungicides are all preventative in nature. Once serious disease is evident in a field, the damage is done and the materials will have little or no economic value. Generally, two applications of most fungicides are required to give excellent control of frogeye leaf spot. However, there are data from Tennessee which indicates that a single application of Topsin- M or Benlate during mid-pod fill can result in significant yield improvement when compared with non-sprayed plots. Logic assumes, however, that this situation would be null and void if serious disease already exists at the time a fungicide application is made. Certain doublecrop fields may be early enough in development and disease progress to benefit from a fungicide application. However, because of the reduced yield potential of most doublecrop soybean fields, there is a question if any fungicide treatment would be economical. This comment is based on the current low price of soybean, the rather high cost of treating fields, and the low probability that most fields will be impacted by frogeye leaf spot because of the "built in" resistance factor. If you do consider fungicide application an option, please note that Quadris and various formulations chlorothalonil (e.g., Bravo) are also labeled for use on soybean foliage in addition to Topsin-M and Benlate, which were mentioned above.

SOYBEAN APHID ACTIVITY IN KENTUCKY