Recently, we have been seeing many cases of lower leaf spotting in tobacco in all areas of the state (Figure 1). These symptoms are most common in crops that were set 3 to 6 weeks ago (late May).
In most cases, the spots do not appear to be the result of an infectious disease, although there may be some target spot involved in a few areas. Some of the spots have the distinct look of phosphorous deficiency, while others tend to look more like weather flecking. We have also seen early signs of potassium deficiency and an overall pale cast in some plants. These symptoms of general nutrient deficiency suggest a slow development of the root system.
In samples sent to the Plant Disease Diagnostic Labs, limited root development relative to the amount of time the plants have been in the field has been a common feature (Figure 2). Some plants show clear evidence of being infected with Pythium root rot at transplanting; the majority, however, have had apparently healthy roots but with very limited growth beyond the transplant rootball.
Although it is not certain why the root growth has been so slow, lower than normal soil temperatures this spring may have contributed to slow root growth. It is expected this situation will be temporary, and in fact, many plants are already showing signs of recovery with the bud and newer leaves having a dark green color with no spotting.
At this point, the following recommendations apply to fields with lower leaf spotting:
- In most cases, side dressing additional nutrients will not be necessary;
- Foliar feeding may provide some visual improvement, but will not likely result in increased yield. Be aware that foliar fertilizers can cause leaf burn; do not apply excessive amounts in any one treatment.
- Lay-by applications of 8 oz per acre of Quadris or Satori would help reduce the chance for significant target spot later in the season. Satori is a recently labeled “generic” for the active ingredient in Quadris.
By Bob Pearce, Extension Tobacco Specialist & Julie Beale, Plant Disease Diagnostician