Massive amounts of rain has led to a number of problems for Kentucky burley tobacco, such as drowning, leaf scald and black shank, as we would expect.
We have also begun to see rapid development of spotting on lower leaves, weather flecking and in many cases severe scorching of bottom leaves.
Before the rains came, we were already seeing foliar symptoms of deficiencies of nutrients like nitrogen, potassium and phosphorus, which we suspected were directly related to poor root development resulting from generally wet soil conditions that had prevailed during the early season.
The presence of symptoms before the rains and the sudden development of severe spotting after the heavy rains have raised concerns that a new and explosive leaf disease is occurring.
However, even though some of the symptomology is similar to what we would see with angular leaf spot (which, in fact, is present in some areas); we have examined numerous samples and are sure that this problem is not caused by a pathogen. Instead, what we are seeing is the result of an interaction between stressed tobacco and the environment.
What's causing this new pathogen to thrive?
So why did these symptoms appear so suddenly and across a wide area of the burley region? The short answer is that we don’t know for sure. The problem may be related to one specific factor, or a combination of factors. Weather flecking may have a lot to do with what we are seeing, and it is possible that we are seeing worst-case examples of weather fleck.
Some of the spotting seems to be actual physical or mechanical injury from rain drops driving into the leaves. The symptoms of severe scorching between the veins of lower leaves and nutrient deficiency symptoms (like potassium and phosphorus) have also been common.
One possibility that would explain this sudden onset of symptoms is nutrient imbalance in the affected plants. Plants with impaired root systems would not have been able to take up nutrients at a sufficient rate to supply the growing points, and would have re-mobilized these from the lower leaves.
More than one nutrient could be involved, but we don’t really know with certainty. We are in the process of having leaves analyzed for nutrient content to try and pinpoint what might be deficient.
The scorching has a passing resemblance to Spartan injury, which can occur when excess rainfall washes herbicide into the root zone. However, these symptoms are showing up in fields that were not treated with Spartan as well as Spartan-treated fields, so our feeling is that we are not dealing with herbicide injury in most cases.
Something can be done
The biggest question of all is what to do in fields where this syndrome is occurring. The difficulty in making a recommendation right now is that we really don’t know the exact cause (or causes) of the problem. Until we get a better handle on the situation, the following are some points to consider:
- Although there’s not a pathogen associated with what we are seeing, we are concerned that extensive damage to the foliage will make it more susceptible to target spot and possibly brown spot. An application of Quadris is recommended for growers who have not applied it already. The standard 8 fl oz./acre rate should be sufficient, and growers who are seeing disease already should consider a rate of 10 to 12 fl oz./acre.
- The nutritional problems that we’re seeing right now are not generally associated with deficiencies in the soil, but instead are a result of the inability of roots to reach nutrients that are already there.
- Side-dressing would be of potential value if nutrients can be placed into the root zone without creating additional damage.
Thoughts on fertility strategies include:
- For tobacco that is waist-high or bigger, do not apply more than 25 pounds of actual N per acre.
- For tobacco that is smaller than waist-high, use no more than 50 pounds of actual N.
- Ammonium nitrate would be the ideal N source, but liquid N (UAN) would be an acceptable alternative.
- For potash, 100 pounds /A of sulfate of potash (0-0-50) should be sufficient.
- Foliar nutrient applications historically have not resulted in yield increases. However, under current conditions a foliar application may be effective to deliver a quick dose of nutrients to the crop.
- The most likely benefit from foliar application of fertilizer would be correction of micronutrient deficiencies.
- It will be less likely to see benefits from N, P, and K with a foliar application.
- Be aware that foliar fertilizers can cause leaf burn; do not apply excessive amounts in any one treatment.