Over the past several days, Extension specialists have been contacted by tobacco growers from several regions across Kentucky about spotting and yellowing symptoms of tobacco leaves (Figures 1, 2, 3). These contacts have been made in regard to field plantings established 2 to 5 weeks ago. Typically, it is the bottom leaves that are most impacted, while the bud and upper leaves may be pale green colored but not showing spots. While the exact cause of this problem has not been identified at this time we do not believe it is due to a leaf disease, but may be a sign of multiple nutrient deficiencies.
- Some growers have asked if the spots may be from frogeye leaf spot. Even though frogeye is not likely to be occurring so broadly and at this severity early in the growing season, growers are encouraged to send a sample to the Plant Disease Diagnostic Laboratories for confirmation. Comparing Figures 1, 2, and 3 to Figure 4, frogeye leaf spots typically have whiter centers, and the dark brown-red ring is narrower than those occurring on the leaves of concern from most growers. Growers should remain on the lookout for disease on stressed tobacco crops and make preparations for fungicide application if necessary. See below for additional thoughts on this.
- Dark tobacco growers may be concerned about the spots being a result of angular leaf spot (ALS). Mid-June is very early for ALS to be occurring on a broad scale, even though one case has been confirmed so far. ALS will generally be causing lesions that become angular in appearance, filling in the space between the leaf veins. These may or may not initiate in a circular pattern, and have relatively little chlorosis early in the season (Figure 5), whereas circularity and chlorosis are the predominant pattern in Figure 3. Angular leaf spot may increase in areas following heavy wind-driven rains so dark tobacco grower should remain watchful.
- The symptoms on these plants are reminiscent of a widespread problem observed following heavy rains around the 4th of July several years ago. In that case, the exact cause of the problem was never positively identified, but it was thought to be the result of multiple nutrient deficiencies due to root loss from soil saturation. In the current situation, it is thought that cooler than normal soil temperatures coupled with intermittent saturation has resulted in impaired root function. Thus, even though sufficient nutrients may be in the soil, the plants are unable to obtain those nutrients. A typical plant response to nutrient stress is to remobilize nutrients from older leaves to supply the growing bud, which is why the symptoms appear to be worse on lower leaves. Altogether, these environmental aspects may result in nutrient deficiencies in tobacco tissue, though this requires further testing. Growers may submit foliar tissue samples to non-UK labs for further detail on tissue nutrient levels. Foliar applications of water soluble fertilizers may appear to reduce symptoms, but have seldom been shown to result in higher measured leaf yields.
While Extension specialists are unsure what the cause of these tobacco symptoms is, growers should monitor the situation and in some cases, take action. With the hope of the weather drying out a bit and soils warming up, nutrient deficiency symptoms should decrease. If the leaf spotting worsens or spreads up the stalk, send samples to the Plant Disease Diagnostic Laboratories through county agents.
In fields with an extended history of tobacco production, plants the size of a basketball or larger should receive a foliar azoxystrobin concentration at 8 oz/A in at least 30 gal/A volume. While reduced sensitivity to azoxystrobin has been demonstrated in the frogeye leaf spot pathogen, applying an early azoxystrobin spray has been shown to decrease frogeye leaf spot at least 30%, even in reduced-sensitivity populations. An azoxystrobin application occurring now is also well-timed to manage early strikes from target spot.
By Bob Pearce Tobacco Extension Agronomist and Emily Pfeufer Extension Plant Pathologist