I have received several calls from county agents in the last three weeks about stunted tobacco plants with a range of symptoms, each wondering about the possible role of systemic insecticides in the symptoms being observed. While there are more reports of stunted plants this year than in the last 3 years, it’s important to remember that systemic insecticides are used on over 90% of the tobacco acres planted in North Carolina each year with the vast majority of these applied in the greenhouse as tray drenches. Fields with enough stunting to result in a phone call to the county agent or extension specialist are still relatively rare.
It’s impossible to say definitively what caused the injury in these fields, but significant stunting, and in very rare cases plant death, is likely a result of many factors including but not limited to: weather, carry over of pesticides from previous crops, insecticide application method, disease pressure (in and out the greenhouse), transplant quality, fertilizer use and application, and insecticide rate. Because many factors that may influence stunting in early season tobacco fields and these interacting factors are so difficult to tease apart, we cannot predict when and how stunting will occur. There are, however, practices that growers can employ to reduce potential insecticide effects.
This year, all the calls I have received regarding stunted plants have been cases where the insecticide rate was 0.8 fl oz Admire Pro 4.6F/1000 plants, 0.9 fl oz generic imidacloprid 4F/1000 plants, or greater. These high rates of imidacloprid are only necessary in areas with high tomato spotted wilt virus (TSWV) risk (historic infection rates greater than 10% in imidacloprid treated plants on a regular basis) or in fields with a history of wireworm pressure. If this is not the case, a lower rate (0.6 fl oz Admire Pro/1000 plants) is sufficient for aphid and tobacco flea beetle control. We have seen an increase in plant stunting with an increase in insecticide rate in research trials, but it’s important to note that early season stunting is usually transient, and by mid season, it is often difficult to pick previously stunted plants out in the field.
Insecticide application method
Insecticide application method can influence the rate applied to areas within a greenhouse, which may result in “tray effects” in the field, meaning that strips apparently stunted plants can appear throughout fields. We recommend using a boom wide enough to cover the enter bed for greenhouse applications. All nozzles should be checked before making an insecticide application to ensure they are the same size, and are not worn (and therefore emitting too much material). Spray patterns should not overlap. Growers are discouraged from applying insecticides in the greenhouse with single nozzles or with backpack sprayers. Insecticides should be also applied 5 days or fewer before transplant. Keeping plants longer in the greenhouse may prevent them from actively growing and increase any potential impact.
Some fields, like those observed in Johnston County (above), also had herbicide injury symptoms, including bleached, twisted, and constricted leaves. Not all locations with reported stunted used herbicide, however.