When tobacco plants show signs of injury or abnormal growth, farmers often blame chemicals. But experts point out that, when used correctly, chemicals don’t harm tobacco plants, and other factors are often to blame for these
Leaf injuries—curling, weather flecks, spots, etching or discoloration—result from a variety of problems. Cold injury, weather fleck and sun scald are three weather-related conditions that lead to damage often confused with chemical injury.
In the greenhouse, hot sun during the day and cooler temperatures at night can result in cold injury in tobacco plants. In a float system, plants can be subject to the evening’s chill because they lack the ground’s insulating effect.
The big temperature drop from a 90-degree day to a 50-degree evening can affect a plant’s bud, which controls its hormones. The resulting curled leaves “can be very deceiving and look a lot like chemical injury,” says Gary Palmer, assistant director for agriculture and natural resources (and former tobacco specialist) at the University of Kentucky.
Palmer notes the best way to diagnose cold injury is to wait and see. If plants start to recover and show new growth, the problem was likely cold injury. A continued decline of the plants would indicate a chemical cause. Making sure greenhouses are well-ventilated can prevent cold injury. “It’s the big change that gets you,” observes Palmer.
Weather fleck appears as brown spots on leaves and resembles a physical burn or contact injury from herbicide or fungicide, says Matthew Vann, assistant professor and tobacco Extension specialist at North Carolina State University. When weather patterns form a cloudy cap in the atmosphere that keeps air from rising, ozone molecules are trapped in the lower atmosphere, and the plant absorbs them as it breathes. Brown spots show up on the leaves when leaf cells die off after
interacting with this ozone.
The damage usually occurs on lower leaves that are thinner and therefore more susceptible, says J. Michael Moore, professor and tobacco Extension agronomist at the University of Georgia. Vann notes that weather fleck usually occurs earlier in the growing season than chemical injury. However, weather fleck could happen after growers have applied herbicide. This timing can lead to the misdiagnosis of chemical injury. Plants typically recover from weather fleck with no
Long periods of heat cause sun scald, in which large areas of tobacco leaves turn brown and dry. If temperatures get high enough early in the day, the leaves may burn to a crisp, says Moore. Growers may suspect chemical drift or improper use of pesticide when these symptoms arise. Moore suggests first checking the plants for stink bugs (which cause a similar injury) then noting the orientation of the injured leaves relative to the afternoon sun. “It will be the sides facing southwest that get midafternoon sun.” In this case, time and hoping for more favorable weather are the only remedies.
Viruses, nutrient levels and genetic mutations can also affect leaf appearance and mimic chemical injury.
Magnesium and manganese are two important elements tobacco plants require in just the right amounts. Absorption of these elements is affected by soil pH. Moore says tobacco growing in soils with high pH and high moisture may not be able to absorb enough manganese and magnesium.
Leaves of manganese-deficient plants will show a lot of flecking and discoloration, with a pale color starting on the bottom. In extreme cases, “flecks coalesce along the veins, the lamina is lost, and it looks like a fish skeleton,” describes Moore. Vann points out that manganese deficiency is rare because most fertilizers contain it.
Magnesium deficiency, or sand drown, appears as an abnormal whitening of the leaves. Magnesium deficiency is also uncommon but may occur in deep, sandy soils under severe leaching conditions.
Meanwhile, a low soil pH can result in plants absorbing too much manganese. This stunts the plants or causes leaves to turn a pale yellow color, with secondary veins remaining green. This condition is also pretty rare but can occur if growers haven’t kept up with liming and soil tests, Vann notes.
Palmer emphasizes the importance of soil testing: “Get a soil test in the fall when you can get lime on, or spring so you can make adjustments.” Because testing pH is difficult to do accurately, he urges growers not to “base lime application on a cheap test” but to get their soil tested at a state or Extension laboratory. If a grower suspects a plant is nutrient-deficient, he should take the plant and a soil sample to a diagnostic lab.
Diseases such as spotted wilt, potato virus Y and tobacco etch can all damage leaves. Viruses known to growers in one region may not be known to those in another, and patterns in leaf damage could be blamed on pesticides.
Black root rot and genetic mutations can cause abnormal growth and stunting of tobacco plants. Genetic mutations may be indicated if only a few plants exhibit abnormal growth and they are surrounded by healthy plants.
Damage to roots from cultivation too close to the tobacco plant can also lead to leaf damage and abnormal growth. Additionally, when field subsoil has a lot of clay and the soil gets too wet, disking can create an impenetrable clay layer, known as hardpan, says Vann. “Then in June and July, when it’s hot and dry,” explains Vann, “the plants can’t get nutrients or moisture from the deep soil.” Plants cannot grow and appear stunted or wilted. He also notes that growers usually notice hardpan problems at layby, which is later in the season than when chemical injury tends to happen.
With all the potential candidates for leaf injury and stunting, how can growers decide what may be hurting their tobacco plants? Moore emphasizes that “good utilization of education personnel is the first line of defense.”
Vann agrees, saying, “One of the best things you can do is contact local Extension agents and check bulletins.” Photos of problem plants can be sent to Extension agents, and links to internet photo databases (such as IPM Images) can be found on state websites.
Additionally, growers should send soil and plant samples to Extension and state labs. It is important to obtain a sample of the affected part of the plant, as well as the soil around the plant. Providing the laboratory with a healthy or unaffected plant for comparison may help technicians isolate the issue faster. Digging, not pulling, up the plant and soil is key so that root pathogens are included in the sample and not shaken loose, says Palmer.
According to Vann, “It is always great to spot-check equipment to make sure it’s herbicide-free or free from fertilizer contamination.” When used correctly, chemicals should not injure the plants. “Tobacco is fairly tolerant to many of the chemicals we use, or we wouldn’t use them,” Palmer concludes.