Beating spots and blight on veggies and tomatoes

Spots and blights on the foliage of vegetable transplants are annoying and difficult to halt. In fact, it can seem like the odds are stacked against you. But there are specific management tips for your day-to-day operations that can help to prevent, or at least minimize, spots, blights, and other disease problems.

Bacterial pathogens have long been a pest to tomato seedlings in the greenhouse and occasionally infect peppers. Some of the spotting or blighting that you might encounter on your vegetable seedlings may be caused by fungal pathogens. Determining whether your plants have been invaded by a fungal or a bacterial pest should be a priority. Trying to “guess” what the problem is by matching pictures from books or the web can be misleading. A diagnosis from a plant pathologist in a clinic is the best way to proceed. While some control strategies are similar for fungal and bacterial pathogens, there are fundamental differences that shouldn’t be overlooked. For instance, the products that may help to limit fungal tomato pests will not work against bacterial pests. Fungicides that help to limit Alternaria early blight or Botrytis leaf blight won’t help against the bacteria that trouble tomatoes and vegetables.

There are three different bacterial pests that cause problems on tomatoes and they are commonly referred to as bacterial canker, bacterial speck, and bacterial spot. Some specific strains of the bacterial spot pathogen can also cause disease on peppers. Bacterial speck causes small, dark-brown spots on tomato leaves. These spots may be surrounded by a distinctive and obvious yellow “halo.” Bacterial spot that occurs on tomato and pepper is not as easy to identify and results in spots or blotches on the leaves and stems that can vary in color from a medium brown to a dark brown or black. These spots are larger in size than those caused by bacterial speck.

Early symptoms of bacterial canker on tomato transplants include small, tan “blister-like” lesions on the leaves and petioles. Oftentimes, these symptoms are mistaken for physical damage caused by wind abrasion, drought, or improper nutrition. As the disease progresses, brown streaking and cankering become more evident. On larger, more mature plants, one side of the infected leaflet may die while the other side of the leaflet remains healthy and green as a result of the bacterial canker pathogen having invaded the internal tissues of the plant. This particular symptom is referred to as “firing” and can be helpful in diagnosing the problem.
 

Search and destroy

Tomato plants with bacterial spotting or blighting should be immediately removed from the greenhouse and destroyed. Removing infected transplants from the greenhouse is the most critical component of managing bacterial diseases once they’ve been introduced.

Once a plant is infected with a bacterial pathogen and the symptoms are obvious, it cannot be cured. In addition, tomato seedlings immediately adjacent to those showing symptoms should also be removed and destroyed even if they appear healthy.

Bacteria move readily in a film of water and can spread through splash droplets. When tomato plants are spaced closely and watered overhead, it is easy for bacteria to be spread from the surface of infected plants to nearby healthy plants. It may take a week or more for the telltale disease symptoms to develop on these nearby plants; in the meantime these plants appear healthy, even though they have been “showered” with troublesome bacteria. Think of these healthy-appearing plants that are next to diseased plants as “guilty by association” and dispose of them. In some situations, all tomatoes within a propagation block or in a greenhouse will have to be destroyed, depending on the severity of disease and whether it was diagnosed at the onset of the first symptoms or much later after the disease has intensified.

Although an epidemic may seem to have appeared overnight, chances are it began in just a few plants and progressed unnoticed for a few weeks. Plug sheets containing infected transplants and plants should not be reused. In theory, the pot can be sanitized if it is done correctly, but in practice, many do not take the time and effort required to effectively sanitize and are better served by disposing of these containers so that the pathogen is not carried over to new crops.

Since bacteria thrive in humid and wet conditions, keeping the greenhouse as dry as possible is important. Water plants at a specific time of day to ensure that foliage dries completely by evening. Good ventilation, air circulation, and low relative humidity (less than 85 percent) are also important in helping to maintain dry foliage. Purging humid air or raising the temperature by a degree or two can help during times of wet, overcast periods.

Bacteria enter plants through natural plant openings that occur along the leaf margin or via wounds. Clipping, pruning, or any other type of injury provides a means for the bacteria to enter the plant and should be avoided. If the plants must be clipped, pruned, or handled in such a way that injury is likely, keep the humidity extra low for at least 36 hours immediately afterwards to discourage bacterial infection. Also, keep in mind that bacteria can be spread on equipment that is used to clip and prune plants.

In greenhouses, the warm, wet, and humid conditions, along with close plant spacing, create a favorable environment for the spread and increase of bacteria. While bacterial pests can also multiply and spread in the field, the relative humidity outside of the greenhouse tends to be lower than the relative humidity inside of the greenhouse, while reduced moisture in the environment limits disease development.

Plant spacing in the field is increased compared to the greenhouse, so bacteria cannot be spread via splash as easily. While it is economical and efficient to spray transplants while in the greenhouse, bacterial pathogens are not readily controlled with currently available sprays.

Some bacterial pathogens have a history of being a contaminant on some vegetable seed. While vegetable seed is routinely tested and treated for these pests, a few contaminated seeds may slip by even the most rigorous procedures and make their way into that warm and wet greenhouse. As a result, some growers treat their tomato seed with either hot water or chlorine as an additional insurance policy. An Ohio State University Extension FactSheet authored by Drs. S. Miller and M. Ivey is available with step-by-step seed treatment instructions (http://ohioline.osu.edu/hyg-fact/3000/3086.html).

If treating seed with hot water or chlorine is not an option, applying a copper hydroxide product every seven days, beginning when true leaves have emerged, may be helpful. Some growers believe that a copper spray is more effective if it is mixed with a mancozeb fungicide, but this has not been widely demonstrated.

By itself, mancozeb does not have activity against bacteria. A combination of mancozeb + copper hydroxide would limit some foliar diseases caused by fungi (such as Botrytis and Alternaria) but there are other fungicides that would work better against these fungi. While Agri-mycin alone or in combination with copper hydroxide may help limit bacteria, this product does not list greenhouse use on its label. Currently, this product can be used on specific vegetable seedlings in the greenhouse since the label does not prohibit this use, but be sure to check with your state authority to ensure that this interpretation applies to your region.

Some bacteria that infect greenhouse vegetables have become resistant to copper and streptomycin. This is a problem that will increase over time. Without laboratory testing, it is im possible to know whether a particular bacterial outbreak will respond to these sprays. This emphasizes the importance of cultural controls such as seed treatment, keeping the plants’ foliage and greenhouse environment dry, catching disease problems early, getting an accurate diagnosis, and disposing of infected plants along with adjacent plants.