Always in demand, tomatoes can be profitable crops for Florida’s farmers. However, tomatoes are susceptible to a number of diseases that can erode profits if they get a foothold in the crop and are not suppressed early in the cropping cycle. Some plant diseases are commonly introduced on transplants and can be avoided, so it is important to examine your plants carefully. If you save seed from previous tomato crops or produce your own transplants, consider surface sanitizing your seeds by treating with dilute bleach or hot water. Several protocols can be found on the web. However, be sure to test the protocol on a small batch of seed first, as tomato varieties can differ in their sensitivity to bleach or hot water.
Establish the crop in an area that has been rotated out of tomato production for a year or longer. Be aware that residue from previous tomato production, any volunteer tomato plants, and some weeds can harbor tomato pathogens. Avoid low-lying areas of a field or fields that had previous problems with diseases caused by soilborne pathogens or pests, unless soils were treated for such problems.
The Basics Of Plant Disease
Some examples of tomato diseases and their life cycles are listed below. Additional resources that provide more complete information are referenced at the end of this article.
Target spot can affect foliage, stems and fruit of tomato. Foliar and stem symptoms begin as small, brown to black spots with light brown centers and dark margins. There may be yellow halos around these spots. These initial lesions are extremely difficult to differentiate from bacterial spot lesions and require additional laboratory testing to properly diagnose. However, as target spot progresses, the lesions enlarge and develop diffuse concentric rings within the lesion surrounding a tan to dark brown center. These lesions can often coalesce, leading to blighted areas on a leaf or along the leaf margin, eventually blighting the entire leaf. Often the pathogen will establish on lower, older leaves before progressing into the interior of the plant canopy where the dense foliage increases leaf wetness favoring spore germination.
Target spot is easily controlled with timely fungicide applications. If target spot is a recurrent problem in your production area, preventative applications of broad-spectrum fungicide may be necessary to protect plants initially. However, as the plant canopy increases, specific-systemic fungicides are necessary to protect the denser inner canopy. Selective thinning of the canopy may also help reduce disease pressure by reducing leaf wetness and to improve coverage with broad-spectrum fungicides. Consult with the UF/IFAS Cooperative Extension Service for recommended fungicides. Gardens planted close to commercial tomato production fields are more likely to be affected by target spot.
Early blight, caused by Alternaria solani and A. tomatophila, is a fungal disease that damages the leaves and fruit of tomato. Similar to C. cassiicola, the fungal spores are easily spread by the wind. However, unlike C. cassiicola, A. solani and A. tomatophila are limited to members of the Solanaceae.
On leaves, symptoms begin as small, pencil-point-size, dark-brown to black spots that are difficult to differentiate from bacterial spot without additional laboratory testing. However, the lesions become more distinct as they enlarge, reaching up to a half inch in diameter and larger with readily visible, concentric rings that look somewhat like a bull’s-eye. These distinctive leaf spots make early blight one of the easier tomato diseases to identify. When compared to target spot, early blight lesions are typically dark brown to nearly black with very distinct rings and lack the tan or light brown center.
To control early blight, start with disease-free transplants and fertilize plants adequately. Inadequate nitrogen levels, in particular, make tomatoes more susceptible to early blight as it leads to premature senescence of lower leaves that are particularly vulnerable to infection. Similar to target spot, persistent problems with early blight will likely require preventative applications of broad-spectrum and systemic fungicides. Gardens planted close to commercial tomato production fields are more likely to be affected by early blight.