Jude Boucher, UConn Extension Educator
The main pests of tomatoes in our area are all diseases. Among the fungal diseases, there are three important/common ones that attack your tomatoes annually or most years: early blight (EB), Septoria leaf spot (SLS), and Anthracnose ripe rot (ARR). There are also three bacterial diseases that are quite common and destructive: bacterial canker (BC), spec (BS) and spot (BLS). All these diseases except for ARR (which attacks the fruit) start by infecting the lowest leaves and then work their way up the plants. In recent years, there is also one water mold, late blight (LB), which has become an annual threat to your tomato crop. LB does not necessarily start on the lower leaves; it can start on any part of the plant, except the roots. It sounds like a lot to remember, but this article will explain how you can manage these problems. What you are simply trying to do is prevent or delay the initial infection and slow the spread of the diseases, so that your plants and fruit stay as healthy as possible, as long as possible.
Alternative Management Strategies
There are many alternative control methods used in tomato production that help control these diseases, some, or many of which, you probably already use. Crop rotation is a very important part of tomato IPM because almost all of these diseases overwinter in the residue of the previous year’s crop. It is recommended to rotate to non-solanaceous crops, and if possible non-cucurbit crops, for two years before trying to grow tomatoes again in the same field. Otherwise, several of these diseases simple move from your old infected plant residue to your new seedlings the very first week after transplanting, and have all season to spread and cause damage.
Many of these diseases are favored by wet weather and long periods of leaf wetness. Choosing open sites on hills where winds can dry the foliage quickly can be almost as effective as a good fungicide program in some years.
The trellised-plasticulture system that many growers use for solanaceous and cucurbit crops combines many different alternative control practices into one pest management/plant production system. The plastic not only prevents weed growth and promotes crop earliness; it also acts as a barrier to help keep ARR and other sporesfrom splashing up from the soil to infect your fruit or new plants. The stakes and trellis system helps hold the plants up off the ground creating a spacial barrier between spores in the soil and the plants, and helps the wind dry the leaves to minimize infection periods. Timely pruning of suckers reduces the amount of foliage and the humidity in the plant canopy, which allows the leaves to dry faster and resist infection. The trickle irrigation under the plastic promotes continuous plant growth and minimizes leaf wetness time and disease infection. And raised beds help keep the plant roots dryer and healthier.
Good cultural and sanitary practices can also play key roles in disease prevention and suppression. Hot water seed treatment can often disinfect seeds that carry bacterial diseases, effectively stopping a plant epidemic before it gets started. Disinfecting benches, flats, and tomato stakes can also help keep bacterial diseases from infecting new seedlings. Not pruning a tomato field infected with bacterial canker can help keep the disease from entering the pruning wounds and killing the whole plant or planting. Hastening the destruction of crop residue and volunteer plants that may act as hosts and sources of infection can be critical for controlling many diseases. Resistant or tolerant varieties can also help minimize disease damage and maximize yield and profits. A healthy soil, without a plow pan that restricts root growth and water movement, can help reduce plant stress, reduce disease, and improve yields.
Scouting for disease
Here is how we try to make tomato IPM simpler. EB and ARR are on every farm each year, and EB is usually the first disease you will see, so you should design your disease control program around these diseases. EB overwinters on the debris of infected tomatoes or related host plants. It requires 2 to 48 hours of leaf wetness at moderate temperatures (61-90oF) for a spore that lands on a leaf to invade (infect) the inner tissue and begin to form a lesion, which will produce secondary spores. The EB leaf lesions are usually oval, dark grey or brown, about ½- to 3/4-inch in diameter, and have concentric rings like a target, so they are easy to recognize. You find these lesions on the lowest leaves on the plant first, usually with a discolored yellow area around the spots. As the lesions become plentiful they kill the lower leaves and spread up the plant killing the foliage as the disease progresses. EB is what kills most tomato plants in most years. You can scout your plantings once each week looking for the yellow on the lower leaves or the lesions by walking every other row, and checking three rows at a time. With more wet periods, these lesions or spots will spread to higher leaves and new plants. So, you start your fungicide program as soon as you see the first EB lesion, and spray at 7 to 14 day intervals, depending upon how wet or dry the weather is, respectively.
There is a computer model (TOM-CAST), based on temperature and leaf wetness time, that can help predict EB infection periods and help with spray timing, but I have found over the years that it is simpler and more reliable to scout your fields. Scouting allows you to find the first disease occurrence as quickly, or quicker than the model would predict it, and the model doesn’t alert you to the presence of other pest problems (e.g. other diseases, insects and animal pests).
Septoria leaf spot is favored by similar wet environmental conditions as EB and also overwinters on infected crop debris, but it can be more aggressive, and fortunately, not as common. Where EB occurs on all farms each year to some degree, SLS only occurs on 40 to 60% of the farms in CT each season. The SLS spots are easy to distinguish from EB because they are smaller (1/4-inch) with an ash-grey center. ARR is what causes most of the round, sunken, water-soaked spots on the surface of the fruit. This fungus persists in soils for years and often infects the fruit while they are still green, but symptoms don’t become obvious until the fruits mature (color) and the fungus resumes its growth.
Bacterial diseases tend to be introduced to a farm on infected seed and may overwinter on crop residue, flats or stakes and may persist for many years, depending upon the sanitary and cultural practices used. Bacterial canker can be distinguished from specand spot because the margins of the leaves first turn yellow, then brown, before spreading inward and killing the entire leaf, and upward, eventually killing the whole plant. Bacterial spot and spec both produce small black spots on the leaves, similar crop damage, and can be hard to tell apart. However, the amount of spot symptoms will decline as the nights and weather gets cooler in late August, while often the amount of spec will increase with these same conditions. Since pest management recommendations for spec and spot are similar, it is purely academic which disease your crop has.
Products and Resistance Management
A good protectant fungicide will help reduce infection and slow the spread of all three fungal diseases (EB, SLS, and ARR). Copper is still the most effective bactericide we have, and can provide tomato plants on both organic and conventional farms with some protection from bacterial and fungal diseases, as well as limited protection from LB. However, copper has the potential to cause phytotoxicity, whether visible or invisible, and can affect yields. For this reason, conventional growers may only want to add copper to their spray mix if their crop has a bacterial disease, and they have the potential of gaining more than they lose by using copper. Organic growers have limited choices of effective products to fight funguses and may find that they increase yields in most years using copper compared with unprotected plants. Some other fungicide options for organic growers include Regalia, Oxidate and DoubleNickle 55, while popular choices for conventional growers include Dithane/Manzate (prior to fruit coloring), Bravo, Cabrio/Quadris (systemic fungicides).
Because LB is a water mold and not a fungus, the usual protectant fungicides (e.g. Bravo and Dithane) have limited effect on this organism. These protectants will provide some disease protection from LB when pressure is low and there are a limited number of spores on the wind finding your field. However, once farms in your area become infected and start producing spores, these common protectants usually will not stand up to this increased disease pressure, especially when wet weather favors infection. Then, you may need to switch fungicides or add another product to your spray mix that is more effective on water molds. Some effective choices for conventional growers include Ranman, Presidio, Curzate, Previcur Flex, Tanos, and Revus Top. The last two products mentioned are pre-mixes, which can provide control of EB, SLS, ARR and LB.
As with any disease spray program, resistance management is critical to make it last. Always alternate between resistance groups (chemical families or modes of action), especially for systemic or mobile fungicides, to help slow resistance to the products. It also helps to choose something with a short days-to-harvest restriction so that the product doesn’t interfere with the frequent harvests this crop requires.