TOMATO FAQ

Common questions about problems when growing tomatoes


TOMATO DISEASES

How to prevent problems during next year's tomato growing season

By: Sandy Feather ©2013
Penn State Extension

Q. Tomato blight damaged my crop this summer. Is there anything I can apply to the soil to reduce the likelihood of this happening again next year?

A. The best course of action this fall is to thoroughly clean out your vegetable garden: remove spent plants, stakes, ties and/or cages. Scrub stakes and cages and allow them to air dry, and launder ties if they are reusable. There is nothing you can apply to the soil now to reduce the likelihood of tomato diseases next year. Rotate crops so that tomatoes and related crops such as peppers, eggplants and potatoes are not grown in the same part of the garden every year.  

peppers.jpg

 Rotating crops in the Garden

Three- to four-year rotations are best if you have enough room in the garden. It is also important to stake or cage tomato plants because that allows increased air circulation around them and it makes them easier to spray if you choose to use fungicides.

Tomato Early Blight

There are a number of “blights” that have impacted tomatoes in our area this year, including early blight, septoria leaf spot and late blight. Early blight is caused by the fungus Alternaria solani and is characterized by leaf spots with a bulls-eye appearance. Spotted leaves yellow and die prematurely, which leads to early defoliation. This reduces yields, and leaves fruits open to sunscald. The fungus overwinters on plant residue and can persist for at least a year. It is also borne on seeds, and can be introduced to the garden on infected transplants. Early blight occurs under a variety of weather conditions, but is favored by periods of leaf wetness from dew and rainfall.

Early blight usually starts on the oldest, lowest leaves when spring rain hits the ground and splashes overwintered spores onto the leaves closest to the ground. Good garden sanitation can help remove much of the fungus, and mulching transplants can reduce splashing. Clean oat straw, composted grass clippings (not from herbicide-treated lawns), shredded leaves and red or black plastic are suitable mulches in the vegetable garden.

Fungicide applications work best when they are used to protect plants from infection, rather than after the fact. Start making applications about two weeks after setting transplants into the garden. Fungicides labeled to control early blight include chlorothalonil (Ortho Vegetable Disease Control), maneb (Dithane), copper-based fungicides (Bonide Copper), and Bacillus subtilis (Seranade).

Septoia Leaf Spot

Septoia leaf spot is caused by the fungus Septoria lycopersici and is characterized by numerous small, circular leaf spots. They are much smaller than early blight leaf spots, and lack the bulls-eye appearance. Infected leaves yellow and fall prematurely, resulting in reduced yields and fruits damaged by sunscald. Like early blight, septoria overwinters on infected plant debris and is splashed onto the lower leaves of new transplants by rain or overhead irrigation. Septoria is favored by wet weather and dew, but is not usually a problem until plants begin to set fruit. Garden sanitation, crop rotation, mulching and fungicide applications all help reduce the severity of this disease. The same fungicides labeled for early blight are also labeled to control septoria leaf spot.

Tomato Late Blight

Late blight is caused by the oomycete Phytophthora infestans, and is one of the most destructive tomato (and potato) diseases. Leaf spots caused by late blight are irregularly shaped, and appear greasy or water-soaked at first. The spots enlarge rapidly during wet weather, merging together to kill entire leaves. You may also see masses of white spores on the underside of affected leaves under humid conditions.

tomato-late-blight.jpg

The causal organism can only overwinter on potato tubers in the northern United States – those overlooked during harvest or in cull piles on commercial potato farms. Otherwise, it overwinters in the southern United States and the spores are blown north on storm currents. Cool, wet weather favors late blight development. If you grow potatoes, avoid planting tomatoes or potatoes in those areas more than once every three or four years. While garden sanitation and staking to improve air circulation are always good practices, fungicide applications are the most important control for late blight. The same fungicides labeled to control early blight and septoria leaf spot are also labeled to control late blight.


TOMATOES AND TMV

The 'wilty gene' causes upward curling of tomato leaves

By: Sandy Feather ©2009
Penn State Extension

Q. I am growing several types of tomatoes in my garden this year. I have noticed that the tomato leaves are tightly curled on some of them, while other plants look normal. Is this a problem with tomatoes this year?

A. Our office has received a lot of questions about tomato leaf roll due to the wet weather most areas have experienced recently. Leaf roll on tomatoes is a physical symptom specific to certain cultivars of tomatoes. Those that contain a specific gene – known as the wilty gene- will develop lengthwise, upward curling of the leaves in response to certain environmental conditions or infection by tobacco mosaic virus - TMV.

tomatoes.jpg

Growth Habit

Leaf roll caused by environmental factors is not a serious problem, and affected plants should produce a normal crop of fruit. They will even develop a more normal growth habit if the weather dries out. Favorable environmental conditions for leaf roll include high temperatures, drought, and prolonged periods of wet soil – and we have experienced all of those during the current growing season. The symptoms often appear when the plants are carrying a heavy load of fruit. It is common for the lowest, oldest leaves on susceptible plants show rolling more than newer leaves higher on the plant.

Leaf Roll Symptom

Leaf roll may also indicate that plants carrying the wilty gene are suffering from tobacco mosaic virus (TMV). Plants suffering from TMV usually will show light-and-dark-green mottling on the leaves in addition to leaf roll. The mottling may also appear on green fruits. Affected plants may appear stunted compared to uninfected plants of the same variety. TMV is very infectious and may be spread by simply brushing up against plants as you work in the garden. Plants suspected of virus infection should be pulled from the garden and sent out with the trash to avoid infecting clean plants. There is no treatment for virus infections in plants.

Virus Indexing

The only sure way to confirm that a suspicious-looking tomato plant is infected with a virus is by virus indexing through a certified laboratory. This can be expensive, and is really meant for commercial growers rather than home gardeners. Penn State's Disease Diagnostic Lab does not perform virus indexing.


TOMATO BLIGHT

Conditions that favor late blight on tomatoes

By: Sandy Feather ©2009
Penn State Extension

This is more of an alert than a response to a particular question. We have been experiencing the right weather conditions for a severe tomato disease known as late blight. This disease has been confirmed in several Pennsylvania counties already this year, and the first confirmed report came from a home garden.

Warm (70 - 80°F), humid days followed by cool (40 - 60°F), foggy nights make for ideal late blight weather. Late blight is highly contagious, and can wipe out tomato and potato crops in short order. It is caused by the fungus Phytopthora infestans. Late blight is the disease responsible for the potato famine in Ireland in the mid-1800’s.

tomatoes-late-blight.jpg

Late blight symptoms on tomato leaves

Late blight epidemics often start in home gardens where fungicide applications are less likely to be made on a regular basis than in commercial plantings. Whenever the disease develops unchecked, large quantities of late blight spores are produced and released into the air. During moist weather, the spores can survive and be transported up to 50 miles on air currents to infect other plantings of tomatoes and potatoes. During favorable weather conditions, unprotected foliage can be infected in three to six hours; symptoms can appear within a week. Those symptoms can expand rapidly during cool, wet weather and cause entire plantings to die within two weeks of infection. The disease is held in check by hot, dry weather.

Life Cycle of the Blight Fungus

The fungus overwinters in southern areas on winter-grown tomatoes and potatoes. In northern areas, it overwinters in commercial potato cull piles, compost piles and potato tubers overlooked during harvest. The late blight fungus requires live tissue to overwinter, so potato tubers are the most likely source in northern gardens. It can be introduced to the garden on infected tomato transplants or seed potatoes. This is one of the reasons it is so important to used certified seed potatoes rather than saving homegrown potatoes to grow the following season. Late blight spores are also carried north on air currents coming from the south.

Symptoms on the tomatoes

Check for foliar symptoms on tomato and potato plants by examining vigorous new growth higher on the plant. This helps avoid confusing late blight symptoms with less serious problems that often develop on the older, lower leaves. Late blight symptoms first appear as somewhat circular, water-soaked spots near the edge of expanded leaflets. These spots expand rapidly during moist weather to form irregular brown, dead areas. There is often a light green margin between the dead tissue in the center of the spot and the normal green tissue outside the spot.

Tell tale sign of TMV

The real diagnostic feature of late blight is the white, downy-looking mold that develops at the margin of the spot on the underside of the leaflet. If the white mold is not obvious, remove the suspicious leaflet and put it in a plastic bag with a moist (rung out well, not sopping wet) paper towel for 24 hours to see if this symptom develops. If it does not develop, late blight is probably not the cause of the leaf spots you are seeing.

Protect your Plants

To protect your tomato plants, avoid growing tomatoes where potatoes were grown the previous year, or growing potatoes and tomatoes next to each other. Late blight is likely to start in the potatoes and spread to the tomatoes. Protective fungicide sprays are the only sure way to avoid this devastating disease. Chlorothalonil (Daconil 2787) and maneb are labeled to control late blight in home vegetable gardens. Organic gardeners can use copper in spray or dust form to prevent late blight. Copper is not as effective as chlorothalonil or maneb, but it is better than doing nothing. Applications should continue as long as weather conditions favor the development of late blight. Follow label directions as to how often the product you are using should be applied.

tomato-soft-spot.jpg

Tomatoes may look good but will eventually get soft spots

One of the difficult things about plant diseases is that fungicide sprays are most effective when they are used preventatively. Once the disease is present, fungicide sprays will not necessarily “cure” the problem, especially one as severe as late blight.

Infected Tomatoes

Plants infected with late blight should be removed from the garden and destroyed. Do not compost them. Cut potato tubers from infected plants in half so that they decompose quickly. Bury infected plants rather than sending them out with the trash. The disease-causing spores could be released if they are exposed at a later date in the landfill.

Green Tomatoes

Mature green fruits from infected tomatoes can be removed and stored for ripening. Avoid storing them under conditions of high humidity (plastic bags or containers) since this will promote spore production. Potato tubers from infected plants can be eaten, but should not be stored for any length of time. Avoid using infected tubers for seed potatoes.


GREAT TOMATOES!

Tomato growth habits and early blight

By: Sandy Feather ©2009
Penn State Extension

Q. Even though I have been raising tomatoes for many years, my plants are growing tall and lanky -- to six feet tall -- instead of being bushy. Also, the leaves start turning brown as soon as tomatoes start to ripen. This doesn't seem normal. What would you recommend Sandy?

A. Tomatoes can be classified by their growth habits. Those that produce tall vines that continue to grow, blossom and set fruit up until frost are classified as having an indeterminate growth habit. They can easily grow 6 to 10 feet tall through the growing season. These tomatoes are the ones most likely to outgrow all but the tallest stakes and sturdiest cages. It sounds like your tomato plants fall into this category.

Indeterminate varieties of tomatoes

Indeterminate varieties should be staked to keep the fruit off the ground to minimize problems with rot. Staking also allows better air circulation, which can reduce the incidence of disease problems. And staking allows more thorough coverage if fungicide or insecticide sprays are needed through the growing season. Examples of indeterminate varieties include Sweet 100 (cherry tomato), Big Boy, Brandywine and Beefmaster. Indeterminate varieties can be used for fresh eating through the growing season, as well as canning and freezing.

tomatoes.jpg

   Determinate tomato varieties

Other varieties of tomatoes naturally have a bushier, more compact growth habit. They are classified as having a determinate growth habit. They grow to a certain size -- 3 to 3-1/2 feet tall -- then produce flowers and fruit. Determinate tomatoes tend to ripen together, which makes them a good choice for gardeners who grow tomatoes for canning and freezing. They are also good for fresh eating. Although many varieties are self-supporting, staking can help support a plant under a heavy load of fruit. Once they produce their main crop, production can fall off sharply, and the plants may go downhill quickly. Determinate varieties are much easier to grow in containers than indeterminate ones. Examples of determinate varieties include Roma VF (many paste tomatoes have a determinate growth habit), Better Bush, Bush Early Girl and Mountain Spring.

Semi-Determinate tomatoes

Still other varieties of tomatoes fall somewhere between these two growth habits, and are called semi-determinate. They will grow larger than determinate varieties, but are not as rampant as indeterminate ones.  They typically grow 3 to 5 feet tall. They should be staked, but are less likely to outgrow their stakes than indeterminate types. They will produce a main crop that ripens together, but will also continue to produce up until frost. Examples of semi-determinate tomatoes include Celebrity and Mountain Pride.

You may be happier with varieties that have a determinate or semi-determinate growth habit. Many seed catalogs include the tomato’s growth habit in the description, and most catalogs can be found online these days.

Browning of tomato leaves

The browning on the leaves is probably caused by one of several leaf spot diseases tomatoes are susceptible to. Early blight is one of the most common tomato diseases that matches the description of your problem. The fungus that causes early blight overwinters on plant debris in the garden. Good sanitation - removing all of the dead annual vegetable plants from the garden, cleaning up tomato stakes and ties, etc. - is an important step in controlling early blight. The fungus can survive up to a year without a susceptible host. It can also be introduced to the garden on infected transplants. It is always wise to practice crop rotation, even in small gardens.

If you grow tomatoes in the same place year after year, you increase the likelihood of insect and disease problems becoming established in the soil. Be sure to rotate among plant families rather than just individual plants. For example, tomatoes, potatoes, peppers and eggplant are all in the nightshade (Solanaceae family). A good rotation for tomatoes would be green beans or cucumbers because they are completely unrelated and are less likely to suffer from the same insect and disease problems. A three to four year rotation is recommended to allow diseased plant matter to decompose completely.

Early Blight

Early blight symptoms start close to the ground and work their way up the plant. The oldest leaves - those closest to the ground - are infected first as spring rain splashes up from the ground, carrying spores of the causal fungus that overwintered on bits of debris from last year’s garden. The leaves develop dark brown spots characterized by dark concentric rings. Usually some yellowing develops around the leaf spots, too. Spots range from pinprick-sized to one-half inch on diameter. Early blight develops under a range of weather conditions, but it is favored by heavy dew and/or rainfall, moderately warm weather (75 - 85°F), and high humidity.

Protecting Tomato Plants

Mulching around plants with an organic mulch such as clean oat straw or shredded leaves, or a synthetic mulch such as black or red plastic, reduces the likelihood of spores splashing up from the soil to infect the lower leaves. Use drip irrigation or soaker hoses rather than overhead watering, if possible.  You can also make fungicide applications to protect the plants from infection as they grow. Repeated applications are required through the growing season, at intervals recommended on the label of the product you choose. Organic gardeners can use a copper based fungicide. Daconil 2787 (chlorothalonil) is also labeled to control early blight in the home vegetable garden.


TOMATO PINWORM

Tomatoes with leaf blotches

By: Sandy Feather ©2012
Penn State Extension

Q. My tomatoes appeared to have some kind of leafminer attack them this year. By the end of the season, the plants looked pretty ragged, and the fruit seemed to be affected, too. Can you tell me more about this pest and how I can avoid problems with it next year?

A. Although there are leafminers that attack tomatoes and related plants, there was an unusual infestation of tomato pinworm in the mid-Atlantic states this year. There have been reports of damage from Delaware, Maryland and New Jersey. Early larval stages tunnel into tomato leaves and create a blotchy mine that is easily confused as the handiwork of a leafminer. Tomato pinworm does not overwinter in colder areas of the country, and is more common in southern states, Mexico, Haiti, the Bahamas, Cuba and Hawaii.

In colder areas, they are more likely to be a problem with greenhouse-grown tomatoes where they are able to overwinter. Infestations in home gardens result from transplants that are shipped in from southern states or from nearby greenhouse operations. Adult moths can be blown north on wind currents.

Description of Pinworms

The adult is a rather non-descript, small greyish-white moth that lays its eggs on the undersides of the leaves of tomato plants (occasionally eggplant, potato and weeds in the nightshade family). Adults are active at night, so they go unnoticed.

When the eggs hatch, the small yellow-grey larvae mine into the leaf and create a blotch-type mine that has a papery appearance. Older larvae may be yellow, green or grey with dark purple spots. They fold leaves over or web leaves together, and live and feed inside. Older larvae also tunnel into the stems and fruit. In large infestations, they can cause significant damage to the fruit, although it is usually confined to the core and the rind. Mature larvae drop to the ground and pupate near the soil surface. There are multiple generations a year, with seven to eight common in their native range.

Infestations can be more severe in home gardens and in organic production where regular pesticide applications are less likely than with conventional commercial producers. Insects that feed inside plant tissue are more difficult to control than those that feed externally. Management focuses on prevention and sanitation. Be sure to clean your garden out thoroughly this fall. To be on the safe side, bag up and dispose of spent tomato plants with the trash rather than composting them. Even though they will not survive a “normal” Pittsburgh winter, do not take the chance.

Pinworm Precautions

Carefully inspect transplants for signs of tomato pinworm and avoid purchasing those that are symptomatic. Once plants are in the garden, monitor your garden regularly for signs of mined or folded leaves. Pick them off and send them out with the trash. The only insecticide recommendation I could find that is suitable for home gardeners is for products containing spinosad such as Captain Jack’s Deadbug Brew. While spinosad is not a systemic product, it does move into the leaves enough to provide some control of leafminers. It may help control the small larvae.