Pests & Disease
Home-grown tomatoes are a source of pride, a thing of beauty, and beyond-description delicious. Whether heirlooms of the sort our grandmothers knew or a tried and true northern variety that gives us success despite June and September frosts, a perfect tomato is an achievement. If that perfect tomato is organic, kept pest and disease free without the use of harmful chemicals, it’s priceless.
To produce that perfect tomato, be alert. Keep an eye on your plant’s health, look for larvae and other insects, watch for signs of disease. And if you find them, come here for advice on what to do. Remember: part of a quick reaction is having the most efficient tools, products, and methods ready for when trouble shows its head. Be prepared.
The first task when facing an unhappy tomato plant is to diagnose the problem. Websites with pictures can be enormously helpful here. One of the best is Texas A&M’s Tomato Disorder’s page, which presents photographs under five headings, green fruit, ripe fruit, stems, leaves, and roots. What you can’t do on that site, though, is type in a suspected problem and call up an associated picture. Two of the best sites come out of Colorado, one from CSU/Denver County Cooperative Extension titled Common Tomato Problems, and one directly from the horticultural department of Colorado State University Extension.
If you see an insect on or near your beloved tomato plants, don’t rush for the nearest insecticide. Many insects are beneficial to the garden or at least neutral. That insect may be feeding on the very pests you’re having trouble with. Even if you’re looking at an enemy, one insect does not make an infestation. It’s best to identify the intruder and the level of damage it’s causing before implementing steps in managing insect pests in vegetable gardens (hat tip to Cornell University).
For a complete description of tomato pests, along with photos and more details on life-cycle and organic control, visit the Pest Problem Solver at www.PlanetNatural.com.
Aphids: These are those dense clusters of tiny insects you may see on the stems or new growth of your tomato plants. While small numbers are not a problem — don’t be afraid to crush them with your thumb — large infestations can gradually injure or even kill plants. Pinch off foliage where aphids are densely concentrated, and throw these discarded bits into the garbage, not on the ground. If the problem then seems manageable, release beneficial insects such as ladybugs or lacewings. If it doesn’t, go for the insecticidal soap that uses natural fats and plant oils (Organic Material Review Institute listed) or natural sprays, many of which are listed for organic production.
Cutworms: These are the tiny grub-like caterpillars that feed on young plant stems at night, frequently felling seedlings by eating right through them at ground level. Prevent damage by placing collars around seedlings. You can make these of paper, cardboard, aluminum foil, or an aluminum pie plate about ten inches long and four high, bent to form a circle or cylinder and stapled. Sink the collars about an inch into soil around individual seedlings, letting three inches show above the ground to deter high-climbers.
Flea Beetles: A potentially devastating visitor, the flea beetle (so-named because it resembles and jumps like a flea) attacks from both sides: adults eat foliage, leaving numerous small holes, while larvae feed on roots. They’re not picky, these beetles; they’ll go for corn, cabbage, lettuce, and all members of the Solanaceae family: peppers, eggplant, potatoes, and tomatoes. Unless levels are very high, damage can be minimized and controlled by using preventative measures.
• Clear away or plow under weeds and debris, in which adults over-winter.
• Place yellow sticky-traps to monitor levels and capture adults.
• Use row covers. Young plants are more vulnerable to damage, so cover them to keep beetles off.
• Dusting plants with diatomaceous earth (a chalky stone composed of marine fossils, ground to powder) helps control adults feeding on foliage.
• To attack the insect more directly, introduce beneficial nematodes into your soil to feed on the larvae and pupae.
• In cases of high infestation and serious damage, botanical insecticides such as pyrethrin can be used.
Hornworms: These destructive caterpillars are so big — three inches long or more — that it would seem to be easy to control them just by picking them off. And so it is, sometimes. The problem is that their pale green color provides excellent camouflage, and the nymph and larval stages are far smaller and less obvious. If there are only a few, picking them off works well. (One site suggests spraying the plant with water, causing the caterpillars to, and I quote, “thrash around,” giving themselves away.) If there are more than a few, other measures may be called for. One of these is Bt, or Bacillus thuringiensis, an organic treatment that can control numerous other problems as well.
Nematodes: This is one of the most dreaded tomato problems. Actually, almost 20,000 different species of nematode have been identified, and billions of these usually microscopic worms occupy each acre of fertile earth, so it is fortunate that only a few cause gardening problems. Some, insect pathogenic nematodes, can actually help control other gardening pests such as fungus gnats or flea beetles. But when a gardening friend says in a voice of doom, “I’ve got nematodes,” he generally means one thing: root-knot nematodes. This particular species invades various crops, causing bumps or galls that interfere with the plant’s ability to take up nutrients and to perform photosynthesis. They’re most common in warmer areas with short winters. Unfortunately, controlling nematodes is not easy.
• Rotation: Since they take several seasons to get established, rotating garden crops denies this pest the chance to get entrenched. It’s crucial, though, that you follow tomatoes with crops that are not vulnerable to the same problem! Members of the same family are of course taboo; this includes peppers, eggplant, and potatoes. However, less likely crops are also vulnerable; these include okra and cotton, in the south, and peas, squash, beets, and numerous others anywhere. If you suspect nematodes — if you ever pull a plant that has odd-looking lumpy growths on its roots — have your county extension agent take a look at it, and get advice about crop rotation in your area.
• Soil sterilization: Completely sterilizing the soil is one option on small plots, but it’s toxic and sometimes expensive. It also means that you’ve killed off all the beneficial organisms in the soil as well as the troublesome ones, so it’s particularly important to follow such treatment with a big infusion of clean compost. It would also be best to add earthworms, and an assortment of micro-organisms as well, since doing so will restore the soil to full health and make it less vulnerable to further incursions by nematodes.
• Nematodes: While eliminating nematodes is extremely difficult, it is possible to limit their damage by using resistant varieties, marked N. Doing so doesn’t kill the pests, but it does keep them and their effects under control.
Whiteflies: These tiny flying insects feed on plant juices, leaving behind a sticky residue or ‘honeydew,’ which can become a host for sooty mold. Rustle the leaves of infested plants, and clouds of these insects will rise. If you have a serious problem, you may be tempted to reach for a conventional insecticide, but don’t bother, as whiteflies have developed resistance to many.
• The best bet is a horticultural oil, which effectively smothers all stages of this insect.
• To deal with lower levels, place yellow sticky traps to monitor and suppress infestations.
• Hosing down plants can be surprisingly effective, especially if you use a bug-blaster, a hose attachment designed to produce an intense multi-directional spray that easily reaches the undersides of leaves.
Tomatoes can be stricken by an astonishing array of diseases. If you want to see the full list, go to the How to Manage Tomato Pests page at UC Davis, which discusses some 30 diseases that can afflict tomatoes. Tomatoes can get early or late blight, either white or grey mold (or both). Then they can have problems with diseases with quirky names like curly top and corky root rot. It’s amazing that tomatoes are ever healthy. But they are, and it’s largely because the problems never get thoroughly established. After all, it’s a lot less work to nip problems in the proverbial bud.
If you’re at all susceptible to anxiety attacks, it will probably be of some comfort to know that disease is generally far less of an issue for back-yard gardeners than for commercial producers. Furthermore, there’s a lot a gardener can do to minimize diseases in vegetable gardens. Here’s how you can protect your tomatoes:
• Give your plants good soil & fertilizer and regular watering; healthy plants are much more likely to resist diseases and other problems.
Tip: Planet Natural offers a large selection of effective, organic herbicides that are SAFE to use around your children, pets, and vegetables — and won’t damage your soil.
• Rotate crops so that soil-borne pathogens never have more than a season to get established.
• Clean your gardening tools and equipment, especially at the end of the season, to ensure that they don’t carry over or spread a disease.
• Remove unhealthy foliage; pull unhealthy plants to cut down on the spread of problems.
• Don’t compost diseased foliage or plants unless you know it is safe to do so.
• Don’t use tobacco near tomato plants, to avoid communicating tobacco mosaic virus.
• Avoid watering the foliage of your plants, especially in humid climates, as many diseases are encouraged by damp conditions.
The last on that list may be one of the most important. Many plant diseases — verticillium and fusarium wilt, early and late blight, and various leaf spots — are all caused by fungi that prefer damp, cool conditions. Experts generally advise gardeners to water in the morning in part to avoid conditions that encourage fungal growth or molds. Using drip watering systems or soaker hoses keeps leaves dry, again reducing attractive sites for the fungus to get established. Though some of these fungi are airborne, many reside in the soil or in garden debris or weeds related to the tomato. It is important, therefore, to keep weeds and brush piles clear of garden plots. It also helps to keep tomato foliage off the ground and to avoid splashing water up from the ground onto foliage while watering. Mulches help achieve both these objectives.
Damping Off: Caused by any of several viruses, damping off disease is a tomato problem that affects young, seemingly healthy seedlings that suddenly develop a dark lesion at the soil line, then quickly wilt and die. Cool, damp soil, overwatering, and overcrowding all increase probability of infection. Use clean potting soil and germination trays and tools to reduce incidence, avoid crowded seed beds, and monitor watering carefully during the first two weeks after sprouting.
Fusarium Wilt: Caused by a soil-borne fungus that targets Solanaceous plants (tomato, pepper, potato, eggplant), fusarium wilt often causes no symptoms until plants are mature and green fruit begins to reach its full size. At that point foliage, sometimes on only one side of the plant, turns yellow, and a sliced stem will show brownish, discolored tissue. Control includes crop rotation, so that the wilt organisms, deprived of a host, will die down in affected soils where it winters. Since cool, damp conditions favor infection, avoid spraying leaves, especially in cool weather. Use resistant varieties.
Mosaic Virus: There are actually several closely related viruses (the tobamoviruses) that cause the wilted, mottled, and underdeveloped fern-like leaves characteristic of the tobacco mosaic virus. All are spread by what are termed mechanical means: something or something that’s been in contact with the virus touches an uninfected plant, and voila — you’ve got an infected plant. Sanitation is therefore of the utmost importance, starting with never smoking near tomato plants, as tobacco can carry the virus. Infected plants should be destroyed. Back-yard plants purchased from a reliable nursery or grown from certified disease-free seed and handled in a tobacco-free environment by only one or two people, are unlikely to develop this disease.
Verticillium Wilt: Like fusarium, verticillium is caused by a fungus that, once established in soil, is virtually impossible to remove. Symptoms are almost identical to those caused by fusarium wilt, but are less lethal. The edges of large, older leaves turn yellow, then brown and crumbly, and stems show vascular damage. Unlike fusarium, verticillium wilt affects a wide variety of crops, but lowers yield without killing plants. Again, avoid spreading infected soil and watering foliage, and again, use resistant varieties.
Blossom End Rot: If your ripening fruit develop a dark spot at the lower end, a spot that gradually widens and deepens, you’re looking at blossom-end rot. It’s an environmental problem most often caused by uneven watering or by calcium deficiency. (These can be related; uneven watering can interfere with the uptake of calcium.) The simplest treatment is therefore pre-treatment: make sure soil is rich in all necessary nutrients (including calcium), and water regularly. Mulches also help maintain even moisture levels.
Catfacing: Catfaced tomato plants are deformed to a greater or lesser extent, having deep grooves or indentations running from the blossom end all the way around to the stem. The condition results from cool weather or insect damage while the plant is in blossom. Tomato varieties with large fruit are most susceptible and tomatoes are often rendered inedible — although considered safe to it. To avoid the problem select resistant varieties whenever possible.
Cracking: Several things can cause cracking in tomatoes. Cherry tomatoes, especially small ones, frequently split at the stem end, sometimes all the way to the blossom end, and it does not indicate any sort of disease or problem. The skin of a tomato becomes less resilient as it matures, so the fruit often outgrows the skin. Pick cherry tomatoes just before full ripeness to avoid this.
Circular splitting at the stem end, (concentric cracking) or cracks running towards the stem (radial cracking) usually result from a sudden increase in moisture after a dry spell. Once again, the tomato fruit expands beyond the skin’s ability to adapt. Keep soil evenly moist to avoid this phenomenon.
Sun Scald: The tomato’s skin will look bruised or leathery, the skin sunken and puckered. It is essentially what it sounds like, a sun-burn, tomato style, and it occurs when fruit is too exposed during hot weather. This problem primarily affects staked and trellised tomatoes, which are more aggressively pruned than are caged or free tomatoes. To prevent this problem, be sure to leave adequate foliage on plants when pruning. Reusable shade cloth can also be used to protect tender vegetable plants. Once sun scald has occurred, you cannot do anything for affected fruit, but you can provide shade for the unaffected ones.