Q. I am worried about another outbreak of late blight, after 2009’s epidemic. Can I do anything to prevent tomato disease?
Q. What are those giant green caterpillars on my tomato plants?
Q. My tomatoes flowered but didn’t set fruit. What’s up?
Q. My tomato plants are sick or the fruit is disfigured (or both). Help!
A. Some surprising late-blight facts and tactics to try:
- Assuming 2009’s late blight was the same strain normally seen, it can only overwinter on live plant tissues, not dead, in a cold zone like the Northeast, which would mean just one potential host: potato tubers overlooked during harvest. Dig and destroy (trash) all overwintered potatoes as they resprout.
- Volunteer tomatoes that sprout in the garden or compost can be the carrier of various other diseases (though not late blight). Always remove volunteer tomatoes as they appear, to prevent other tomato afflictions.
- Seed that was saved is not a potential transmitter of late blight, which cannot get inside a seed or survive on it outer surface. (Some other tomato diseases do taint seed.)
- Other fungal diseases are soil-borne and do overwinter even in a cold zone like mine. A three-year rotation sequence is the best control—not replanting the area with tomatoes and potatoes and their relatives for that time (though peppers and eggplant are less readily infected).
- Some such diseases (not late blight) can even contaminate cages and stakes and tools, which should be disinfected with a 1:9 bleach:water solution. First wipe or brush off any caked-on soil; soak the implements for 10 minutes.
- Resistant tomato varieties such as ‘Mountain Magic,’ ‘Plum Regal’ and ‘Legend’ can offer some protection from late blight, says the University of Massachusetts Extension on their great late-blight fact sheet. Investigate appropriate resistant varieties to offer some protection against other diseases; this chart of disease resistance by tomato variety may help.
- Tomato grafting, as we have discussed—using a more vigorous rootstock to support less-robust growing heirlooms in particular—is another possible tactic, but not as easy as merely sowing seed.
- Cornell’s late-blight fact sheet answers many other questions about this disease; a great reference, should you wish to know more.
- At plant-shopping time, buy local (trucked-in seedlings sold at big-box stores were implicated in the spread of the 2009 late blight outbreak) and carefully check seedlings for any spots on stems or leaves. Even better, grow your own.
A. Be vigilant about watching for tomato hornworms and their telltale droppings or first signs of their chewing damage. The droppings are easier to see than the green caterpillars, who normally start their eating at the tops of plants. Pick them off and destroy them.
back to top
A. Sometimes, despite all the love you give them, tomatoes fail to set fruit. Assuming you did not give the plant too much Nitrogen, it may be weather-related: Nighttime temperatures that remain above 70 or temperatures below 50ish interfere with pollination. Fruit set can also be hampered by over-feeding with Nitrogen or by irregular watering.
Hot, dry conditions at blossom time prevents proper pollination and can causes buds or tiny fruit to drop. If it’s early enough, hopefully a next round of flowers appears during more favorable weather. I know some gardeners who hose down their plants if the weather is inhospitable, hoping to encourage fruit set.
back to top
A. There are so many potential tomato problems it’s a wonder we ever ripen a red fruit…but we do. Here’s the barest minimum of explanation to why some tomato problems occur, followed by links to expert diagnostic sites that may help you get even more specific.
Rot on the bottom: The bottom of a tomato fruit is the blossom end, where the blossom used to be before the fruit expanded. Blossom-end rot can appear as leathery and sunken, or be watery-looking; the end is discolored, and dark. The cause: not enough calcium, caused by water stress. Some gardeners work lime or calcium into the beds as a preventive measure, but does it help? Here’s what I learned in 2012 when I had BER on some early fruits.
Green on the top: On the top or stem end, problems such as “greenback” can occur when areas around the stem remain hard and green, unripened. Too much sunshine can sometimes be the culprit there.
Seeing spots? Various fungal diseases, cankers, viruses and bacterial conditions can show up as spots on tomato skin, whether red or green. If your tomatoes get anthracnose (sunken round spots that then go dark in the middle, sort of a bull’s-eye effect), alternaria canker (also known as blight, with numerous sunken gray-brown marks on fruit, both green and red, and lesions on plants, too), or black mold and ghost spot (watery spots with dark centers), crop rotation might have helped prevent it, and is a must next year.
A three-year cycle is ideal; two is good; skipping a year helps with some conditions and not others, but is better then no rotation.
Tomato foliage can also experience all manner of spotting, and many such afflictions are symptoms of the same cankers, blights, fungi and viruses above. Sometimes leaves start to fall off after yellowing, moving up the plant, other times it’s top-down. Bacterial wilt is a top-down deal; fusarium and verticillium begin at the bottom of the plant.
Tom Stearns of High Mowing Organic Seed says rotation won’t do the whole trick, though, since certain fungal pathogens, such as septoria and early blight, reside in the soils of the Northeast, mid-Atlantic and upper Midwest in particular. That means that no matter how much you move your tomatoes around year to year–how much you rotate your crop–you can’t completely avoid these troubles.
A barrier of clean mulch applied at planting time can reduce some spores that splash up from the soil onto plants. Stearns uses black plastic, and stakes and prunes his tomatoes (always removing the lower leaves that would otherwise act as a “ladder” for spores). Get his whole management program–he calls it “tomato hygiene.” Heirloom tomato expert Amy Goldman is using a new (reusable) material, ground cloth, the stuff of greenhouse floors and nursery pathways.
Cracks in the fruit often develop when soil moisture is uneven, and lots of moisture becomes available suddenly after not enough. Fruit swells faster than the skin can expand. Too much Nitrogen can bring on cracking, as can some fungal diseases and even merely plant genetics (large-fruited beefsteak types are said to be more susceptible).
Other damage to fruit can come from hail, from sucking insects, maybe from fire and brimstone, huh? It really is endless. So much so that there’s a whole annual conference called the Tomato Disease Workshop for breeders, farmers, and other professionals to try to work on problems together. One recent year’s event, the 25th edition, coincided with the third international version and took place in Italy. Think pomodoro! (Apparently tomato troubles are a global preoccupation, not just domestic.)
My favorite diagnostic sites: The University of Maryland’s simple pdf, as well as Texas A&M and also Cornell University, each has a great, photo-driven diagnostic tool that you will want to bookmark. Rutgers has a great PDF on fungal diseases of tomatoes. And Cornell’s chart of the disease resistance of various tomatoes was one of the most extensive I could find.
back to top