“Hi! Doing good? Please, can you tell me if this is tomatoes mosaic virus or others, or simply burn from the heat lamp? How can I send you pictures, please? Thank you for caring! Anne ;-)))” Question from Anne of San Diego, California
Answer: It is hard to tell at this stage. There are points where physical damage, such as foliage burn or herbicide damage, can certainly look much like a disease. Nutrient deficiency (click here to learn about tomato nutrient deficiencies) can also look much like a disease. With that said, it is rare for seedlings to get a mosaic virus in the home. These diseases are often vectored by insects, though if you are a smoker, tobacco mosaic virus is very stable and can be transferred from contaminated cigarette tobacco to a seedling, though this is rare, too. I recommend placing the seedlings in question apart from the others, feed them, and watch them. If they pull out of it, then it was not a disease. If the foliage looks more and more diseased, then dispose of the plant. Click here to for a good look at advanced mosiac virus in tomatoes.
I have tomato seedlings that have tiny dark spots on the top of the leaves after it got down to 40 degrees the other night. I live in South Florida. Could this be cold damage? Question from Susan of Pembroke Pines, Florida.
Answer: The most common physical tomato leaf response to chilling is physiological leaf roll, which your tomatoes don’t seem to have. Unfortunately, cold stress, among other environmental stresses, can invite diseases to rear their ugly heads more quickly. It looks very much like your tomato may have Septoria leaf spot, a fungal leaf spot disease that starts on older leaves and spreads. Another possibility is bacterial spot. The way that you can tell the difference between the two is by looking at the center of the spots. Septoria blight will have tiny black specks in the middle of the spots, while bacterial leaf spot will not.
High humidity and rain will encourage the growth and spread of both diseases. Here is more about each.
Septoria Leaf Spot
The spots of this disease are generally circular and about 1/16 to 1/4 inches across. They have dark brown edges and grayish tan middles with tiny black spots in the center. An infected leaf will have many spots, which appear on the lower leaves first and move up to the younger leaves. Badly infected leaves eventually turn from yellow to brown. This disease rarely impacts fruit.
The first management step is to remove diseased leaves and dispose of them far from the garden. Then use clean shears to remove the most infected branches to increase air flow, which will discourage further fungal disease spread. Next, mulch the plants with straw or Black Gold Garden Compost Blend and bottom water to keep the disease from spreading. Refrain from overwatering! Then apply a copper-based fungicide every 7 to 10 days all season long.
Finally, really clean your garden at the end of the season, and rotate your tomatoes. This disease overwinters on infected tomato refuse or nearby weeds.
This disease is a little worse because it can damage fruit as well as foliage. The spots on infected leaves are dark, small, irregular and eventually join together, causing overall leaf yellowing and decline. No little black spots will be visible in the center of the spots.
Care for your plants as you would for those with Septoria leaf spot. Another recommendation is to spray plants with a preemptive streptomycin antibiotic before transplanting. During periods of early flower and fruit set, you should also spray with a pesticide that contains mancozeb plus copper. This spray is not approved for organic gardening, so if you garden organically, another option is to replant your tomatoes for the season, while you can.
Contaminated seed or plants can be a source of the disease, so be sure to purchase your tomatoes from reliable, disease-free sources. The disease will also remain in the soil and old plant debris for a year or more, so clean your garden well at the end of the season and rotate your crops.
Mothers always admonish us “not to hang out the dirty laundry”, which is code speak for keeping unpleasant family secrets out of neighborhood gossip. This same problem is afflicting the raised bed garden world where nobody wants to hear the downside of these tiny plots of food plants. At the heart of it is the very old concept of crop rotation. Even the ancients knew that crops grown in the same place year after year developed big problems without rotation.
Why Rotate Crops?
The science is quite simple because diseases and pests can accumulate every year you cultivate a small vegetable garden. It may not be visible the first or even the second year, but by year three it can strike with a vengeance. This is how long it takes for your soil to foster a killer dose of pests and pathogens.
Imagine a 4-foot-by-8-foot planting area (the size of a typical raised bed) that you grow tomatoes in year after year. A mature tomato can occupy half that square footage, with its roots fanning out over the area underground. To properly rotate your crops, you must not grow tomatoes in the same place you did the previous year. In this case, the only choice is to plant on the opposite end of the raised bed. That leaves only two possible spots for the tomatoes.
Farmers know to rotate their fields over four years to ensure healthy crops. That’s four entirely different growing areas for crops in the course of four years; three years with the crop being grown in a different spot and a fourth year for a given growing area to lay fallow to “rest”. But farmers have space for rotation. What about home gardeners working within the confines to small raised beds? Without sufficient space for rotation, the results can be devastating.
For example, a microscopic pest called root-knot nematode often strikes when tomatoes are grown in the same location or very close-by year after year. This pest is invisible to the eye and is common to most soils but present in such small numbers that it doesn’t cause serious problems. But when tomato crops are not rotated, these nematodes multiply to fatal proportions. As a result, tomato plants begin to turn yellow and die because too many nematodes have invaded their roots. No matter how much you water or fertilize, nothing will change this downward spiral.
You won’t know the cause until you pull up an ailing or recently deceased plant to see first hand what the roots look like. Root-knot-nematode-infected roots look bizarrely knotted and swollen and are obviously unable to support the plant. As tomato fruits mature and heat stimulates growth, the plants can’t keep up. There is no cure for badly infected plants, and fear of this organism is why gardeners often plant big African marigolds (Tagetes erecta) around food plants because it is known to discourages nematode populations.
Replace Old Soil
Rotation is key, but soil replacement can also work wonders towards keeping raised bed problems at bay. Replacing your garden or potting soil with fresh every other year will help discourage the accumulation of diseases and pests, such as the root-knot nematode. Quality garden soil like Black Gold Garden Soil is rich, fertile, and fresh. It reduces the need to rotate crops, will fortify your garden to increase yields, and rid your raised bed of “dirty laundry” that could mean the downfall of your summer garden plantings.
The more cautious your are about ensuring healthy soil for your raised garden, the better. For example, if you move into a new home with an existing raised garden, it’s best to assume its potting soil is nutritionally exhausted and that crop rotation has not occurred. Plus, it’s just a good idea to replace the soil anyway because the removal process may give you clues to other problems, like drainage or the presence of aggressive underground grass roots. Excavation will help you discover if there are problems lurking below.
Small gardens don’t have to be hotbeds of pests and diseases, if you know how to compensate with rotation and garden soil replenishment. Rotate your crops liberally (preferably on a 4-year cycle), replace old, spent soil, and reap the harvest!