Q: What causes curled, twisted leaves on my tomato all of a sudden? Is this a virus? It’s only on the youngest growth.
A: Plant viruses and virus-like organisms (phytoplasmas) can cause weird symptoms, like distorted growth or yellowed patterns on the leaves. A more common cause of abnormal foliage in tomatoes, especially this time of year, is herbicide exposure.
Like other pesticides, herbicides fall into certain “modes of action” categories, based on how their ingredients function. Growth regulator herbicides that function similarly to plant growth hormones tend to be the culprits in this case, which is why they interfere with new growth. In comparison, chemicals that interfere with photosynthesis or the formation of chlorophyll can give young leaves a yellower or whiter bleached look.
If you didn’t use any weed killer, perhaps a neighbor, farm, or golf course upwind of you did. Applications on a warm or breezy day may volatilize, where the gaseous residues drift onto vulnerable plants. Tomatoes are sensitive to herbicide drift and will typically show exposure symptoms first.
If symptoms are mild and subtle, all you can do is wait to see if the plant outgrows the exposure, since nothing can remedy the damage. If herbicide is the culprit and the exposure was great enough, recovery will not be possible, and the plant should be replaced. Viral infections cannot be cured either, though not all plant viruses are fatal to the host, or even always symptomatic. Even so, remove plants believed to be infected with a virus so they don’t contribute to pathogen spread. Plant viruses need a living host to survive, so the debris of infected plants can usually be composted.
Q: I used to put down grub control each year, but in my new yard, I’m not sure if it’s needed. What do you suggest?
A: I would not apply any insecticide, especially routinely, unless a pest has been identified and is observed to be causing intolerable levels of damage. (Even in that situation, other management options often exist before resorting to pesticides.) Often, lawn browning, thinning, or a general lack of vigor is presumed to be grub-related when grubs are either absent or not abundant enough to cause symptoms.
Cool-season turfgrasses like tall fescue (the most common lawn type in Maryland) naturally go dormant for the height of summer, as heat and drier weather force them to pause growth. Dormancy can result in some browning or a lackluster appearance, so it’s not necessarily due to grub problems or disease. (That said, Brown Patch disease is one of our most common summer fungal issues for turfgrass, especially if it’s irrigated regularly. Even so, it does not require treatment.)
The insecticides in grub control products are applied to turf areas, but might be absorbed by the roots of nearby trees, shrubs, and perennials, including those grown for wildlife benefit. We don’t know the full impacts of those treatments on these other organisms, including whether soil applications risk harm to desirable invertebrates like fireflies (whose larvae hunt pests on the soil surface), centipedes (which also prey on pests), the solitary wasps that specifically hunt grubs to consume, and other species.
Grub control is sometimes mistakenly used to try to rid a lawn of mole activity, but these animals consume an array of other invertebrates, so the removal of grubs won’t necessarily get rid of them.
I would not automatically apply any pesticide (organic or otherwise) to the lawn in your situation. Use our Lawn Care and Maintenance recommendations to support good growth and resistance to most problems. If you can, replace sections of lawn not needed for its foot traffic tolerance or pathway delineation with other groundcovers or plantings. Fortunately, a mix of different species is resilient to issues like grub root feeding, so grubs are not likely to be a problem in lawn alternative plantings.
University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.