Q: What’s going on with my coneflower? The flowers have weird deformities and the plant seems to be struggling.
A: These look like the symptoms of aster yellows, a disease caused by a type of virus called a phytoplasma. They behave similarly to viral diseases, though are actually somewhat related to bacteria; still, they are often lumped together with viruses. Although named for its impact on aster-family plants (which includes daisies like Echinacea and vegetables like lettuce), it can infect other unrelated species like carrot, potato, grains and lawn-weed plantains. This phenomenon occurs with true viruses as well — naming for the first plant they’re identified on (cucumber mosaic, tobacco rattle, etc.) even though their host range can be much broader.
Transmission occurs in several ways, but is generally akin to the way human bodily fluid exposure is needed to transmit certain viruses in people. For plants, these fluids are sap or cellular juices. Usually, sap-feeding insects (aphids, leafhoppers, thrips, etc.) serve as the vector since the virus or phytoplasma can’t survive outside of plant tissue. Their mouthparts act like a contaminated needle, picking-up viral/phytoplasma particles in one plant and injecting them into another as they feed. Sometimes viruses are passed-down in a plant’s seeds, but aster yellows renders blooms more-or-less sterile.
The key vector for aster yellows is the aster leafhopper. They are a native insect which overwinters in the Gulf states, mitigating north each year, though some can also overwinter as eggs in more northern latitudes. This means that you can’t practically eliminate them from the garden, because migration flights or storm front winds will bring them right back in future years, sometimes arriving before the locals have even hatched.
Weed management in the garden is important, as infected weeds serve as reservoirs for disease, not all of which will have identifying symptoms. Still, these leafhoppers can pickup the virus elsewhere in their travels, perhaps hundreds of miles away at a prior pit stop.
Despite the doom-and-gloom impression from what I just described, don’t be too alarmed. Plant viruses and similar organisms are numerous and diverse, and new ones are being discovered all the time, but as a whole they aren’t causing catastrophic plant losses left and right. Debilitating viruses in particular aren’t as prevalent as some fungal and bacterial diseases. It’s impractical to test for viruses in most cases, and lab tests (if they exist for the virus in question) can be expensive – well more than the cost of just replacing the plant as a precaution.
Peculiar symptoms identify suspect plants, though environmental conditions and the plant’s stage of growth can influence symptom manifestation or prominence. Sublethal herbicide exposure can mimic viral symptoms, since the chemicals acting like plant hormones interfere with normal growth. (As they do to kill weeds.)
A common viral characteristic is unusual ringed or marbled stark yellow markings on foliage. As the name implies, aster yellows can yellow foliage, but one telltale trait is blooms developing weird features. These include tiny partially-formed secondary blooms atop normal-sized blooms, green petals (virescence, though normal for a couple Echinacea cultivars) and the mutation of flower parts into leaflike tufts (phyllode).
Plant viruses and phytoplasmas cannot be treated or cured; pruning-out symptomatic parts won’t eliminate it. Remove the entire plant and dispose of it so it doesn’t risk spreading the virus further. Often they cause the decline and death of the host anyway, though how quickly can vary. Hot weather can temporarily deactivate aster yellows, but this won’t resolve the situation.
Q: Each year it seems some of my tomatoes fall victim to animal bites, cracking, or some other issue just before I can pick them. Should I be covering them? Applying something? Does repellent work?
A: I wouldn’t want the foul flavors of common animal repellent products near my harvest (plus you’d have to verify the product label allowed for applications on food crops), but fortunately there’s an easier solution – pick them a bit early. Some animals are eating the fruits because they’re thirsty, but regardless of the reason, there’s less to attract them if the juiciest fruits simply aren’t there. If birds or squirrels are the issue you could try covering plants with bird or insect netting, though make sure birds, snakes, or other animals don’t get caught in the net.
Once fruits are at full size and have just started to color-up, pick them and let them finish ripening indoors. (Keep them on the counter, don’t refrigerate.) Taste-tests have demonstrated that tomatoes which finished ripening off the plant weren’t very discernible in flavor or texture from those picked at full ripeness. If in doubt, experiment with a few and see if it works for you.
Tomatoes are sensitive to ethylene, a natural plant hormone that contributes to ripening. This gas can be harnessed to ripen mature green tomatoes at the end of the growing season by putting the tomatoes in a brown paper bag with a ripe apple or banana as a source of ethylene.
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.