Q. I read your column two weeks ago with interest, as my lawn has had a major infestation of crabgrass this year. Can I overseed those areas and still use a crabgrass pre-emergent herbicide next spring?

A. Reseed by Oct. 15 if you can. Fall is an ideal time to plant new lawns or to overseed and repair damaged areas. Air temperatures are cooler and less stressful for tender grass seedlings. However, the soil is still warm, which speeds germination and root development. Finally, there is much less weed pressure when you plant grass in the fall rather than spring. Weed seeds are "programmed" to germinate when they have the best chance to produce a new generation (seeds). A handful of winter annuals such as henbit (Lamium amplexicaule), deadnettle (Lamium purpureum) and annual bluegrass (Poa annua ssp. annua) germinate in fall, but the vast majority of weed seeds germinate in spring.

If you reseed in fall, you can apply one of the traditional crabgrass pre-emergent herbicides next spring with no problem. These include Dimension (dithiopyr), Halts (pendimethalin), Barricade (prodiamine), Team (benefin and trifluralin), or corn gluten meal. If you reseed in spring, there is a product called Tupersan (siduron) that will prevent most of the crabgrass seed from germinating while allowing most of the good grass seed to germinate. This product exploits the fact that crabgrass is a warm-season grass while most common lawn grasses are cool season.

Q. I have been hearing about a virus that affects cannas. I have a pretty good collection of some of the fancy-leaved varieties that I would hate to lose. How can I tell if mine have the virus, and what can I do about it?

A. A number of different viruses have shown up in cannas in the past few years. Canna mosaic virus (CMV) and Canna yellow mottle virus (CaYMV) have been considered the most important.

CMV causes the area between leaf veins to turn yellow; these areas eventually turn brown and die. It can also cause stunting and eventual death of infected plants. CMV is also known as bean yellow mosaic virus, and it can infect gladiolus, petunias, lupines and flowering tobacco (Nicotiana spp.). CaYMV symptoms include yellowing of veins, yellow or brown spots on the leaves, or a distinct mottling on the leaves. White streaks in the flower petals (known as color breaking) are another symptom of both viruses.

Viruses are systemic, so infected plants should be dug up and thrown away; there is no chemical control for plant diseases caused by viruses. Canna rhizomes that are infected with virus should be buried (in an area where you will not plant cannas in the near future) or burned. Dividing infected rhizomes can spread both viruses. CMV can also be transmitted by aphids, but insects have not been identified as vectors for CaYMV. If you are shopping for cannas, ask for rhizomes that have been tested and found to be virus-free.

A definitive identification of viral infection requires laboratory analysis. Many university disease diagnostic laboratories do not perform virus indexing, including Penn State's. Testing is available through the University of Minnesota. Fees vary depending on the test, with a $37 minimum. Information on taking samples and submitting them to the University of Minnesota can be found at pdc.umn.edu/.

Viruses are often transmitted by propagation from cuttings, on tools, or by human contact with infected plants. Wildlife also can spread diseases by getting infected sap in their claws, fur, feathers, beaks or teeth, then moving on to a healthy plant. Most viruses require live tissue to survive and do not persist in the soil as long as ALL infected canna roots and pieces have been removed from the planting area. Since cannas are not considered winter hardy in our climate, those pieces will be long dead by spring.