If I understand correctly, nematodes are not just on hostas, but brought into our gardens by several different plants. I'm guilty of purchasing companion plants at big box stores and small town nurseries. I've read that nematodes will not harm your plant or its growth. It will make the leaves look sick in the late summer, but won't kill the plants. Now HVX is a whole different story! I've had two plants infected with HVX in the past and destroyed them as soon as I saw it. I was fortunate that they were both still in pots waiting for that perfect spot in my gardens...turns out the perfect spot was the garbage can!
