Hello and thanks for your help. Something is attacking my basil plants – the lower leaves are okay but the tops are curly, shrivelled and twisted, I’ve never seen anything like it. I can’t see anything on the leaves and wonder if it’s disease rather than a bug. Not every plant has it though. They’re in the sun and well watered. Any advice appreciated.
There are many insects in the garden that are fond of basil, Ocimum basilicum, including Japanese beetles, slugs, snails, nematodes, aphids, spider mites, whiteflies, earwigs, flea beetles and woodlice. Carefully check all the basil plants to see if you can find insects on stems, the underside of leaves or in the soil. Also, if your plants are in pots, lift the pots to see if there are insects hiding underneath. Doing this will help in determining if you have an insect infestation or a disease problem. Many of the insects listed above can be controlled by vigilant picking, a vigorous water stream or insecticidal soap spray application.
A link to good information on Japanese beetles can be found here.
There are also several diseases, fungal, viral and bacterial that can affect basil. These include Botrytis (gray mold), peronospora belbahrii (downy mildew), fusaruim wilt, necrotic spot virus and alfalfa mosaic virus, to name a few. For the home gardener, if you have any of these diseases, the best remedy is to discard the infected plants.
Some good cultural practices such as rotating where you plant the basil each year, cleaning up leaf debris from around the base of plants, and watering the soil rather than overhead watering in order to keep leaves dry, should help control the spread of disease. Also, if you are growing basil in pots, emptying out the soil each year in the fall, washing pots thoroughly before storage and replacing with new soil in the spring will be of benefit. Basil plants are heavy feeders so an application of a 10-10-10 fertilizer every couple of weeks would be good.
Here is an article with some good pictures on the necrotic spot virus.
On a personal note, the basil plants I have are not healthy either this year (2015). My supplier said they have lost 1/3 – 1/2 of their stock to the same virus that affects impatiens (downy mildew). I cut back my original basil plants, new growth came up, initially green but then yellowed and shrivelled. It would seem that getting a good crop of basil this year is going to be a challenge.