For the most part we don’t think too much about an aphid here or there on the lemon verbena or a slug hiding in the marjoram. At the end of the growing season many plants get powdery mildew and that’s just part of the cycle. But if there is a whole infestation of aphids, as there has been once or twice on the lemon verbena, then Super Gardener has to come to the rescue.
If there aren’t too many, rubbing them out with your fingers is easily done. If that’s too gruesome then shake them into a pan of soapy water (1 tsp dish soap, pinch of cayenne and a quart of water). Or put the soapy water in a spray bottle and spray it on the plant. Commercial insecticidal soaps are also available in garden centers. Once the aphids are gone, dusting the plant with flour or diatomaceous earth will help them stay away. Companion plants such as garlic, chives, catnip and yarrow also help to repel these little sap suckers. We have found yarrow to be especially beneficial by our roses. Frogs and ladybugs find aphids to be tasty and will help keep the population down, so encourage them to hang out near and on your plants.
Usually older leaves are affected first, then younger leaves. Petioles (the little stem that holds the leaf to the main plant stem) remain green even after the leaf dies. We found downy mildew a death sentence and didn’t see any way to stop it. It spreads very quickly from water splashing onto the plants. However, several garden sites have a few recommendations: prune effected areas and plants as soon as the mildew is found. A copper spray may be used as a preventative.
There are a few other preventative measures and we have put those into place since that deadly summer as all the different basils we grow are very important to our business. We use wider spacing between plants, whereas we used to simply scatter plant the tiny basil seeds we take more care to spread them out to promote good air circulation and leaf drying. We water early in the morning to allow leaves to dry thoroughly. There is one thing we do not do that is recommended, avoid overhead watering. In the big garden where the basils grow we have overhead sprinklers, so if you can water differently it would be a good idea.
It appears that downy mildew on basil has been problematic for quite some time. Rutgers University has been working several years on a resistant basil and has developed a variety that is resistant to downy mildew. Johnny’s Seeds now has Prospera and Rutgers Devotion and we have purchased both to add to our seed repertoire.
As with downy mildew, removal of infected plant parts is a place to start. Spraying with a copper based fungicide can also be beneficial. Because the mildew is “host specific” the fungus on the monarda won’t infect the catnip. But once it gets on a plant it typically returns each year since the spores live in the debris around plants. There are a few preventative measures that can help and as with downy mildew giving plants plenty of circulation is important, no overhead watering and avoid fertilizing if the mildew is present. If you know a plant is susceptible, try a preventative spray starting early in the season and continuing throughout the summer.
Perhaps our attitude towards powdery mildew is somewhat causal – because it happens late in the summer we often just ignore it. We find it on the comfrey, bee balm and almost into fall on the calendula. By the time it appears we have mostly completed the harvesting of these plants, so we see it as a natural progression of the herb garden, with plants telling us they are done for the season.
A couple of recommendations for dealing with these fungal problems: grow some herbs in containers, water from the base of the plants, clean debris from around the container each year. Some gardeners would recommend completely digging up the infected bed and moving any uninfected plants to a new area. The old area could be treated with a fungicide and not used for several years. Another choice would be to cut down the herb that’s there and dispose of the infected plants but not in the compost. Burn them or put them in the garbage. Then try treating the bed with one of the newer biological fungicides that are bacteria based. These products are acceptable for organic gardening and edible crops where most fungicides are not. An example is Serenade found in most garden centers. It is OMRI approved and won’t harm bees or beneficial insects. Our kind of commercial spray if we have to use one. Next mix the product according to directions and drench the soil all around the growing area and just a bit beyond. Finally, begin spraying with the product according to directions regularly as the plants begin to grow. Only water in the morning, and only water once a week.