Yellowing and Dying Back of Basil Plants
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Basil will yellow for several reasons, some are easily rectified and some are incurable depending on how far along in the growing season you've gotten.
Diseased Plants will turn Yellow
Root rot [damping off] is a common reason for yellowing of basil plants. Basil requires ample water, but if you've over done it, or mother nature hasn't cooperated, it is also prone to root rot. Root rot is a fungal disease and persistently soggy soil promotes it. One of the early symptoms is the yellowing of foliage.
Root rot fungi enter the plant through small capillary roots and eventually infests not only the roots, but at times the entire plant, if it lives that long. As the fungi reproduce they hamper the plants ability to take up water and nutrients and it eventually dies off.
Downy mildew, Alternaria and Pythium are fungi that cause yellowing of basil leaves. Other symptoms include leaf drop of older leaves, stuned growth, twisted shoots and at times blackened and decaying foliage from the ground up.
Water basil only when the soil surface is dry, not parched, just dry. Under most circumstances a deep watering once weekly is sufficient. In hot climates or in the dog days of summer a tad more is advisable. Basil grown in containers should have adequate drainage and quality potting soil that retains adequate moisture as well as sufficient airflow. Some mulch on the surface helps retain moisture while allowing for air flow and evaporation.
If you're lucky and catch it in time you can sometimes stop the spread by removing affected foliage and applying a sensible watering regimen. Any foliage removed, as well as entire plants that can't be saved should be burnt or disposed of and tools used should be sanitized.
If you have a large enough crop of Basil and related plants to might want to look into solarizing the soil, which will help kill off the fungus.
So far as fungicides are concerned, there is no chemical cure for plants already affected. Prevention of future outbreaks can be done primarily by allowing for good air circulationa and aeration of soil. There are a few fungicides sold as a preventative agent for damping off.
Cool Temperatures can cause Yellowing Basil Leaves
Climate - If it's early or late in your growing season and your Basil plants are yellowing, there's a good possibility that fungus may not be the issue. Basil leaves tend to yellow when subjected to cool weather. Basil thrives in warm temperatures and heat. It will turn black and die off in temperatures approaching the mid 30s F. Temperatures should be at the very least in the 70s around the clock. Optimal Growth Temperature 65 to 85 F
Nutrient deficiency is rarely a problem with Basil. It is hardy in this respect and does okay in poor soil, so long as it is well draining. An all purpose fertilizer or even some miracle grow will suffice.
Nutrient uptake caused by disease can however cause yellowing, See: Root Rots Above
Pests can weaken plants and cause a Yellowing of Basil Leaves
Insects such as Aphids, white flies and other sap sucking insects can cause a yellowing of Basil leaves. They are easily controlled and damage to plants minimized if caught in time. See Aphids Whiteflies
Root knot nematodes are microscopic soil dwelling worms. They cause damage to the root system which can lead to a yellowing of foliage in Basil the same way root fungi can. Nematodes can not be fully eradicated but can be controlled somewhat.
Compost added to the soil helps a little, it contains microbes that are harmful to nematodes.
Marigolds over time will curtail their spread in the soil as will beneficial nematodes. So far as the current season crop - well unfortunately it's advisable that you harvest them early, lick your wounds but come back stronger next season. There are resistant varieties you could try as well.