As you might expect, discovering that the leaves on any plant are starting to turn yellow is never going to be a good sign. In actual fact, it is usually going to result in you being made aware of a real problem that has to be rectified although, in some cases, by the time your plant gets to this stage it can sometimes be too late to change anything.
Now, in the case of basil there are a number of reasons as to why leaves may indeed turn yellow and by being aware of the reasons it can be expected that you will then be in a position to make the relevant changes and prevent it from happening in the first place. After all, you have put a lot of effort and time into growing this herb so you do not want to then do anything that effectively destroys it, do you?
Reason 1: Nearing the End of the Growth Cycle.
The first reason for the leaves turning yellow is that the plant is coming to the end of its growth cycle. By this stage it is running low on various nutrients and the plant just does not have the same kind of energy as it once did. It is finding it difficult to maintain the growth that it had previously achieved and at the same time it is unable to absorb the nutrients to the same extent. With this, there is little that you can do unless you grow the herb indoors at which point the conditions that it is under can be perfect for longer periods of time.
Reason 2: Problems with Watering.
The amount of water that basil needs in order to stay healthy is actually more precise than you would imagine. You are basically looking at something in the region of an inch and a half of water each week but it is how you water it that can lead to yellowing leaves.
If you add too much water then you get something known as root rot which leads to those yellow leaves. Basil prefers slightly drier soil so avoid overwatering or you are just going to kill off the plant.
Reason 3: Temperature Issues.
If the temperatures fluctuate too much and from one extreme to the other then the basil can go into a state of shock and that can lead to the leaves becoming damaged. If the night time temperature drops below
Reason 4: The Growing Conditions.
By growing conditions we mainly mean the light that the basil is able to get during the day. This herb loves full sunlight and you have to try to give it in the region of 6 to 8 hours per day. Less than 6 hours on a daily basis means the plant is unable to produce its chlorophyll to the same degree so if you are growing it indoors or want to grow it over winter then LED grow lights are a necessity.
Reason 5: Sun Scald.
Sun scald can easily lead to all kinds of problems, one of which are the leaves of the basil turning yellow and brown. If you water the plant from above and the water goes on the leaves, then the sun beats down on it then it can effectively scald the leaves of the plant resulting in them turning yellow. The way to avoid this is to just water at the base and avoid splashing the plant.
Reason 6: Problems with the Soil.
Issues with the soil is a common cause of yellowing of leaves with basil. This herb likes things to be very well draining so if you use soil that is too heavy then the roots are not going to be best pleased. If the roots are not happy then they will be unable to pick up the various nutrients and this is then going to be shown in the leaves of the plant as they will be unable to sustain themselves are they are, in effect, being starved. Make sure that you use the correct soil to stop this from becoming an issue.
Reason 7: Problems with a Lack of Nutrients.
Basil is a heavy feeder and if you are unable to provide it with an adequate level of nutrients, then you are going to see those leaves changing color in a short period of time. Basil needs a rich and fertile soil and if they are low on potassium then this can result in parts of the leaves changing color. Make sure that you feed basil on a regular basis in order to really avoid this problem. The type of fertilizer that you should use is a well balanced one that is pretty much an all-purpose fertilizer as the plant will suck up the nutrients and thrive in its new conditions.
Reason 8: Problems with Fungal Disorders.
Fungal disorders are going to be a major cause of yellowing leaves and the type of fungal disorder that is the most common cause is downy mildew. You can spot this as it moves across the plant quite quickly and aside from it making the leaves yellow it also has a brown or gray growth on the leaves. If you do not spot this problem as quickly as possible then the only option is going to be to remove the plant entirely and dispose of it and never use the soil that it was in again. You should also be aware that fungal disorders are not just because of moist conditions as it can also be spores in the soil that eventually leads to the problem developing.
Reason 9: Problems with Aphids.
The problem with aphids is that they are miniature in size and yet they can cause so much damage in a relatively short period of time. These tiny insects will suck the sap out of the leaves and by doing this it is going to lead to them turning yellow or brown. You need to check the underside of the leaves along with the stems and remove them by using a soapy solution or you are going to have a lot of damage to your basil plant.
Reason 10: Root Knot Nematodes.
Now, you might not have heard of this problem but root knot nematodes will go ahead and eat away at the roots and deprive them of the nutrients that they need to survive. If you dig up basil that has been attacked in this way then you will see that the roots are damaged and in next to no time the leaves change to yellow and brown and the plant is going to die. Harvest the leaves that are still healthy and throw away the plant as it is your only option.
The Best Treatment for Yellowing Leaves.
But what about the best treatment for yellowing leaves? How are you going to tackle this problem in the first place when it always feels as if you are fighting a losing battle? Well, there are a number of things that you can do to help the situation and even though nothing is going to be able to provide you with a cast iron guarantee, anything that can lessen the chances will certainly be useful to you.
1. Dealing with Water and Sun.
When it comes to the watering of the plant then make sure that you only water when the top one to two inches of the soil are dry. Basil is going to be able to cope with this and even then you should not give it a drenching. Instead, a deep water is only going to be required every 10 days or so unless you are in sky high temperatures. When watering, always do so at the base and never over the plant due to the way in which the sun can scald the leaves.
Basil needs up to eight hours of sunlight per day so keep it in a sunny position or the leaves will change color. If growing indoors, then use LED grow lights to make life easier for yourself.
2. Its Fertilizer Needs.
Basil loves nutrients and an imbalance in the nutrients in the soil is not going to lead to a healthy and happy plant growing. Instead, you have to make sure that the soil is fertile before you go ahead and plant the herb first of all and then top up the fertilizer throughout the year to keep things in tip top condition. Go for a slow release version so it can benefit over an extended period of time.
3. Treating Bacterial Diseases.
Bacterial diseases are a major issue and there is nothing that you can actually do to stop them as they can come in the air or the soil as well as cross contamination via your own self. However, the main thing to do when you see that your basil has been infected is to take the appropriate action as quickly as possible. Snip off the leaves that are affected to stop the spread of the bacteria and then disinfect the equipment that you used even using rubbing alcohol before you then use the shears again or you run the very real risk of adding the bacteria back to the plant later on.
4. Treating Fungal Diseases.
In order to treat fungal diseases then the best thing is prevention first rather than the cure. You should avoid using fungicides on them and the best course of action is to avoid watering the leaves and make sure that a lot of air can circulate around the plant. This is going to help reduce the chances of several of the main types of fungus from developing as there will be less moisture on the leaves. Furthermore, do not place them too close together as they should be in the region of 24 inches apart to stop the spread of the fungal infection.
5. Treating Foliar Nematodes.
Finally, you should also be aware of how to treat foliar nematodes and the best course of action is to avoid putting water on the leaves as that is often how trouble begins. However, once it is into the plant then there are no chemicals available that can help you and your only option is to go ahead and remove the basil plant and dispose of it.
So, there you have it, 10 different reasons as to why basil leaves can turn yellow and also the way in which you can help to prevent the most common reasons. Of course, there may be times where the yellowing of leaves just happens no matter how careful you are but at the end of the day as long as you are careful in your approach then there is no reason why you cannot have your basil growing without it running into this particular difficulty.