While Citrus are generally easy to grow, they can have problems under certain conditions. For example if certain pests are introduced, or if the citrus are kept too wet and humid over a long period of time and fungus sets in.
One general treatment I like to provide them is to spray their leaves with water occasionally. This tends to knock off any pests, cleans the leaves, and refreshes the plant. When grown outdoors this is naturally quick and easy to do with a misting nozzle on a hose. When grown indoors I used to give my smaller plants, the ones that are easy to move, a quick spray in the shower with cool water.
A common sign of potential problems is leaf drop. Citrus will drop leaves as soon as there is an imbalance, although that is often not a problem as it can be due to the changing seasons and is natural. Citrus almost act like they are deciduous, in fact one (the trifoliate orange) is fully deciduous. There are four kinds of leaf drop I have seen:
- Sudden, with the leaves looking healthy and the parts coming off easily. This is usually due to shock. Shock usually due to conditions changing rapidly. The leaves almost pop apart in your hands. The citrus is trying to eject the leaves to restore balance.
- Slow, with the nutrients appearing to get sucked back into the plant as the leaves turn yellow and show other signs of micronutrient deficiency, even though the soil has plenty of nutrients. This usually signals the plant is either going dormant or preparing for a burst of new growth. However, it can be due to disease or a nutrient deficiency so it is good to check conditions. Make a close and careful inspection for spider mites (which are tiny and hard to see) and scale (which are camouflaged and very hard to spot until they die and turn brown). Also make sure you are providing sufficient nutrients.
- The leaves hang on until dry and crispy. Usually I see this when there is a fungus infection, or when the plant is too close to a heater or vent. Usually the whole branch will die. If not cut off then sometimes the whole tree will die.
- Leaves show brown patches that grow until the citrus decides to drop the leaf. A sign of disease on the leaf. I always remove all signs of disease as soon as they are spotted, otherwise there is a chance it will spread. Dispose of diseased tissue in the trash or burn it.
Keep in mind some pests and diseases do not cause a sufficient imbalance to cause leaf drop, so don't assume that because there is no leaf drop that there is no problem. If you want to be thorough then the citrus should be checked every so often.
Another common sign of an imbalance is leaf curl. Here is a good article on curling citrus leaves. There are many different kinds of leaf curl I have seen:
- Edge roll upward, like making a taco, and have a dull appearance: usually associated with dehydration. This can be because the citrus is too dry, or it can be due to root rot. Check the soil. If the soil is quite moist, or you water heavily and the symptoms do not correct themselves within a day or two, then you most likely have a root root problem. Make sure the plant has really good drainage!
- Leaves cup upward: usually associated with too much nitrogen vs other nutrients.
- Leaves cup downward and leaves are larger than normal: usually associated with a copper deficiency.
- Leaves are misshapen and curl in odd ways: usually associated with a pest problem. Several different pests suck leaf juices. This will usually result in an imbalance that shows up as odd leaf curls and other damage to the laves. Look really closely at the leaves and stems (some pests are really hard to spot). Mosts sap suckers can be controlled with an oil spray.
- Leaves are yellowish and bent down at the tip: most likely a potassium deficiency.
The pages below list some of the problems I have encountered and their treatment in greater detail.