Our Tasmanian horticulture students start out as novices but soon become “citrus whisperers”. Here’s how they do it!
It’s such a common question, “What’s wrong with my lemon tree?”. Well, a little detective work can give you a quick idea of what your citrus may be lacking. In this article we are talking about nutrient deficiency, but pests like scale and the black sooty mould that goes with it, are common in citrus varieties and can be addressed with a regime of white oil application.
Peter Cundall’s own recipe is 2 Cups of vegetable oil plus half a cup of detergent, mixed well. Dilute 2 desert spoons of the oil mix to 1 litre of water. Spray entire tree, including both upper and lower sides of the leaves. Repeat every couple of weeks as needed.
Why are the leaves of my lemon yellow?
But what about those yellowing leaves? The colour and patination of the leaves can tell you a lot about what is going on inside the plant.Here are a few common deficiencies and their symptoms:
Nitrogen Deficiency in citrus:
General yellowing of nitrogen deficient leaves.
www.idtools.org
A general yellowing of the foliage, starting with the older leaves. The leaves are evenly yellow across the whole surface, there is little mottling or chlorosis in between the leaf veins. You will see nitrogen deficiencies particularly in early spring and winter because of the tree’s low nitrogen reserves and cooler soil temperatures.
Application of a nitrogen rich fertiliser like pelletised chicken manure will help correct the problem.
Iron Deficiency in citrus:
Iron deficiency showing chlorosis increasing L to R. http://ag.arizona.edu/pubs/crops/az1007/az1007-2.html
Yellowing of new leaves, sometimes leaving the veins green (interveinal chlorosis). In extreme cases the leaves can turn through yellow to white. Iron deficiency often occurs in winter, due to low soil temperatures and low root activity but can also be caused by high soil pH and poor drainage.
Iron can be added to the soil by adding iron sulphate or iron chelate, but addressing the high soil pH is a better long term solution.
Zinc Deficiency in citrus:
Zinc deficiency indicated by the mottling chlorosis and small leaf size.
http://www.plantmanagementnetwork.org/
A yellowing of newly produced leaves. They may be mottled and are smaller than usual. Interveinal chlorosis occurs in moderate cases and in more severe cases the veins turn yellow, especially near the tip. Leaves may start to die and brown off at the tips and twigs may die back.
Zinc can be added as a trace element in a foliar spray or as part of a kelp solution spray. Chelated zinc can be added to the soil.
Manganese Deficiency in citrus:
A Manganese deficient leaf (L) and healthy leaf (R)
http://ag.arizona.edu/pubs/crops/az1007/contents.html
Interveinal chlorosis is present on the new foliage with the veins appearing as green but mottled. The leaf size is normal.Manganese deficiency shows mostly in winter but often self corrects in spring.
Manganese can be added as a foliar spray of trace elements.
Magnesium Deficiency in citrus:
Magnesium deficiency with yellowing leaving dark green triangles on the leaf margin. http://www.agnet.org/
Yellow blotches starting near the centre of mature leaves. These will combine to leave a dark green triangle along the midrib of the leaf.
A foliar spray of magnesium nitrate will help, but an easier fix is to water in an Epsom salt solution.
Still unsure?
If you are in doubt, our instructors, your nursery staff or the Royal Tasmanian Botanical Gardens crew can easily point you in the right direction. Just take a picture of your tree or pick off a leaf and take it in. Good luck!