Sunday, June 11, 2017

Yellow leaves on Citrus

Taking a step back, there are a few fundamental reasons why citrus leaves go yellow. They all center around not having enough nutrients. The options are outlined below. A photo of the whole plant, and a close up of the leaves, would be very helpful to determine exactly what option it is: 

a) The leaves are naturally dying. This is the time of year that some leaves will die and drop off citrus. It happens all year round, but more so now. These will be old leaves, and the tree is removing their nutrients, so when they fall, the tree retains as much as it can. If this is the case, it should only be a few leaves, and they are most likely be where they don't get much sun ie in the middle of the tree. 

b) Lemon / Citrus borer. A NZ native, that loves to lay eggs on citrus trees. The grub then tunnels down branches / twigs. This can either cause the branch to die due to the bug chewing through the nutrient supply, or from weakening the branch, which causes it to break in the wind. If this is the case, then a sector of the tree will be yellowing, and only this sector. 

c) Magnesium deficiency. This shows up on old leaves, and is a more blurry yellow, kind of like a smeared yellow paint, compared to the zinc / manganese issue in (d). As others have said, epsom salts which are high in soluble magnesium will help this. 

d) Zinc and/or Manganese deficiency. This produces yellow leaves with green 'veins' which looks different to the Mag lack mentioned above. Since these two minerals are used at a high level by citrus, I would hope that it is in citrus ferts, so make sure you read the packet for what is in it. If not you can buy a trace element mix. 

e) Nitrogen deficiency. This shows up with new growth. Instead of being large leaves, dark green (not as dark as the old leaves still), they are small and yellow. Hence people suggesting nitrogen fert. 

The question I have is why doesn't the soil have enough mineral and biological life to supply the citrus needs. So some things to poke around at. Concrete, concrete is high pH and tends to turn soil alkaline (high pH) where as citrus like slightly acid (lower pH) soils. Thus if you have buried concrete (common if a house is a new build, and they just tip the excess wheel barrow of cement some place and it then gets covered with fresh top soil! Or close to pathway etc. If this is the case then think about getting a pH test kid (cheap) from bunnings etc. You can use iron sulfate to drop the pH. Little and often is better than a lot at once. 

Also is the soil cold. Citrus like it warm, and NZ is marginal for citrus. So if you have clay soil, or water logged soils. Can you increase drainage / temperature. For example things like gypsum are supposed to clump the clay into particles to make for easier drainage. Can you prune back trees etc, for more sunlight. Think about planting on a mound if has wet feet etc. 

Chemical lack. You probably not going to do a soil test, but if your citrus is lacking in zinc, it is likely your whole place is. So a trip to farmlands, for a larger volume might be wise, so your other trees / plants are not working hard to get it. 

Biological. Citrus trees have mycorhizal relationships with fungi. This is a symbiotic relationship where the fungi provide the tree with minerals, in return they get carbo's for living on from the citrus. Things like round up or other fungicides kill / harm the mycorrhizal fungi. So reducing / eliminated these can help. Also having mixed species of plants growing under / around the tree ie not just ryegrass. This means that the mycorrhizal fungi can move about so to speak from plant to plant in the soil, so they can connect with your tree better. 

Of course building organic matter is also very helpful for the soil biology and then tree health. 
Wide shot  of Meyer lemon with yellow older leaves

Close up shot shows that this is due to Zinc / Manganese 

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