Citrus is one of the most common household fruit-bearing trees to grace a garden, be it lemons, limes, oranges or mandarins. From the softly scented blooms to the juicy fruit, they are a delight to have. Yet, somehow we manage to stumble across a hiccup or two when it comes to fruiting trees. Hundreds of diseases and pests plague citrus fruits, but knowing the most common problems will help you manage your tree.
1. Citrus Psyllid
The most commonly asked question includes baffling warts created by the plant in response to an insect called a psyllid. The warts or galls do not stop the tree from producing fruit, but the insect may carry a parasite that causes Greening disease, also known as HLB or Yellow Dragon disease. It causes the rind to appear green, with the fruit taking on a distorted appearance.
Note a tree with a simple psyllid problem can survive and produce fruit, whereas a tree with Greening disease requires removal.
- Symptom - Warped leaves
- Classification - Insect
- Identification - Tiny flying insects or orange eggs on tips of leaves
- Season of interest - June-November for the African citrus psyllid, Trioza erytreae
- Treatment - Natural predators occur throughout SA, remove eggs if visible.
For a CABI datasheet see here.
For more on Diaphorina citri see the following article:
Sooty mold causing a black dusting because of the scale insects on the leaves.Image by Scot Nelson CC0 1.0
2. Sooty mould
The appearance of black dustings on fruits and leaves is due to a fungal infection known as sooty mould. It is a secondary infection, which means it comes on the heals of other sucking insects that leave behind a sugary substance for the fungal to feast off of. Though they are collectively known as sooty cold, the fungus that causes the appearance can be multiples species from different genera (2).
In most cases, the black dusting merely reduces access to sunlight. Afflicted leaves can be removed or washed with a bar of horticultural soap. It is more important to find and treat the problem insect.
- Symptom - Black layer on leaves/fruits
- Classification - Fungus
- Identification - See symptoms
- Season of interest - Tracks insect emergence in Spring and fall
- Treatment - Neem oil or insecticide aimed at the specific insect (ex. Aphid, scale, whitefly etc.)
Warning: Insecticides will target a broader range and will take time to break down before the fruits can be consumed. Please follow MSDS guidelines for use on edibles.
3. Endoxerosis/Internal decline/Fruit drying
Internal decline refers to fruits that appear dried or misshapen when sliced open. The exact cause seems to be under debate, but the cause seems to be a combination of abiotic factors, including 1) high temperatures close to harvest, 2) water stress and 3) low potassium concentrations (3).
As yet, there are no recommended agricultural practices to circumvent the problem. Some recommend improving drainage as well as water management during times of heatwaves.
- Symptom - Dried, shrunken or discoloured core/fruit
- Classification - Environmental
- Identification - Only through cutting open of fruit
- Season of interest - Year-round
Treatment: Leach the root zone, improve drainage if potted.
4. Citrus thrips
The SA citrus thrip or Scirtothrips aurantii is an insect that gets around. It affects a range of crops including:
It causes a ring scar around the top of the fruit or silvering of the leaves (4). You may also realise that the fruit becomes malformed or fewer fruit all together the following season. CABI notes that its natural hosts might be Acacia and Combretum trees, but it is rather opportunistic and will jump to a new food source if available.
- Symptom - Ring scar on fruit or silver leaves
- Classification - Insect
- Identification - A tiny insect, hard to see with the naked eye.
- Season of interest - Year-round, Peak growth during October and November.
- Treatment - Koppert Attracker or directed insecticides
A proportion of deadwood is considered to be normal and should be pruned back to allow the canopy to develop. A gradual decline and dieback may point towards a more severe root/disease problem (5). It will also be accompanied by other symptoms such as leaf discolouration, fruit drop etc.
Many diseases, pests, and environmental factors can cause dieback so it is best to approach it by systematic elimination. If the tree is in well-drained soil, not under heat stress or nutrient stress, look for insect or fungal activity. If neither is present, it may be a root problem. Gather as much information regarding the leaf, bark, and fruit problems to narrow down the culprit.
This is one of the times where I would advise you to work on a case-by-case basis and post it to the community for extra advice.
- Symptom - Deadwood
- Classification - Multiple causes
- Identification - Dead branches, only a problem in trees experiencing large diebacks of 50% or more.
- Season of interest - Year-round
- Treatment - Problem specific.
We touch on the five problems above, but the list is by no means exhaustive. We excluded several nutrient deficiencies (that cause yellowing of the leaves), butterfly larvae (see The Citrus Swallowtail Caterpillar), viral diseases and root nematodes that can affect a tree. It is better to preempt any hurdles from the get-go by buying trees from reputable sources, providing the right soil conditions and minimising exposure to other pest host plants.
For a comprehensive photographic guide to Citrus scarring see the University of California's publication on the subject.
(1) Berg MA van den; Deacon VE; Steenekamp PJ, 1991. Dispersal within and between citrus orchards and native hosts, and nymphal mortality of citrus psylla, Trioza erytreae (Hemiptera: Triozidae). Agriculture, Ecosystems & Environment, 35(4):297-309. Cronje, P., Le Roux, H., Julius, C, & Bence, J. (2012).
(2) Zekri, M., & Rouse, R. E. (2002). Citrus problems in the home landscape. EDIS, 2002(3).
(3) Endoxerosis of lemon fruit: Proposed re-classification of inspection guidelines. CRI Cutting Edge. link
(4) Grout TG, 1994. The distribution and abundance of phytoseiid mites (Acari: Phytoseiidae) on citrus in southern Africa and their possible value as predators of citrus thrips (Thysanoptera: Thripidae). Experimental & Applied Acarology, 18(2):61-71.
(5) Le Roux HF, 1995. Control of the citrus nematode in South Africa. PhD Dissertation, University of Pretoria, Pretoria, South Africa.