Orange Tree Diseases and Pests

Plant Protection in Orange Trees

Orange trees are susceptible to various pests and diseases that can significantly impact their growth, productivity, and fruit quality. From damaging insects like aphids to destructive fungal and bacterial infections, identifying and managing these threats is essential for every orange grower. This article presents the most common and important pests and diseases affecting orange trees. By following best practices, integrated management, and working with local agronomists, orange farmers can protect their trees’ health and production and manage pests and pathogens efficiently and sustainably.

Major Pests infesting orange trees

Aphids and other Sap-sucking Insects Infesting Orange Trees

Aphids (mainly Aphis spiraecola) are small, soft-bodied insects that feed on the underside of leaves, often resulting in the curl of orange leaves. The honeydew excreted by the aphids can cover leaves, twigs, and fruits, leading to the growth of sooty mold. This alone does not cause significant damage but can stress the trees and decrease their photosynthetic ability. However, aphids can also transmit the Tristeza virus. Aphids mainly affect young trees with soft leaves and are more abundant during the development of new shoot growth. However, their population rapidly decreases as the tree matures and the leaves harden. Aphids have numerous natural enemies that can keep them under control. To support and protect the population of these beneficial insects, the farmer should be very careful when applying plant protection products. Low-toxicity pesticides (pyrethrin-based) are sometimes used. Growers are strongly advised to consult a licensed agronomist before applying pesticides and always use suitable and certified products following the guidelines on the label.

Major bacterial diseases of orange trees

1) Blast or Black Pit (Pseudomonas syringae)

It causes water-soaked or reddish-black lesions in the twigs and leaves of most citrus species (oranges included). Symptoms include small light brown to black pits or larger sunken spots on the fruit, usually 5 to 20 mm (0.2-0.8 inches) in diameter. These pits increase in size during postharvest storage. Ideal conditions for the spread of this disease include prolonged cool and wet conditions and temperatures of 8-20°C (46.4-68 °F). Infection occurs when the bacterium invades the tree tissues through injuries caused by wind, hail, thorns, or heavy rain. Farmers interested in cultivating orange trees should carefully examine the area’s environmental conditions and avoid locations with increased weather variations since citruses are particularly susceptible to such conditions. Many farmers apply copper-based fungicides to control the disease. The advice of local agronomists should be used in all cases.

2.) Citrus Canker (Xanthomonas campestris)

Citrus canker (CC) is a disease caused by the bacterium Xanthomonas axonopodis (syn X. campestris). This species includes three pathovars (bacterial sets of strains with the same/similar characteristics). Strains of this bacterium also cause disease in sour and trifoliate oranges. This pathogenic bacterium first appears on the lower leaf surfaces, developing light-colored, pin-point lesions that become raised, tan-colored pustules over time. By that stage, the lesions are visible on the upper leaf surface and might also be surrounded by a chlorotic halo. Eventually, the lesions take a corky or spongy crater-like appearance. Their size depends on the citrus cultivar and the tree's age, but it generally ranges between 2 and 10 mm (0.08-0.4 in) in diameter. Similar lesions occur on twigs and fruits. Unfortunately, it can be quickly spread from infected areas of the tree to non-infected areas through rainwater and by wind-blown rain (the pathogen can travel easily up to 32 m or 104 feet), resulting in the premature fall of leaves and fruits. Under favorable conditions, defoliation, shoot die-back, and fruit drop can occur. To control the pathogen, orange farmers can inspect the nurseries and the orchards often, quarantine the infected trees, and apply copper sprays (usually 3 times a year at 30-day intervals), always following the advice of a local agronomist.

Major fungal diseases of oranges

1) Phytophthora root rot or gummosis in orange trees

Root rot and gummosis are fungal diseases that affect orange trees and are caused by Phytophthora species (spp). They lead to severe soilborne infections affecting orange trees’ roots, trunk, branches, shoots, and fruits worldwide. P. citrophthora (common in Mediterranean areas) and P. nicotianae (syn P. parasitica, common in subtropical areas) are 2 of the Phytophthora species causing the most economic damage.

Infection of the root results in discoloration, softening of root tissue, and decay of fibrous roots. At this stage, the orange tree roots appear to show damaged lesions, and Crown Rot (Collar Rot) may also occur. Contrastingly, when the fungal spores enter the tree through wounds or bark cracks, they develop into gummosis and root rot. Root rot induces water stress, potentially resulting in young orange trees’ death. Mature trees display reduced vigor, decreased fruit production, and twig dieback. Low-hanging fruits may also become infected from new stems (propagules) during rain splashing. This causes brown rot in fruits and leaves to fall. Disease worsening occurs due to susceptible rootstock, excessive irrigation, high water salt accumulation, or frost. The lack of immediate action against infected fruits might cause fungal establishment of branch infections and lead to even more twig dieback.

Farmers should pay close attention during the rainy season. Soil amendments that increase soil aeration are necessary to prevent the disease. Well-drained soils are a plus since this fungus enters the tree via the root system. Additionally, it is important to use disease-free cuttings and materials in plant nurseries, use resistant rootstocks (in areas with known problems), and apply suitable fungicides with the advice of an agronomist.

2) Alternaria Rot or Sooty Mold (Inner Black Rot) (Alternaria citri)

This post-harvest disease is caused by the fungus Alternaria citri. Common symptoms are the premature coloring of fruits on the trees and the development of light brown to black lesions of the rind (peel of fruit) and internal black discoloration, mainly along the central axis of the fruit during storage. This disease is mainly a stem-end rot of fruits that have been stored at low temperatures for long periods. The disease only develops following damage from sunburn, drought, frost, or over-maturity of the stylar end of the fruit (i.e., the naval).

The most important viral diseases that affect orange trees

1) Tristeza (Citrus Tristeza Virus (CTV))

Citrus Tristeza Virus (CTV) is a phloem-limited virus that belongs to the Closteroviridae family. It infects all citrus species, hybrids and cultivars, and other closely related Citrus and Fortunella genera of the Rutaceae family.

CTV is considered the most important viral disease of citrus worldwide. Symptoms of CTV depend on environmental conditions, rootstock, and virus strain and include overall tree decline, stem-pitting, and yellow stains on seedlings.

Infection usually results in quick decline, wilting, and death of the tree. Once the infection has occurred, the phloem vessels (the ones that carry nutrients) become dysfunctional, leading to girdling at the bud union, a reduction of the root system, dull-green to yellow foliage, leaf drop, twig die-back, and the production of low quality, unmarketable pale fruits. Symptoms also include stunting and small yellow leaves in sour orange trees. In extreme cases, there is a distribution of deep pits, longitudinal depression in the bark, distortion and twisting of the main branches, and the development of many fine pits.

Most CTV introductions in a country arise from propagating virus-infected planting materials. Contrastingly, local virus dispersal is due to vector transmission. Seven aphid species are considered CTV vectors:

Aphis gossypii
Aphis spiraecola
Aphis craccivora
Toxoptera aurantia
Myzus persicae
Macrosiphum euphorbiae
Toxoptera citricida

Other factors affecting the rate of CTV spread include aphid population density, environmental conditions such as moderate temperatures, irrigation, and nitrogen fertilization that results in new foliage flushes. CTV has caused extensive crop losses, especially in regions where the most susceptible rootstock is used, which is sour orange. Farmers that cultivate orange trees could use the above description of the symptoms and the pictures indicated to identify potential Tristeza infections on their trees. You must always seek the advice of your local agronomist. The biological control of aphids, virus vectors, is one of the most effective ways to treat the disease.

References

https://ipm.ucanr.edu/agriculture/citrus/phytophthora-root-rot/
https://edis.ifas.ufl.edu/publication/HS261
https://ipm.ucanr.edu/agriculture/citrus/phytophthora-gummosis/
https://iocv.ucr.edu/citrus-diseases/fungal-diseases-citrus
https://www.intechopen.com/chapters/53686
https://www.agric.wa.gov.au/pest-insects/citrus-pests
Sharma, Priyanka & Roy, Monish & Roy, Bidhan. (2021). A Brief Overview of Major Citrus Diseases and Pests and Its Management. 10.9734/bpi/ctas/v1/13548D.
Solel Z, Salerno M, 2000, Mal Secco. In Timmer LW, Gransey SM, Graham JH (Eds) Compendium of Citrus Diseases, Americal Phytopathological Society Press, St. Paul, MN, pp33-35
Stephen, H. Futche and Timmer L., W. 2008, A guide to citrus disease identification, Institute of food and agricultural sciences, University of Florida.