Compared to other citrus, pummelos are relatively tolerant to most pests and diseases. Leaf miner (Phyllocnistus citrella), leaf-eating caterpillar (Papilio demoleus, P. polytes), fruit-borer caterpillar (Citripestis sp.), scale insects (Chloropulvinaria psidii), red mites (Panonychus citri), fruit flies (Bactrocera dorsalis), nematodes (Pratylenchus sp., Tylenchulus semipenetrans, and Meloidogyne sp.) and rats (Rattus sp.) can be a problem.

Bacterial canker (Xanthomonas campestris p.v. citri) is common to most citrus but is not so serious in pummelos. Root rot (Fusarium solani, Phytophthora sp. and Pythium sp.), gummosis (Phytophthora nicitianae var. parasitica, P. citropthora, P. palmivora or due to viroid: Cachexia xylopose), and brown rot (Phytophthora sp.) appear to be serious in some locations of growing centers in Southeast Asia, but generally they are not a problem. Pummelo tolerates greening disease, but yields are reduced somewhat and the fruit may apprear lop-sided rather than symetrical.

Pests: Pomelo is affected by numerous species of insects, mites that infest the leaves, flowers, bark, fruit, and branches of citrus.

Insects and mites:

Insects: Several insect pests that attack pomelo tree, leaves and fruit include the fruit flies (Monacrostichus citriola, Bacrocera papayae and Bactrocera carambolae), citrus leaf minor (Phyllocnistis citrella), citrus butterfly (Papilio demoleus malayanus), citrus fruit borer (Citripestis sagittiferella), aphids (Toxoptera citricidus and Aphis gossypii), leaf footed bug (Mictis longicornis and Mictis tenebrosa), green stink bug (Rhynchocoris humeralis) and Asian citrus phyllid (Diaphorina citri).

Mites: Several species of mites are pests of citrus, most notably the citrus rust mite, Phyllocoptruta oleivora, which causes minimal damage to foliage but extensive damage to fruit. The damage is generally minor in regard to production but causes a russetting of the fruit, making it unmarketable. Other mite pests are the citrus bud mite, Eriophyes sheldoni, the red spider-mite, Panonychus citri, and the broad mite, Polyphagotarsonemus latus.

Scales, mealybugs, and whiteflies are very common insect pests that feed on the foliage, fruit, and roots of citrus. These are the Californian red scale, Aonidiella aurantii, which attacks fruit, foliage and twigs; black scale, Saissetia species; the citrus mealybug, Planococcus citri; the citrus whitefly, Dialeurodes citri; the citrus blackfly, Aleurocanthus woglumi; and the woolly whitefly, Aleurothyrixus floccosus.

Generally, they pierce plant cells with their needle-like mouthparts and suck out the liquid; many then secrete honeydew. Ants, such as the long-legged ant, Anoplolepis longipes, feed on the honeydew and protect the pest insects from predators.

Phyllocoptruta oleivora		Planococcus citri		Aonidiella aurantii

Consequently, sooty mold (a fungus) grows on the honeydew and reduces light penetration to the leaf (and thereby photosynthesis) but does not infect the leaf.

Eriophyes sheldoni		Anoplolepis longipes

Aphids, psyllids, and sharpshooters: This group of insects causes similar damage to that caused by the group above but is notable due to their ability to transmit diseases agents such as viruses and bacteria. They usually feed on new leaves and stems and can cause significant damage; however, the diseases vectored are usually more serious than the feeding damage. Many species of aphid affect citrus; one of the most widespread is the brown citrus aphid, Toxoptera citricida, which is known to carry citrus tristeza virus. Diaphorina citri, the Asian citrus psyllid, is a carrier of the pathogen causing greening disease.

The glassy winged sharpshooter, Homalodisca coagulata, is also a pest of citrus.

Toxoptera citricida	Diaphorina citri (Asian citrus psyllid causing greening disease)

Fruit flies: The Oriental fruit fly, Bactrocera dorsalis, and the Mediterranean fruit fly, Ceratitis capitata, are the main fruit fly pests of citrus. The major problem they pose is that they deposit their eggs in the fruit as soon as there is any color break, and the larvae burrow into the fruit and feed. This makes the fruit unpalatable as well as having the potential to introduce these pests, which cause serious damage to numerous agricultural products, into new areas.

Bactrocera dorsalis		Fruit damaged by C. sagittiferella

Fruit Borer: Citrus fruit borer (Citripestis sagittiferella) attacks the growing young fruit and cause it to drop prematurely. Symptom of attack is exhibited by numerous holes with fras with yellow exudate. The fruit surface will become soft and rot which will further attact attack by the fruit flies. The extent of damage to the fruits can be quite significant but can be controlled by one spray of carbaryl (Sevin 85S- 9g/45 liter of water) or fenthion (Lebacyd 550C 10 ml/10 liter of water) directed to the fruits prior to wrapping. At the same time, all dropped fruits must be collected and burried.

Fruit damaged by fruit borer		Wrapped fruits

Other insect pests: The citrus leafminer, Phyllocnistis citrella, Chinese rose beetle, Adoretus sinicus, thrips such as Scirtothrips citri, and other insects feed on citrus cause varying levels of damage.

Mites on lemons showing fruit bronzing		Symptom mealybugs attack

Nutrient deficiency leaf symptoms, probably iron		Leaf miner attack on pomelo leaves

Diseases and disorders

Physiological disorders (not caused by pathogens) in pomelo

Fruit cracking (see photo below): It is a serious problem to fresh fruit growers. Young fruits cracking could be due to Boron deficiency which can be prevented. But for fully developed fruits, fruit cracking is due to moisture imbalances as this fruit is very sensitive to variation in soil moisture. Prolonged drought causes hardening of the peal and if this is followed by heavy irrigation or a downpour then the pulp grows and skin cracks.

Cracked pomelo

Root rot: There are many possible causes of root rot, but one common cause is watering too frequently. Root rot can be a direct result of the lack of oxygen in the root zone due to over-irrigation or an indirect result when over-irrigation predisposes the roots to infection by a root rotting pathogen.

Nutrient deficiencies: Nutrient deficiencies can be caused by leaching due to excessive irrigation or rain, by naturally low soil nutrient levels, by soil nutrient imbalances, improper pH, or insufficient or incorrect fertilizer application. Nitrogen, zinc, magnesium and iron deficiencies are common and correctable.

Diseases caused by pathogen

Pomelo is readily exposed to diseases of the leaves, roots, wood and fruit; some of the more common ones are listed below.

Root rot: There are many pathogenic species of the Phytophthora fungus causing root rots that prevent the plant from taking up sufficient water and nutrients. The leaves will turn yellow, wilt, and may drop; the tree slowly declines, and often will die.

Phytophthora gummosis is another disease caused by Phytophthora spp., affects the base of the trunk and lower limbs from which a clear gum is secreted. The bark dries upward, hardens, and cracks; the lesions spread and often girdle the branch or trunk. Most scion wood is very susceptible to this disease; therefore be sure that the graft union is well above the soil level at planting.

Melanose: Melanose is widespread but is a problem only when inoculum levels are high and there is extended rainfall during early fruit development. The symptoms begin as small, brown, sunken spots, which become raised as they develop, on the leaves and fruit. On the fruit the spots may combine and expand to become relatively large diseased areas, depending on the stage of fruit development when they are infected.

Melanose on grapefruit leaves		Melanose on grapefruit

Greasy spot: Greasy spot, a common disease in hot, humid areas, is caused by the fungus Mycosphaerella citri. It produces brown to black lesions on the undersides of leaves, which appear as grease soaked spots, and very small lesions on the skin of the fruit. If severe, the disease causes defoliation leading to a significant reduction in yield.

Greasy spot is a disease of citrus caused by the Mycosphaerella citri		Citrus scab, caused by Elsinoe fawcetti

Scab: Citrus scab, caused by Elsinoe fawcetti, is the most widespread of three scab diseases. Citrus scab is a common problem all over the world, wherever conditions are suitable. The optimum temperature for germination is in the range 21-27ºC. However, this fungus can thrive in a much wider temperature range of 18-30ºC.

Spores of this fungus are produced by scab pustules on the leaves and fruit of citrus trees. They are dispersed onto healthy tissue by wind and rain. Overhead sprinkler irrigation at flushing, or when the fruit are very young, increases the risk of scab infection. Generally, citrus leaves or fruit must remain wet for at least three hours after a spore has been deposited for the spore to be able to germinate and develop.

Infection causes a small bulge on one side of the leaf and a corresponding depression on the other side. Raised, brown pustules form on the fruit rind, becoming corky as they develop. The symptoms resemble scaring from wind; scab and wind scar may occur together where a leaf is in contact with the fruit.

Citrus Scab on the Leaf & fruit of Tankan Orange

Black spot: Black spot infects leaves and fruit but is only a problem as a disease of the fruit. Leaves infected by Guignardia citricarpa may develop small necrotic spots with a gray center but most often do not show symptoms. On fruit, the black spots may take various forms, making the disease difficult to identify. The spots make the fruit unmarketable as fresh fruit but they can be used for processing. When the infection is severe, fruit may drop prematurely.

Black Spot by Guignardia citricarpa				Citrus canker by Xanthomonas axonopodis

Citrus canker: All the previously discussed diseases are caused by fungi; citrus canker is caused by a bacterium, Xanthomonas axonopodis pv. citri. Symptoms of canker are lesions on young fruit and leaves from which bacterial ooze is exuded under humid conditions. On leaves the lesions begin as circular, pinpoint-size spots which enlarge and become irregularly shaped pustules surrounded by a characteristic yellow halo.

An even more reliable symptom is a water-soaked edge that develops around the lesion. The size of the leaf spots can vary with cultivar and time of infection but will be approximately the same on each leaf. On fruits the lesion size varies but will be otherwise similar to the leaf lesions.

Greening: Greening (huanglongbing), one of the most devastating citrus diseases, is caused by a bacterium which grows in the bark, leaves, and veins of infected trees. It had previously been thought to be caused by a virus, but the pathogen has now been identified as Liberobacter asiaticus. Affected fruit do not color properly or remain green, consequently the name, "greening". Other names for the disease, "yellow dragon" and "yellow shoot", because newly infected trees produce shoots that are yellow. Leaf symptoms on chronically infected trees may resemble nutrient deficiencies. Trees will die back and decline severely.

Citrus greening on pomelo caused by Liberobacter asiaticus

Citrus Tristeza virus		Citrus nematode, Tylenchulus semipenetrans

Tristeza: Tristeza virus is one of the most serious pathogens of citrus and is widespread throughout citrus-growing areas. Symptoms are highly variable among citrus species and cultivars and are affected by the strain of the virus and the environmental conditions. Stunting, stem pitting, vein clearing, leaf cupping, yellowing, and reduced fruit size are common symptoms. Vein clearing (disappearance of green color in or along the leaf veins, visible when the leaf is held up to light) and stem pitting (small holes in the stem underneath the bark) can often be used to diagnose this disease.

Tristeza is often severe; resulting in significantly reduced yields and often tree death.

Psorosis-ringspot: Psorosis-ringspot, a widespread problem particularly in old-line trees, is a complex of several diseases. Although not fully understood, the disease is believed to be caused by a virus or virus-like pathogen. Bark scaling and flaking on trunks and limbs of sweet orange, grapefruit, and occasionally mandarins is a classic symptom of one form of the disease. Young leaves may show various symptoms including yellow flecks, leaf mottling, or distinct light green patterns, and young shoots may die back. Mature leaves may have raised ring-spots (a yellowish ring with green tissue in the center) or large, irregular yellowish or light green patterns; fruits may also show ring-spots.

Nematodes: Several nematode species infect citrus and cause mild to serious damage depending on the species of nematode and citrus, the age of the plants at infection, and the environmental conditions. The only species that is found worldwide is the citrus nematode, Tylenchulus semipenetrans , which is most damaging in dry areas with moderate temperatures. Due to the low rate of population growth of the citrus nematode and the slow development of symptoms, the disease it causes is called citrus slow decline. Other nematodes that are pathogens of citrus are the burrowing nematode, Radopholus species, causing spreading decline; the lesion nematode, Pratylenchus species, causing citrus slump; and the root-knot nematode, Meloidogyne species, causing rootknot. Because these nematodes infest the roots of the plant, they interfere with the uptake of water and nutrients,causing symptoms such as reduced leaf and fruit size, wilting,canopy thinning, and a general, slow decline.

Postharvest diseases: Some common postharvest fungus diseases of citrus are stem-end rot (Lasiodiplodia theobromae or Diaporthe citri), green mold (Penicillium digitatum), sour rot (Galactomyces citri-aurantii), anthracnose (Colletotrichum gloeosporioides), Alternaria stem-end rot (Alternaria citri), and brown rot (Phytophthora palmivora and P. nicotianae).

Stem-end rot of citrus caused by Diaporthe citri & Alternaria citri

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