Abstract

In most areas, the oil palm appears to be free of economically important diseases caused by viruses. Nevertheless, the so called “chlorotic ring spot” (anillo clorótico) that affects nursery palms has caused considerable losses in Ecuador. So far, this disease has been found only in that country and in India. Other conditions that produce virus-like symptoms are also described, but most of them are not significant in economic terms or else their viral nature has not been clearly demonstrated. So far, there is no evidence of seed transmission of any of these conditions.

Introduction

Seed transmitted pathogens are an exception in oil palm (Chinchilla and Umaña 1996), and diseases caused by, or associated with the presence of viruses are very uncommon in most palms (Chase and Broschat 1993). Some early reports of viruses in oil palm were either not subsequently confirmed, or else were considered more of a curiosity without any economic importance, such as the disorders described by Plumb et al. (1977) and bronze streak described by Turner (1981). The conditions known as “blast” in nurseries and “mancha anular” in young plantings in the field, are thought to be caused by viruses but some experimental evidence is lacking.

Nonetheless, in Karnataka state in India, in 1994, and particularly in Ecuador (pacific coast) in 1995 new virus-like symptoms were observed in oil palm nurseries (Solomon and Babu 1998, Chinchilla et al . 1995), which made some people think that we might be on the verge of a more serious problem that could jeopardize the industry. Several years have passed since these reports surfaced, and the problem is now less frequent. This has made the oil palm growers feel more relaxed and restart the planting programs that were once considered to be at risk.

Previous reports on virus-like symptoms in nurseries

Turner (1981) had described the conditions known as “bronze streak, infectious chlorosis and ring spot” in oil palms at the nursery stage in Africa and South East Asia. All these symptoms were said to disappear once the palms were taken to the field. Poor nursery management was associated with these problems.

Plants affected by bronze streak showed, among other symptoms, reduced growth and chlorotic streaking of younger leaves, some of which also showed a narrow white interveinal streaking of the distal pinnae. Chlorotic streaks were sharply defined and became proximally discontinuous to produce an interrupted row of minute discrete chlorotic spots. The tips of the apical pinnae could become wholly chlorotic. Some symptoms of “infectious chlorosis” were similar to bronze streak, and included the appearance of yellowish-white interveinal chlorotic streaking at the tip of the apical pinnae of the youngest expanding leaves.

The first symptoms of  “ring spot” appeared on the spears as faint white interveinal chlorotic spots seen on transmitted light, more common on the edge of the folded pinnae. When the spear opened, the chlorotic areas became pale green, producing chlorotic circular or oval rings, normally between the major veins. Symptoms were evident on older leaves in transmitted light. By the time the disorders were described, there was no evidence of transmission of any kind.

In Papua New Guinea, Plumb et al. (1978) described what they called a symptomless infection, in which they found three types of virus associated, but no specific symptoms could be associated with the presence of any of such particles. Palms having virus particles were particularly stressed (cause or effect?). Transmission attempts to other hosts or even oil palm failed.

Orange spotting

This is a general term for a condition found in adult oil palms, where most of the canopy (usually starting from older leaves) develops a spotting that changes with leaf age from yellow to dull orange. In older leaves, where spots coalesce, the entire leaf takes on a reticulate orange pattern. The condition, which typically occurs on isolated palms in commercial plantings, has been associated with particular progenies and is related to inbreeding depression (Hartley 1988, Turner 1981). Confluent orange spotting is a different condition associated with potassium deficiency, and normally only affects older leaves of a group of plants growing nearby. In this condition there is a marked dying off of leaflet tips and margins. When confluent orange spotting is severe, necrosis also develops on the central portion of the lesions, which does not happen with orange spotting.

Hanold and Randles (1991) found a type of orange spotting in oil palm associated with the presence of viroid particles related to the Coconut cadang cadang viroid in the Philippines. Bright orange leaf spots developed on inoculated palms, and these lesions were larger and more numerous on older leaves. The infected trees stopped producing bunches and were said to die after some time. Other palm species were also inoculated with the viroid and developed symptoms. No evidence for seed transmission is available for this condition, which lacks importance in economic terms.

Chlorotic ring (anillo clorótico)

This disease, as described in nurseries in western Ecuador (Chinchilla et al. 1995, Genty 1996) poses a potential risk for the industry. A similar condition (if not the same) was described in Karnataka and Andra Pradesh states in India in 1994 (Solomon and Babu, 1998). Morales, F. (2000) describes the disease in Tumaco, Colombia and presents new information on the causal agent.

Disease incidence in India was rather low (0.0001-0.01%) (Solomon and Babu, 1998), but quite high in some nurseries in Ecuador, reaching 80% in some cases in 1995. Since this outbreak in Ecuador, in 1995, disease incidence has steadely decreased during the last years, and is considered to be under control - even though nobody can exactly pinpoint the specific measures leading to this control.

Symptoms. One striking feature of this disease is the combination of at least three types of symptoms in infected leaves: chlorotic streaking, mosaic and most noticeable of all, ring spots.

Fig. 1a. Mottling

Fig. 1b. Streaking

Fig. 1c. Rings on young leaf

Fig. 1d. Rings on old leaf

The chlorotic streaking (figure 1) corresponds to very narrow, discontinuous, yellow streaks along the main veins of the leaflets. This streaking is normally more evident toward the leaf tips. The mosaic (mottling) corresponds to alternating dark green tissue with chlorotic areas. This symptom prevails in older leaves. The ring spots consist of an irregular ring (halo) of chlorotic tissue surrounding a central clear green area. The shape of the rings ranges from almost circular to oval, and contiguous rings may merge or form a pattern of concentric rings, particularly around the center part of the leaflet. The halo has a pale-whitish appearance in very young leaves (spears), but takes on a yellow-orange coloration on old lesions on lower leaves. This is particularly evident on infected palms that were transplanted to the field. On the spears, ring spots seem to progress from leaflets at the base of the leaves toward those in the middle and tip. Petioles on affected leaves may also show symptoms.

No rotting or desiccation of tissues occurs and affected plants seem to proceed with an otherwise normal vegetative development. When taken to the field, these palms do not recover, but keep showing symptoms, where older leaves (with very old lesions) take an orange appearance. Yield will be probably affected, but bunches have been observed developing on these severely affected palms.

Causal agent. Symptomatic leaf tissue from India and Ecuador showed the presence of flexuous filamentous, rod-shaped viral particles, and cytoplasmatic inclusions (pinwheel, scrolls). Based on the morphology of cytoplasmatic inclusions, Solomon and Babu (1998) tentatively placed the virus within sub-division I of Potyviridae. Leaf extracts from diseased plants reacted positively with Potyvirus antisera (Reddy 1996). Viral particles observed on diseased tissue in Ecuador also belong to the Potyviridae (Chinchilla et al . 1995, Rivera et al . 1996).

Epidemiology and control. There is little information concerning the management of this disease. Early rouging of infected nursery palms is definitely a wise decision. In general, the main transmitters of potyvirus are insects, particularly aphids, so it is thought that these insects are involved in the transmission of chlorotic spot in oil palm nurseries. The distribution of the first infected palms in the nursery is completely random, and some evidence for insect transmission comes from the apparent reduction in disease incidence when weeds (particularly grasses) are eliminated from nursery surroundings. In Ecuador, it has also been observed that incidence is higher in nurseries established in low-lying areas near water reservoirs where grasses are common.

Most commonly, symptoms are first visible on palms when they reach four months in the main nursery, and the incidence can be quite high when plants reach 7-8 months of age, when 25% or more of the plants may show symptoms.

There is some circumstantial evidence that shade may help to reduce incidence, and this could be part of the reason why incidence is low at the pre-nursery stage where shading is used. Practical experience tells us that the problem does not occur, or is not serious, in the pre-nursery stage, so delaying transplanting to the main nursery has been suggested as a measure to reduce losses. However, this may cause much unwanted etiolation. In some countries of Africa, where the condition known as “blast” (thought originally to be caused by a virus) is prevalent, the use of shade is combined with insecticides to manage the disease.

Diaz et al. (1998) conducted several experiments in Ecuador trying to reduce the incidence of the disease. They found that the use of several insecticides had no effect, but some positive response was observed when rice husk was used as an insect repellent mulch.

Evidence gathered so far indicates that this is not a seed-borne virus (Chaves 1995, Chinchilla 1994, 1995). In one situation, a particular lot of germinated seeds was divided in half, each being planted in a different area. Disease incidence reached 80% in one location but was nil in the other (Genty 1996).

Blast

This condition can be very destructive in nurseries in Africa and Malaysia. A disease with similar characteristics has been described in Colombia. However, blast symptoms could be easily confused with dry bud rot (ring spot disease or “mancha anular”). The real causes of the disorder are not yet clear, so control is somewhat empirical. Losses of 5-80% of nursery plants were common in the past (Turner 1981).

Symptoms. The first symptoms observed are a loss of the normal gloss of the leaves, which take on an opaque appearance and a flaccid consistency. Younger leaves develop a yellowish coloration at the base, and lower leaves become brown. A violet-like coloration develops toward the tips of the leaves. Affected leaves rapidly become necrotic and brittle. Together with these symptoms, a brown to black wet necrosis develops at the base of the spear. The roots develop a wet rot, even at the very early stages of the disease, starting at the tips of the primary roots. The cortex is completely destroyed, and when the seedling is pulled up, the central cylinder becomes detached (Renard and Franqueville 1989). The rotting of the roots seems to stop at the bulb of the plant (Robertson et al .1968), where a brownish-orange (sometimes black) coloration develops. Death of the plants may occur in about two weeks. Partial remission of symptoms sometimes occurs; the plant turns yellow but does not die, however, its growth is severely affected. Seedlings are more susceptible at the 4-6-leaf stage (Franqueville et al. 1991).

Originally, root symptoms were thought to be caused by a combined infection of Rhyzoctonia lamellifera and Phythium splendens , but this theory was later discarded. The causal agent (non-cultivable mollicute?) seems to be transmitted by sucking insects ( Recilia mica : Jessidae). This insect normally develops in grasses ( Paspalum sp, Pennisetum sp), and thence the association of blast with the presence of these types of weeds in and around the nursery (Renard and Quillec 1979, July 1979).

Control. Adequate irrigation, the use of mulch in the bags, and good weed control in and around the nursery keep the disease at a low level. Another recommendation is to clear the nurseries of all grasses, using herbicides, and leave a strip of at least 30 m left around them. Aldicarb is also applied during periods of greater risk (dry season).

There is no known risk of movement of the causal agent of blast in seeds (Frison et al . 1991).

Dry bud rot (mancha anular)

Ignorance of the causal agent of this disturbance has raised many doubts regarding its relationship with other described diseases such as some forms of spear rot and even blast. Diseases with similar symptoms have been described in several South American countries (Peru, Ecuador, Colombia, Brazil) and in Africa (Ivory Coast). The incidence is higher and the symptoms are more severe in palms between 2 and 4 years in the field; the incidence is frequently 10%, but may reach 25% or more, in specific areas. (Renard and Quillec 1984; Van de Lande 1986; Turner 1981; Dzido et al. 1978; Renard and Franqueville 1989).

In nurseries, the disease is concentrated near the borders, whereas in the field the first cases appear dispersed in the plantation. In the second year after planting, the incidence in some areas may reach as much as 25% or even higher. E. oleifera may also suffer infection in the form of dry bud rot (Renard and Franqueville 1989).

Symptoms. In nursery seedlings, small rounded or oval stains or spots of a yellowish or whitish color, may appear in the lower leaflets of the still unopened spears. In the basal part of the spears, still having a creamy color, the lesions have a brownish coloring.

The chlorosis then extends to the opened young leaves and a dry bud rot develops. The growth rate is reduced, new leaves produced are shorter and may have wrinkled areas in the rachis. In the basal part of the leaves, the stains are surrounded by a brilliant yellowish halo, measuring 2-5 mm in diameter, which extends along the main vein.

Fig. 2a. Yellowing in young leaves

Fig. 2b. Spear rot

Fig. 2c. Decolorations on bulb.

In young palms in the field, the growth of the spear is reduced and a brilliant yellow coloring develops at the ends of the youngest leaves (generally number 1-3) (Fig 2). The chlorosis extends to older leaves, and subsequently all the young leaves dry up. Small round or oval (0.8 x 0.85 mm) greasy and yellowish small stains develop at the base of the basal leaflets of the spears during the first phases of the disease. These stains can also be sub-quadrangular and at times they may be ordered in a row resembling the links of a chain. As the stains undergo necrosis, dry rot develops covering the spears and then advances until reaching the meristem in some cases. The lower leaves turn brownish and dry up. The inflorescences and youngest bunches present yellow-orange stains and also rot.

In the initial phases of the disease, the development of necrotic lesions in the basal part of the rachis of the youngest leaves can be observed internally. The basal bulb of the plant develops greasy spots in its upper part and a violet-colored area, which extends in a radial manner, especially near the roots' insertion zone. Reddish-violet stains may also appear in the upper part of the young stem (Renard and Quillec 1984; Dzido et al .1978). The death of the palm can occur between three months and one year from the time the symptoms first appeared. Dzido et al. (1978) observed a watery rot at the level of the tertiary or quaternary roots and a blackening of the central cylinder of the primary and secondary roots.

Tolerance to the disease is developed with age and plants five years of age or older are apparently resistant. As the plant develops in the field, the symptoms are less severe, and in some cases the formation of the characteristic annular stains stops. Symptoms on these palms may be confused with other diseases. In some cases, the recovery of the plant is accompanied by the production of a cycle of smaller leaves.

The symptoms in E. oleifera are characterized by a rapid drying of the spears and by dry bud rot (Renard and Franqueville 1989).

Control. Apparently, the American oil palm ( E. oleifera ) has some resistance to this disorder and the OxG hybrids show a broad range of responses to the disease.

In the field, the disease is prevalent in low-lying, poorly drained areas, with a strong infestation by weeds, especially grasses such as Panicum sp. The incidence is lower in areas with good weed control. This situation has led some people to think that some sort of vector might be involved in disease transmission. Renard and Quillec (1984), were able to reproduce the symptoms of the disease using two cicadae: (Homoptera): Sogatella kolophon and S. cubana .

Disease incidence may be diminished by establishing a legume cover crop, maintaining a balanced fertilization and good weed control (particularly grasses). The early eradication of all diseased plants has also been recommended. In areas with a high incidence it was recommended to keep the soil free of weeds (bare soil) during the dry season (Renard and Quillec 1984). In the nursery, weeds must be controlled (especially grasses), which must be eliminated in a surrounding border of at least 15 m. A monthly application of a systemic insecticide such as Aldicarb also helps. (Renard and Quillec 1984).

There is no evidence for seed transmission of the causal agent of “mancha anular”. In one situation, a particular seed lot was planted in two locations, where the disease was absent in one of them, but was very high in the other (Morales 2000).

Conclusions

The movement of germplasm between countries is always a matter of concern since pathogens can be introduced to new environments where a lot of potential damage can be caused to economically important crops (McGee 1997). So far, seeds are the preferred planting material in oil palm and large volumes of germinated or preheated seeds move in the international market. Viral diseases in oil palm are rare and, so far, the only seed-borne pathogen of any concern in oil palm is the fungus Fusarium oxysporium f. sp. elaeidis. Seed-borne viruses have yet to be proven to exist in oil palm. During the germination process, oil palm seeds are subjected to a heat treatment (39° for 45 days), which few viruses could withstand anyway.

The ring spot symptoms described by Turner (1981) may indicate that the condition now called chlorotic ring spot (anillo clorótico) in India and Ecuador may have already been present for many years in oil palm nurseries in some countries, but at a very low incidence. The two known outbreaks of “anillo clorótico” in nurseries in such distant places as India and Ecuador are not easy to explain, neither the reasons for the rather rapid loss in incidence after only a few years. The observed patterns in both countries may indicate that attacks could be related to particular environmental conditions and the forced movement of vectors from native reservoirs of the virus to a non-host (the oil palm). Once environmental conditions went back to “normal” large migrations stopped which was associated with a decline of the problem.

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