Abstract

Symptoms of Common Spear Rot were consistently associated with those of Crown Disease in both nursery and field conditions. Several environmental factors seemed to determine incidence. In particular, some agronomic practices that seemed to favor a more active-succulent growth apparently predisposed the plants to the disorder.

Incidence was rather low in the nursery stage, and most palms with symptoms in the nursery appeared diseased again in the field. However, once in the field, those plants that did not show recurrent symptoms had what appeared to be a better vegetative growth (larger PxS value, more leaves produced/month, larger rachis and petiole length) than the control palms (healthy in both the nursery and field). Before the appearance of symptoms, affected palms had better growth parameters than palms that did not develop the disorder. However, growth was eventually depressed in diseased palms.

Yield (accumulated fresh fruit bunches during the first 38 months) in a group of plants that had symptoms in both the nursery and field was estimated to be close to 4.5 t/ha less than in a group of healthy palms. However, the impact on yield in a commercial plantation would be much lower, since incidence under these circumstances is very low.  A high precocity in the group of plants which showed symptoms in the nursery but were healthy in the field is probably the cause of a subsequent reduction in yields. This caused the accumulated yield for these palms to be lower than that of the control plants.

Selection against susceptibility at the nursery stage could lead to the discarding of some potentially valuable vigorous genotypes, which may have good yielding potential, but show high incidence due to a predisposing factor.

Introduction

A combination of symptoms of Common Spear Rot (CSR) and Crown Disease (CD) is the most frequent disorder of the oil palm during the first two years after field transplanting. The symptoms of these disorders have been widely described (Kovachich 1957; Bull and Robertson 1959; Duff 1962,1963; Turner 1981; Chinchilla 1987; Monge et al . 1994). CSR and CD have generally been reported as two different conditions. However, there is also evidence that these disorders are closely related, and could have a common underlying cause (Turner 1981; Chinchilla 1987; Monge et al. 1992).

As the palm ages in the field, disease incidence normally lowers, but in some susceptible progenies, some palms can show symptoms even at six years or older. Under these circumstances, the negative effects of the disorder on growth and yield are quite clear. Nevertheless, little is known of the effect of the most frequent situation, where plants recover and show a normal appearance after displaying symptoms for some time.

In this work, a comparison of early growth and yield components is made, between groups of plants that showed symptoms in different stages of their development, including the nursery phase.

Materials and methods

The data for these analysis were obtained from casual observations of disease incidence (both CSR and CD) in plots belonging to an experiment with advanced--nursery planting material. The primary objective of this experiment was to study the effect of several nursery practices on the growth of Deli x AVROS palms both in the nursery and in the field. Bags of two different sizes (40x53 cm and 51x61 cm) were spaced both at 90 and 137 cm. The plants received either a basic fertilization program or a double dose.

Each plot consisted of 25 palms distributed in a triangular pattern with 9 central plants. When the nursery plants were about 18 months of age, the 9 central plants of each plot were taken to the field and planted in a triangular (9m) pattern.

A relatively high incidence of CSR was observed in some plots during the nursery stage, which was quantified according to each treatment. A first set of data (Trial A) was obtained from the palms that appeared diseased  in  the experimental units (9 plants) of some of the nursery treatments.

Border palms of the experimental units that developed symptoms in the nursery were separated and planted together in the field. Another group of healthy nursery plants was planted as a contiguous block that served as a control treatment. This was called Trial B.   

Vegetative growth was evaluated monthly in the field, following the methodology suggested by Corley and Breure (1981). Yield was determined weekly for each individual palm. The incidence and severity of the symptoms of CSR/CD were evaluated during the nursery and field stages.

To facilitate the interpretation of the results obtained at the end of the field observation period, the plants were separated into four categories:

1. Plants that had symptoms in the nursery as well as in the field.
2. Plants that showed symptoms in the nursery, but were healthy in the field.
3. A group of plants, taken at random from  the different treatments, that did not show symptoms either in the nursery or in the field (control).
4. Plants  that  were  healthy  in  the  nursery, but showed symptoms in some of the field evaluations.

The number of plants in each category varied with evaluation dates, since new plants developed the disease, and others, previously considered as recuperated, showed symptoms again (recurrence).

Mean separation between categories (treatments) was made using the "t Student" test or  the Least Significant Mean, using the procedure GLM (SAS Institute 1988).

Results and discusion

Nursery Stage

The presence of CSR/CD is a rare occurrence at the nursery stage; but the combination of a susceptible cross and certain agronomic practices seemed to favor the development of symptoms in some of the nursery treatments.  

The first diseased plants were detected when they reached 7 months of age, and the highest incidence occurred around five months later. Some of the nursery treatments had a significant effect on the incidence of the disease, and this effect was maintained even after field transplanting ( Table 1 ).  

Most of the plants with symptoms were found in those treatments that favored a faster vegetative growth during the first months of the nursery stage. Plants from the treatment that combined a short bag spacing (90 cm), and a large bag size (51x61 cm) were particularly affected. These plants had a fast initial rate of growth, started competing for light sooner, and were highly etiolated during the last months of the nursery stage. Details on the vegetative growth of these plants have already been published (Chinchilla et al. 1992). The negative effects of strong competition for light (etiolation) in the nursery, or the combination of this factor and a high incidence of CSR\CD in both the nursery and the field, is probably the cause of the poor performance of these plants with respect to growth and yield in the field.

The presence of symptoms in the nursery eventually affected negatively all the growth variables during this stage ( Table 2 ). A few plants were severely affected and were discarded before field transplanting. Nevertheless, the effect of a relatively mild attack of the disease in the nursery on the growth and subsequent yield of the palms in the field was not yet clear, so trials A and B were started.

Field stage

Trial A: vegetative growth

The incidence and severity of CSR/CD was evaluated in the nursery on three occasions, and in six instances in the field. The evaluation of the effect of the disease on growth and initial yield was complicated, due to the fact that an attack could have had consequences that were expressed in yield up to two years later, as is the case in sex differentiation. Furthermore, disease severity and persistence of symptoms varied widely between palms within each evaluation. Finally, some plants showed just one single attack, while for others, recurrence of symptoms was a common event. Different attacks were separated by a few weeks or even months.

Normally, growth measures are programmed to be taken in a systematic form (biannually, for example), and, during a given visit, a diseased palm may be showing the negative effects of the disease, and not the type of growth that preceded the attack. On the other hand, a susceptible plant may restart a more vigorous growth after recuperating from a mild attack of the disease. These complicated responses could be what is shown by the comparison of growth and yield between healthy and diseased palms.

The first disease evaluation in the field was done about six months after transplanting. Of 30 plants found with symptoms, 18 (60%) had not shown the disease in the nursery, and only 3 of those were found again with symptoms in subsequent evaluations. However, most palms (11 out of 12) that had already shown symptoms in the nursery appeared diseased again in one or more of the subsequent field visits.   

Probably the most intriguing characteristic of the growth data is the apparent better vegetative development in the group of plants that showed symptoms during the nursery stage, but were free of the disease in the field ( Table 3 ). Moreover, during the first two growth evaluations (190 and 371days after transplanting), the highest value of the petiole cross section (PxS) belonged to plants that had had symptoms in the nursery and were now showing just the first symptoms of the disorder in the field. The leaf emission rate also tended to be higher in those palms with symptoms in both the nursery and the field.

During the visit to the nursery made when the palms were approximately 18 months old, the  majority of the palms that would form the field--diseased category had already shown symptoms. This is reflected in a deterioration in some of the growth variables of the plants with symptoms. However, there are no clear differences between  the palms that are now healthy, and those that were either healthy or diseased in the nursery ( Table 3 ).  

The leaf area was smaller in the affected palms. However, the data from Table 3 should be interpreted with caution, since much of the leaf area is normally lost during an attack of CSR\CD, and this can not be quantified. The reduction in leaf area is partially due to a reduced number of leaflets per leaf, as well as to smaller size.  

The relationship between an apparent better vegetative growth and a higher incidence of  CSR/CD has been observed in plants in the field (Thompson 1928, cited by Turner 1981; Turner 1981; Monge   et al. 1992). This relationship could be due to the fact that a very active growth, particularly a high elongation rate of the petiole and rachis, may be associated with an abnormal lignification of the developing tissues (Heusser cited by Turner 1981; Monge et al. 1992). This phenomenon may induce the bending of the rachis, and favors the attack of opportunistic microorganisms, such as Erwinia  spp and Fusarium spp., which eventually cause rotting of the tissues.

Of all the palms that showed symptoms in the field, only 32% had shown symptoms in the nursery. However, 59% of the plants with symptoms in the nursery appeared again diseased in the field.  

Most diseased plants in the field were detected when they were 18 months old ( Fig.1 ). This pattern has also been observed in other experiments (Breure and Soebagjo 1991; Monge et al .1992; Sterling and Alvarado 1996), and is found again in Trial B. According to this, it would not be justified to continue with disease evaluations in the field after the plants reach this age.  

Trial B: vegetative growth

In this trial, 144 plants which had shown CSR symptoms in the nursery, were sown in a block. Another group of 53 healthy palms was planted next to the first block as a control. From this second block, only four plants later appeared with symptoms in the field. However, 71% of the plants that had symptoms in the nursery showed the disorder again in the field. This high proportion of plants with recurrent attacks is a clear indication of the strong genetic component in determining susceptibility to the disorder. However, the fact that 29% of the plants did not show symptoms again in the field also indicates that there is an important environmental effect in determining the response of the plant. In Trial A, 41% of the plants previously diseased in the nursery did not show symptoms in the field.

The different proportions of plants that appeared with symptoms in the nursery, as well as in the field in Trials A and B, could have been due, in part, to the arrangement of the plants in each trial. In Trial B, the plants that had shown symptoms in the nursery were sown in a block. This arrangement could have created conditions for the development of a high inoculum potential of the opportunistic microorganisms associated with the rottings that characterize both CSR and CD. This could have brought about the susceptibility response in some plants that would not have shown symptoms otherwise. In Trial A, the plants that were diseased in the nursery were distributed in the field in separate plots, following the arrangement of the original nursery treatments, and so they were surrounded by healthy plants.

The trends of the growth data in Trial B were very similar to those observed in Trial A; plants that became diseased in the nursery, initially had an apparent better vegetative growth when compared with palms that remained healthy ( Table 4 ).

During the evaluation done about six months after field transplanting, most of the diseased  palms found were just showing the first symptoms of the condition. At this stage, the category of palms with symptoms in both the nursery and the field had the longest petioles, followed by those palms which showed symptoms in the nursery but were healthy in the field. A similar trend is observed for the PxS value and the leaf emission rate. These trends were maintained through the subsequent evaluation. However, when the palms were about 18 months old, most of the plants in the diseased category had already been showing symptoms for a prolonged period of time. This eventually caused an obvious adverse effect on growth. However, there were no differences in growth between plants that were now healthy, and those which did or did not show symptoms in the nursery. The control palms (healthy in both the nursery and field) had the lowest PxS value. The leaf emission rate was highest in those palms that were diseased in the nursery and had several recurrent attacks in the field.  

Yield

In Trial A, the group of palms that were healthy in the nursery and diseased in the field, had a similar yield to plants that were diseased in the nursery but healthy in the field ( Fig. 2 ).     The initial low yields in this last group of palms, were probably not due to a delayed effect of the disease that appeared in the nursery, but rather the consequence of a very high precocity, which temporarily exhausted the reserves of the plant afterwards.

In other experiments, as well as in some commercial areas, this trend toward a significant reduction in yields in young harvesting lots showing high precocity and high initial yields has been noted. When the yield peak starts during the dry season, as was the case in these experiments (December -- February), the subsequent detrimental effect on yield is even greater, possibly due, in part, to bunch abortion (Richardson 1987). The fall in production that occurred after the first yield peak was associated with a significant reduction in the number of bunches produced per plant ( Fig.5 ), but not in bunch mean weight ( Fig.4 ). Mean bunch weight during the last months was greater in the plants that were healthy in the field, and had had symptoms in the nursery. Mean bunch number per palm was smaller in this group of palms.

Conclusions

The CSR/CD affected negativelly the vegetative growth and the initial yields of individual palms. However, those palms that were diseased in the nursery but healthy in the field showed a growth in the field even superior to those of healthy plants. Moreover, the accumulated yield of the initial months from this first group of plants was superior to that of the control palms (healthy in both the nursery and field). It seemed that the vegetative growth of palms that would eventually develop symptoms was faster and more exuberant than that of plants that remained healthy.

The growth parameter that determines, or could be taken as an indicator of, susceptibility to CSR/CD has not been identified yet. Three possible candidates are: the leaf emission rate (Monge et al . 1992), the PxS value, which is closely related to plant vigor (Richardson 1986), and the petiole elongation rate. This last variable is not routinely determined. Nevertheless, any correlation between these growth measurements and disease incidence must be interpreted with caution, since it could just indicate that the environmental factors that determine the fluctuations in incidence are the same ones that affect the changes of these variables.  

The hypothesis to test is that a susceptible palm with a very active growth (high PxS value, TEF and petiole elongation rate), in a given physiological moment, could be more predisposed to develop the symptoms of CSR/CD. Such an active growth could not maintain a balance with the lignification process, which could lead to the development of the bending of the rachi--ses, and the invasion by opportunistic microorganisms, such as Erwinia spp and Fusarium spp. The present observations, as well as others obtained in other experiments (Turner 1981; Monge et al . 1992, 1993) may support this hypothesis.

A fast aerial growth may upset the root to shoot ratio, particularly when the plants are growing in soils that do not favor a good root system, such as poor soil aeration.

During the first 38 months after field transplanting, the control plants yielded an estimate of approximately 4.5 t of FFB\ha more than plants that showed symptoms both in the nursery and field. However, the real economic impact of the condition of CSR/CD can be disregarded under most circumstances, since incidence in commercial plantations is normally  very low.

The most sensible strategy to reduce the problem of CSR/CD is the selection of resistant genotypes. To reduce the time of the selection process, it is necessary to develop a methodology that would permit the detection of the susceptible crosses early in the nursery stage. However, to be successful with such a method, it is necessary to determine and quantify the environmental factors that determine the palm response, both in incidence and severity. However, the results of these trials indicate that this procedure could unnecessarily exclude some valuable crosses that may show susceptibility to the disorder, but that could have other characteristics linked to high yields.  

Since susceptibility to Crown Disease, at least, is possibly controlled by only a few genes (Blaak 1970; De Berchoux and Gascon 1963; Gai 1969; Soh 1969), and yield is a polygenic character, the possibilities of a real linkage between susceptibility and yield potential are very low. In other words, even if a particular cross is partially susceptible, it can still be worthwhile to save it in a breeding program. An attempt can be made to incorporate resistant genes, while retaining its characteristics of excellent vigor and high yield potential.

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