Abstract

Symptoms of Common Spear Rot (CSR) and Crown Disease (CD) in oil palm are commonly associated. Susceptibility to these growth disorders can be estimated in the field  when plants reach 13-18 months after transplanting from the nursery. However, many plants may escape from the disease, if they are not growing under the appropriate predisposing environmental conditions.

Even though, CSR/CD is of very low occurrence at the nursery stage, the susceptibility reaction can be brought about by certain agronomic factors (predisposition). A deficient soil aeration (poor drainage) in the nursery bag was associated to a high disease incidence. Frequent pruning of the roots within the bags had a similar effect. High doses of fertilizer, particularly nitrogen, also seemed to favor the expression of susceptibility, by promoting a more vigorous and succulent growth of new leaves. Monthly applications of boron tended to reduce both incidence and severity of the attacks. Wounds applied to the young spears did not promote the disease, which is an indication that the microorganisms isolated from the rotting of tissues in both CSR and CD are opportunistic-secondary pathogens.

The results indicate that it is feasible to develop a procedure to select for resistance to CSR/CD at the nursery stage by identifying (and quantifying) the environmental factors that bring about the susceptibility reaction. Selection at the nursery stage would be faster and more reliable, since environmental factors are easier to control in the nursery than in the field.

Introduction

The Common Spear Rot (CSR) and Crown Disease (CD) are two common disorders of the normal growth pattern of the oil palm during its juvenile stage. Eventhough some authors attribute these two conditions to different causes, there are many common elements between them, so they can also be considered as different expressions of the same causative complex. Both  symptoms of spear rotting (CSR) and rachis bending (CD) frequently occur in the same palm at one particular time, or in different stages of the development of the disorder.

A breeding program in oil palm requires to identify and to eliminate those crossings that in the field show a high incidence of CSR/CD. The selection process can be shortened if the susceptible materials can be identified during the nursery stage.

Due to some similarities between Common Spear Rot, Lethal Spear Rot (PC), and other related disorders, the knowledge generated from the study of the factors that predispose to CSR, could be useful to understand the elements that determine the prevalence of such other disorders.

As is the case for PC, the CSR /CD does not have a causative agent so far clearly identified.  It is known for both conditions that susceptibility is genetically controlled (De Berchoux and Gascon 1963; Soh 1969; Gai 1969; Blaak 1970; Monge et al. 1993; Sterling and Alvarado, 1996). However, it is also clear that several factors (both environmental and agronomic) determine both incidence and severity of the disorders (Duff 1962, 1963; Turner and Gillbanks 1974; Turner 1981; Chinchilla 1887; Breure and Soebagjo 1991; Monge et al. 1992, 1993; Sterling and Alvarado 1996). Nevertheless, contrary to the genetic factor, which role has been proven in the case of the CSR /CD, the influence of the agronomic factors has not been documented, but associations have been based almost entirely on casual observations.

Two elements that hinder to establish the relationships between environmental factors and CSR/CD incidence is the difficulty to separate and quantify such elements, and then the need to exclude from an experiment, the contribution of the genetic factor. To achieve this last requirement, it is necessary to identify a highly susceptible genetic cross.

A third factor limiting the experimentation in this field, is the fact that the symptoms of CSR /CD are expressed normally 18 or more months after field transplanting (Monge et al. 1993; Sterling y Alvarado 1996; Breure y Soebagjo 1991). The ideal situation would be to have a highly susceptible cross, and that such susceptibility could be expressed at the nursery stage. This would allow to carry out several experiments in a relatively short period of time, and to reduce research costs.

A genetic cross with these characteristics had being identified by the Breeding Department of the Palm Research Program of ASD de Costa Rica, and was used to conduct an exploratory  experiment with the objective of beginning to identify some of the environmental factors associated with the CSR /CD at the nursery stage.

Materials and methods

The selfing of the palm C810:643T, that had shown a high incidence of CSR /CD in previous field experiments, was planted in a pre-nursery in November 15, 1993. Transplanting to the nursery was done in February 2, 1994. The nursery bags (51X61 cm) were spaced to 0.8m in a triangular pattern. Nine nursery treatments were assigned following a complete block design, with three replications. Each plot was formed with 25 plants, 9 of which were central.  Nevertheless, and due to the nature of the treatments, which were applied to individual palms, the interpretation of the results was made considering all the plot of 25 palms. The analysis of the useful plot solely did not change the main trends. The treatments were the following:

• Excessive  drainage  in  the  soil  of  the bags, achieved with the use of a mixture of 2/3 of rice husk, and a third of loamy soil (35% sand, 46% silt, 19% clay).
• Poor  drainage, by using a fine texture soil: 13% sand, 45% silt, 42% clay. When the roots grew through the bottom of the bags they were pruned, to avoid them to explore the soil of the nursery bed. During the dry season, the bags were put within other larger plastic bags, which were maintained filled with water, in an attempt to keep the soil saturated.
• Monthly pruning of all the leaves that emerged between evaluations, except leaf number one and the spears. This treatment started  in August, approximately five and a half months after transplanting to the main nursery. The pruning knife was disinfected with formaldehyde before pruning each plant.
• Monthly pruning of the roots inside and outside of the bag. The bags were lifted during each monthly visit to cut the external roots. Additionally, a knife was vertically inserted into the soil of the bag trying to intercept 50% of the roots, in a pattern that was changed in each visit. This treatment was begun in July, approximately five months after transplanting to the nursery.
• Mechanical damage to the bud, to offer a doorway to possible opportunistic pathogens that have the potential of causing spear rot. The treatment consisted of practicing in each monthly visit, 10 twinges with a needle to leaf zero and other spears. The needle was disinfected between plants. The treatment was begun in August.
• A high  doses  of  fertilizer,  by doubling the base program of Table 1 .
• Application of boron from August, 1. ( Table 1 ).
• Excess  of  nitrogen from March, 2 ( Table 1 ).
• A base fertilization program ( Table 1 ). These plots also received fungicide sprayings.

Treatments 3 to 9 were planted  in a silty-loam soil: 20% sand, 53% silt, 27% clay. Treatments 1 to 5, and 9 were fertilized with the base program. The observations on incidence and severity of PCF/CD were done monthly as of July: five months after transplanting. During those same dates growth measurements were taken following the procedures of Corley and Breure (1981).

The symptoms were evaluated on the leaves produced between monthly visits. In each affected leaf, it was documented the presence of bent rachises (Crown Disease) and the severity of the rots, according to the following criteria:

• Moderate damage: less than a 25% of the leaf affected by rots.  
• Severe damage: up to a 50% of the leaf affected.
• Very damage severe: more than a 50% affected.

The data were analyzed considering the percentage of incidence, the mean number of leaves affected in a palm, accumulated incidence in the period and recurrence of symptoms (plants that showed symptoms during more than one visit). Finally, it was calculated an infection index, that combined incidence and severity. The index was calculated for each treatment as follows:

Infection index = Incidence * weighted damage

                                            100

in which:

incidence = incidence in the plot (25 plants)

Weighted damage on the leaves = sum of the proportions of the averages of leaves in the moderate, severe and very severe categories (average of leaves in the category/total of leaves), multiplied by 100, 200 and 300 respectively.

When the palms reached 15 months of age, the competition for light caused severe etiolation, so it was decided to cull a row of each two, in two of the replications.

Results

Due to the character of the treatments, not all of them were started at the same time. Nevertheless, when the treatments were grouped according to the accumulated incidence of CSR at the end of the observations, (considering or not the first two evaluations), the ranking of treatments according to incidence and severity was maintained with very little variation for most of them.

Soil aeration and disease

Keeping the nursery plants in conditions that favored a deficient drainage in the bags had a deep effect on the incidence and severity of the CSR from the very beginning of the evaluations ( Fig. 1 , Fig. 2 and Fig. 3 ). This treatment reached the highest level of CSR in all the period. Most cases appeared before plants were 11 months of age. By this time, the external plastic bags used to keep the soil saturated with water during the dry months were placed. This factor or others not identified yet were associated to a reduction in the vegetative growth and in the rate of appearance of new cases of CSR during the last evaluations ( Fig. 4 , Fig. 5 and Fig. 6 ).

The number of plants showing recurrent symptoms (diseased in more than one visit) was also greater in this treatment of poor soil drainage ( Table 2 ).

In spite of the apparent excess of water in the bags, these plants did not show the characteristic yellowing associated with poor drainage. This may indicate that the palms somehow were able to uptake part of the nitrogen supplied.

A greater incidence of spear rot associated with drainage problems has been mentioned by numerous authors (Turner and Gillbanks 1974; Turner 1981), but there is not much experimental data available. Monge et al. (1993), observed a tendency for affected palms in the field to be grouped in areas with a high water potential. Diseased palms had a higher stomatic resistance than healthy ones. In the case of the PC, with some similar characteristics to CSR, also it has been found a clear association with poor drainage.

The results obtained in the plots with excessive drainage are more difficult to interpret, since according to the growth measurements ( Fig. 4 , Fig. 5 and Fig. 6 ), it was apparent that the roots of these plants punctured the bags when they were 8-9 months of age, and eventually explored the soil of the nursery bed. This allowed these palms to partially obviate the water supply problem. Corresponding with this fact, the incidence of CSR was high until October, and then fell drastically when growth improved at the beginning of the rain period in 1995.

Nutrition and disease

The double fertilization treatment caused an exuberant vegetative growth: longer raquises, larger leaf area, particularly at the beginning of the rainy season. Disease incidence and severity also increased during this last period (Figs. 1 , 4 , 5 and 6 ).

The fertilization-base program chosen corresponds to a recommendation commonly used in oil palm nurseries. Nevertheless, the natural fertility of the soil used to fill the nursery bags was not determined, and there exists the possibility that such program could supply some elements in excess or created nutritional imbalances. Furthermore, the nutritional requirements of different genetic crosses may vary greatly, and so the rates and sources of nutrients used could have been inappropriate for the particular cross used in this experiment.These are factors that have to be considered in future experiments, since they could be the cause of a high disease incidence in some plots.

A particular treatment which promotes an accelerated vegetative growth of the young tissues, could favor the entry of opportunistic microorganisms, which would take advantage of an abnormal (or incomplete) lignification. This aspect is illustrated with the presence of a high incidence of CSR/CD in the treatments with high doses of fertilization. Once the spears open to form mature leaves, and the lignification process is completed, all further bending of rachises stop and so does the necrosis of leaflets.

Monge et al. (1992, 1993) found a similar situation in a field experiment, and associated a greater incidence of CSR with an agronomic management that maintained an apparent better vegetative growth. Unfortunately, in any of these experiments the physiology of the diseased plants was fully studied, particularly during the period immediately anterior to symptom appearance, neither was determined the plant nutrient status, to determine if these palms were actually receiving a balanced nutrition.

Applying an excess of nitrogen caused initially, a reduction in vegetative growth compared to that of the basic and double fertilization programs. Nevertheless, the incidence of CSR rose quickly from September. A low disease incidence during the first months, would be explained assuming that the first doses of N could not be effectively used by the plants at these early stages of growth. Later on, the high amounts of N applied caused a new more succulent growth, which was more susceptible to the onset of the CSR attack, particularly during the first months of the rainy season. It is well known for most plants that excessive doses of N cause poor tissue lignification (Graham 1983).

The flaccidity at the base of the spears is characteristic of palms with CD. This phenomenon is associated to an abnormal lignification of these young tissues. As a consequence of reduced mechanical support the spear may break at this point. However, breakage may not always occurs, and due to its own weight, the spear bends as it elongates . The final result is a mature, hardened bent leaf (Monge et al. 1992, 1993; Heusser cited by Turner 1981).

Several mineral elements are involved in the hardening process (lignification) of the young tissues, being the boron one of the main ones (Graham 1983). It is interesting that adding boron was associated to a reduced incidence of CSR and CD. The application of boron was begun in August. Until this month, the plants assigned to this treatment had a relatively high incidence of CSR. This situation was drastically reversed, and these plants presented one of the lowest incidence after boron was incorporated into the fertilization program (Fig. 1 , 2 and 3 ). Breure and Sebagio (1981) presented some evidence that boron somehow affected the expression of CSR in susceptible palms.

Climatic conditions, leaf emission rate and disease incidence

The incidence of CSR followed a seasonal fluctuation, in which an increase in incidence coincided with periods of high rainfall and relative humidity and less hours of sunshine. A greater incidence during the last part of the year was also probably associated to a higher soil water saturation  in the bags (reduced aeration). The fluctuations in incidence followed rather closely the changes in the leaf emission rate (LER) of the plants, which was stimulated by a higher relative humidity.

Reductions in the incidence of the CSR were associated to a lower LER in September and during the dry months. The increase in incidence in the months of October and November coincided with a greater LER in those months. LER increased with the first rains, and this was associated to a higher incidence of CSR. These relationships have made to think that a higher leaf emission rate is an indicative of a state of predisposition to CSR (Monge et al. 1992, 1993). However, this relationship can be misleading, since the same factors favoring a higher LER during some times of the year, are the same that promote the appearance of the disorder.

Some important exceptions exit to the apparent direct relationship between LER and disease incidence. The plants which roots were frequently pruned and those growing in the excessively drained soil probably suffered the most the effect of water deficit during the dry season. The root pruning treatment had one of lowest LER in December, but the incidence of CSR in this month was the second highest, and continued relatively high throughout the dry period.

The frequent leaf-pruning treatment initiated by mid August, had an intermediate LER during the rainy months, and one of the highest during the dry season. Nevertheless, this treatment maintained the lowest CSR incidence during the whole period. The treatment with excess of nitrogen had initially one of the greater LER, but the incidence of CSR was low in those early months.

Correlation between disease and climatic factors

Sunshine periods were negatively correlated with CSR incidence. This was a result of a higher rainfall, which correlated positively with a higher incidence. An increase in sunshine hours and the mean temperatures (less rainfall) was associated to a better vegetative growth of the plants, which would be also associated with better soil aeration. The LER was positively correlated with rainfall, and negatively with sunshine hours, evaporation and temperature ( Table 3 ).

The poor drainage treatment had the highest incidence of CSR, and presented the highest correlation values between incidence and number of hours of sunshine and maximum temperature. Furthermore, in this treatment were found significant correlations between rainfall, mean temperature and the infection index. Correlation was also positive between disease severity and rainfall, indicating that these environmental factors determine, not only the number of plants that develop the disorder, but also the severity of the attack in individual palms. These relationships between disease and climatic factors were not significant in the root pruning treatment. These plants however had their water relationships severely disturbed.

Other treatments

The mechanical damage to the bud with a needle, did not seem to affect the disease progress curve over time. This would be expected if the microorganisms that cause spear rot are secondary pathogens, that will not be successful causing an infection, unless the plant has previously entered in a predisposition stage. Nevertheless, once the palm is affected, the mechanical damage seemed to increase the severity of the attack. This was expressed as a greater severity index in plants of this treatment ( Fig.3 ). Furthermore, the damage caused to plants during the procedure of puncturing the bud, somehow affected the plant vigor in a negative way.(Figs. 4 , 5 and 6 ).

Pruning the young leaves monthly caused a weak growth of the plant, that was associated to a lower CSR incidence.

Regular root pruning may have caused a similar effect to poor soil aeration, which also impeded the plant to maintain an adequate  root/canopy ratio. A poor root system, does not allow the plant to fully explore the soil and reduce its ability to uptake nutrients in a balanced way. The vegetative growth in these plants was relatively poor (Figs. 4 , 5 and 6 ), and this effect accentuated with age, as the nutritional needs of the plants increased. All this was associated with an increase in disease incidence and severity during the last visits.

Crown Disease

The presence of this condition in commercial  nurseries is a rare event. However, incidence was expected to be higher in the susceptible cross used in this experiment. The rather low actual incidence at the nursery can be expected if the bending of the rachises actually occur due to an abnormal lignification of the young tissues, which makes them more flexible than normal. Rachises of nursery plants are short and of low weight, which would normally prevent them to bend, eventhough they are more flaccid (Monge et al. 1993, 1994).

When the palms reached 15 months of age at the nursery stage, two replications of each treatment were thinned. This reduced the etiolation process in these plots, and was associated to a lower incidence of the Crown Disease symptoms. The number of accumulated cases of CD in the thinned plots were half of those that appeared in the replication without thinning. This last plots suffered from severe etiolation.

The competition for light caused an excessive elongation of the rachises, and the tissues were less rigid. This was expected to induce a higher incidence of Crown Disease, which actually happened. This response was in agreement with the theory of an abnormal  lignificación associated  to the curvature of the leaves in palms with Crown Disease ( Table 4 ).

After field transplanting, leaves become longer and heavier, which increases the chances of the appearance of one or more bent raquises in plants with CSR/CD. However, in still older palms, leaf weight will prevent the presence of such symptoms, since these leaves will normally break at the weakest point, where a poor lignification had occurred. This would explain the prevalence of the symptoms of only spear rot without bent raquices in older palms.

Another aspect that may prevent the presence of bent raquises in affected palms is the severity of the spear rot. Quite commonly this rotting is very extensive causing complete destruction of the spears in an early stage. These factors could be masking the real incidence of CD in some treatments, particularly those in which the severity of the symptoms  was greater, such as that of poor drainage.

In spite of those limitations to determine the real response to CD according to each treatment, it was clear from the results that Crown Disease and Common Spear Rot were related (Tables 2 and 4). The most frequent situation found under field conditions (first two years after transplanting), is that of palms that in different stages of the development of these disorders, show either CD or CSR symptoms or a combination of the two (Monge et al. 1993, Turner 1981). This is good evidence to suppose that CD symptoms and CSR are simply different manifestations of the same underlying cause; the differences may be brought about by specific growth conditions in a given moment and by the genetic makeup of  the plant.

CD and CSR are normally treated as two different conditions. Nevertheless, lesion aspect (leaflet and raquis rots), and associated microorganisms are very similar if not the same (Monge et al. 1993; Turner 1981). The difference between the two disorders is then attributed to the presence of one or more bent raquises in CD, but this characteristic is also not always reliable.

Conclusions

The elucidation of the predisposing factors for Crown Disease/Common Spear Rot and other disorder, such as PC, is not easy, due to the fact that several environmental factors and their interactions seem to be involved in the response of the plant. However, this is the most common situation for the so called "declines" or progressive deterioration that suffer some crops and forests (Manion and Lachance 1992). In all these cases no single factor (environmental or agronomic) or pathogen, has been implicated as the sole cause for such disorders.

The importance of the genetic makeup of the palm determining susceptibility to the Common Spear Rot/Crown Disease has been clearly established. This has made possible to select progenies, where the probability of the appearance of a high incidence of the disease is very small. Nevertheless, the process of selection has been slow since the environmental factors that also determine incidence and severity in different genotypes are not well known. The results of the present experience, and some others (Chinchilla et al . unpublished) are encouraging, since it was found that in fact, incidence is determined by some environmental factors that can be identified and measured.

It has been observed an apparent association between some stress factors and a greater incidence of CSR. In particular, Duff (1962, 1963) concluded that those progenies with a more vigorous growth were less susceptible to this disorder. Consequently, this author indicated that any factor that interfered with the normal growth, increased the probabilities of the plant of developing the disorder. Other authors found what seemed to be a similar situation in CSR (Turner 1981).

Nevertheless, these apparent relationships between stress in general, and greater susceptibility to the disorder do not always occur, and some treatments that impose an excessive stress, and impede a normal development of palms (such as continuos pruning of younger leaves), highly reduce the chance of these palms of getting the disorder.

On the other hand, it was observed a clear trend where palms with an apparent normal vegetative growth had a relatively high incidence of CSR. This compels to conclude that it is not just the stress by itself what makes the plant more susceptible, but the effect that a given type of stress (or combination) causes on the physiology of the plant, the root/canopy ratio, and how this stress affects the ability of the plant to defend itself from the many microorganisms acting as opportunist pathogens.

Transplanting from the nursery to the field represents a severe stress to the plants. Neverthless, CSR/CD does not increase inmediatelly following this practice, but the first peak of incidence is observed during the second year in the field. Such peak normally coincides with the onset of the rainy season and the fertilizer applications, which promote a more vigorous vegetative growth. A second peak, usually occurs during the rainiest months of the year, when soil aeration decreases.

Response to Crown Disease can be similar to CSR. Some treatments that promoted an apparent better vegetative growth, also showed a higher incidence of CD. On the other hand, those palms fertilized with boron had a very good vegetative growth, similar to the best treatments, and a quite low incidence of Crown Disease. It is well known that boron plays an important role in cellular division of young tissues and in the process of lignification.

An important finding was the identification of soil aeración as a determinant of susceptibility to CD/CSR. Improving soil aeration is a key issue to solve in oil palm plantations in Tropical America. This aspect will not only increase productivity as a direct effect, but will also reduce the chance of the presence and development of several serious diseases of this crop. The existence of such phytosanitary problems as PC, which is also closely related to a poor soil aeration, justify by itself alone a permanent check of this aspect in any commercial oil palm plantation.

Eventhough, it was clear that some management factors were associated to CD/CSR, they were not fully quantified in this experiment. The test has to be repeated quantifying in the soil and in the plant the effect of each treatment. For example, tensiometers should be used in the nursery bags to have an estimate of water availability for the plant and the degree of water saturation in the soil. Furthermore, the water status of the plant must be measured throughout the experiment (relative water content, water potential, stomata conductance).

Plant nutrient status should be followed through repeated leaf analysis. The fertilization treatments could be planned so that they can induce imbalances between specific elements; mainly nitrogen, potassium and other cations as calcium and magnesium.

There were differences in CSR incidence between plots even before some treatments were started. The peaks of maximum incidence occurred at different times in different treatments. In the bags with poor drainage this peak occurred early in the season, which indicated that the trigger for susceptibility (poor aeration) was present since the beginning. On the other hand, the peak of maximum incidence occurred late in the treatment of excess of the N fertilizer. Most plants with symptoms did not appear until several applications of fertilizer were done, and a N overdose was eventually reached. The trigger seemed to be the beginning of the rainy season when all environmental factors allowed the start of a rapid vegetative growth.

This above behavior is a clear indication of the effect of the environment on the appearance and progress of CD/CSR. It is also clear that not all management and environmental factors involved have been identified. The next logical step is to determine such elements and their interactions, so these and other disorders such as PC, can be better understood and managed.

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