SUMMARY

Effects of ethephon, gibberellic acid (GA3), and stem maturity on defoliation and shoot regrowth in guava (Psidium guajava L. 'Beaumont') was investigated. A factorial experiment with four levels of ethephon, three levels of GA3, and two stem maturity was conducted on potted guava plants. Ethephon, GA3, and stem maturity had significant effects on percent defoliation. Increasing ethephon concentration increased pecent defoliation from 26 to 94%. GA3 had 79% defoliation compared to 64% without GA3. Semi-hardwood stem had 80% defoliation, whereas softwood stem had 69% defoliation. There was a significant interaction between ethephon concentration and stem maturity on the number of new lateral shoots (> 1 cm) per initial leaf count. Mean shoot length was greatest (10.1 cm) at 600 ppm ethephon.

INTRODUCTION

In Hawaii the peak harvest period of guava occurs eight to nine months after the rainy season (November to February). Rainfall stimulates new vegetative flushes that bear the flowers. Because processors want a consistent supply of fruit year round, growers use a crop cycling program to insure good production throughout the year. In this program, different areas of the orchards are pruned, irrigated, and fertilized, or chemically defoliated at different times of the year, which results in a crop about seven months later.

A 24 to 25% urea solution is a very effective defoliant of guava, but it causes excessive re-vegetation at the expense of flowering and is difficult and time-consuming to get into solution at that high concentration. There is no information on different combinations of ethephon and gibberellic acid in defoliation and re-vegetation of guava. Thus, the objective of this study was to determine the effect of ethephon and gibberellic acid on defoliation and vegetative regrowth of guava.

PROCEDURE

Rooted stems cuttings of 'Beaumont' guava were transplanted to 15-cm pots in a media of 1 soil : 1 perlite : 1 verimulite (by volume) plus 6.7 kg Osmocote 16N-7.1P-13.3K per m³ of media. Plants were trained to a single leader and grown in a sarahhouse (32% shade) where they attained a height of 80 to 90 cm before the start of the experiment. The softwood and semi-hardwood sections of the leader averaged 10.5 and 15.6 leaves per plant, respectively.

A 4 x 3 x 2 factorial experiment in a completedly randomized design with four single plant replications was used. The factors were ethephon at 0, 600, 1200, and 1800 ppm; gibberellic acid (GA3) at 0, 50, and 100 ppm; and stem maturity of softwood and semi-hardwood.

Urea at 3.2% (w/v) and X-77 surfactant at 0.2% (v/v) was added to all solutions. Upper and lower leaf surfaces of plants were sprayed to run-off with a hand sprayer at one liter of solution per four plants. After being sprayed, plants were returned to the saranhouse. The percent defoliation of the initial leaf count was determined after 21 days. The number of new lateral shoots greater than 1 cm in length was taken after 38 days. The length of new lateral shoots greater than 1 cm was taken after 66 days. An arcsine transformation was carried out on the percent defoliation data before analysis.

RESULTS AND DISCUSSION

Ethephon, GA3, and stem maturity had significant effect on percent defoliation after 21 days. There were no interactions among these factors. Increasing ethephon concentration increased the percent defoliation from 26 to 94% (Table 1). GA3 increased the percent defoliation compared to no GA3. Percent defoliation was higher for the semi-hardwood than the softwood sections of the stem.

There was a highly significant interaction between ethephon and stem maturity for the number of new lateral shoots per initial number of leaves after 38 days. Increasing ethephon concentration decreased the shoots/initial leaf count ratio for the softwood section of the stem, but increased it for the semi-hardwood section (Fig. 1). Pruning or chemical defoliation stimulates production of more laterals, but it is the labor-saving aspect of chemical defoliation which merits attention in guava crop management.

Ethephon had a highly significant effect on mean shoot length after 66 days. As ethphon concentration increased, mean shoot length increased, reached a maximum at 600 ppm, and decreased thereafter (Table 1).

The addition of GA3 to ethephon aids in defoliating guava and results in gereater new vegetative regrowth. This enhancement effect, which depends on the stem maturity, improves the efficiency of ethephon and shows potential for agricultural use. The dependence of the shoots/initial leaf count ratio on stem maturity indicates the necessity of altering the formulation, depending on the amount of softwood and semi-hardwood stems on the trees.