Factors influencing the occurrence of premature and excessive leaf abscission in the avocado (Persea americana Mill.) cultivar 'RYAN' and possible preventative measures

Abstract

Premature and excessive leaf abscission during flowering time in the late avocado (Persea americana Mill.) cultivar ‘Ryan’ is a considerable problem for avocado growers. They are especially concerned that premature and excessive leaf abscission will have a negative effect on yield. No previous investigations have been performed where premature and excessive leaf abscission in avocado has been studied in detail. This study therefore aimed to investigate the pattern of premature and excessive leaf abscission in ‘Ryan’, and compare it with two other important commercial cultivars, ‘Fuerte’ and ‘Hass’, which do not display this phenomenon. Time course studies of leaf abscission in the orchard were performed during 2006 and 2007 to determine the pattern of leaf abscission on ‘Ryan’, Fuerte’ and ‘Hass’. This also included anatomical studies to determine the time of leaf abscission zone formation. Possible stress factors, which accelerate leaf abscission were also investigated, namely unfavourable climatic conditions (temperature, solar radiation, rainfall, relative humidity and evapotranspiration), nutrient imbalances, excessive flowering and leaf area. The possible impact leaf abscission may have on yield was then assessed by determining levels of reserve carbohydrates in the bark of the tree. In addition, practical solutions, i.e. the application of fertilizers, plant growth regulators (PGRs) and kaolin, were investigated in order to reduce or eliminate premature and excessive leaf abscission. This study was carried out over the period 2005 until 2007, with experiments being modified on an annual basis as information was gathered on the phenomenon. Experiments began in 2005 with a study on the pattern of leaf abscission in ‘Ryan’, which revealed an increase in leaf abscission just prior to flowering. However, this increase was not significant. During 2006, the leaf abscission pattern for ‘Ryan’ was compared with the leaf abscission patterns of ‘Fuerte’ and ‘Hass’. Leaf abscission for ‘Ryan’ was significantly higher than for ‘Fuerte’ and ‘Hass’ during 2006. During 2006 ‘Ryan’ displayed two periods of high leaf abscission, namely the spring flush between bud dormancy and bud swell, and a drastic increase in spring and summer flush leaf abscission between inflorescence development and full bloom. These periods of increased leaf abscission were absent during the 2007 season. In addition, ‘Fuerte’ and ‘Hass’ did not display these peaks of high leaf abscission, with leaf abscission occurring in these cultivars at higher rates from full bloom onwards. Premature and excessive leaf abscission is therefore not an annual event in ‘Ryan’ and is in all likelihood influenced by external factors. Anatomical studies did not reveal any results in terms of initiation of leaf abscission, with only the protective layer of the abscission zone being visible after leaf yellowing occurred. During 2006, two peaks of extremely low temperatures (<4°C) occurred just prior to the acceleration of leaf abscission. During the second period of low temperatures, the solar radiation:temperature-ratio was also considerably higher. These periods of low temperatures were absent during 2007, indicating that cold and light stress could be contributing to premature and excessive leaf abscission in ‘Ryan’ in 2006. In addition, ‘Ryan’ flowered excessively during 2006, which could have been triggered by low temperature stress just prior to flower initiation. A significant correlation was found between excessive flowering and excessive leaf abscission in ‘Ryan’ during 2006. The occurrence of reduced flowering in ‘Fuerte’ and ‘Hass’ may possibly be due to these two cultivars being more tolerant to stress, and it is possible that ‘Ryan’ is genetically more prone to excessive flowering than ‘Fuerte’ and ‘Hass’. Excessive flowering could accelerate leaf abscission by causing an unusually high demand for water, nutrients and carbohydrates, resulting in the acceleration of leaf abscission. No significant relationship between nutrient levels and excessive leaf abscission was found for either 2006 or 2007. In addition, no significant correlation could be found between leaf abscission on a branch and the total leaf area of that branch during the 2007 season. During 2007, leaf abscission was low and it is possible that a significant correlation could be found in a season with excessive leaf abscission. During 2005, chemical applications to reduce leaf abscission did not yield any significant improvement in leaf retention. In fact, the 50 g/tree Solubor® and 50 g/tree Solubor® in combination with 2 kg/tree dolomitic lime had a significant negative effect on fruit set, possibly because too high concentrations were applied too close to fruit set. Chemical applications during 2006 were therefore made at bud dormancy and bud swell, as it was found that leaf abscission occurred before flowering time. However, no effect was observed on leaf retention or fruit set. During 2007, emphasis was placed on treatments that might reduce stress, as it became evident that stress could be responsible for premature and excessive leaf abscission in ‘Ryan’. Most treatments showed a slight positive effect on leaf retention, but no significant results were obtained possibly because that particular season was a season of low leaf abscission. Further research on application of stress-reducing treatments is therefore recommended. Best farm management practices including optimal fertilization and irrigation is therefore vital to prevent stress, accelerating leaf abscission. Copyright