Rootstock Improvement for the Australian Avocado Industry - Phase III

The avocado industry has supported the development and evaluation of new rootstocks for Hass and Shepard avocado. Field sites containing various rootstock/scion combinations have been established around Australia. This project will evaluate tree, fruit yield and and postharvest fruit quality attributes from these field sites

Changing sink strengths influence translocation of phosphonate in avocado (Persea americana Mill.) trees

Translocation of phosphonic acid (H3PO3) in cv. Hass avocado trees was studied after trunk injection with 20% H3PO3, formulated as potassium phosphonate, at three stages of tree phenology during the growing season. Initially, translocation was solely acropetal in the xylem, and H3PO3was detected in the leaves 24 h after treatment. Several days after injection, H3PO3 concentration in the bark of trunks and in roots increased, indicating basipetal phloem transport of H3PO3 from leaves. The rate of accumulation and the final concentration of H3PO3in the roots were directly related to the sink strength of the shoot at the time of injection. For example, trunk injection at the beginning of spring growth flush, when renewal shoots were strong sinks, resulted in low H3PO3 root concentrations (25 8g gfw-1 by 30 days after treatment. These results suggest that strategic timing of injections according to phenological events may greatly improve fungicide efficacy when targeting specific organs for protection

EFFECT OF SOIL BORON APPLICATION ON GUMMOSIS AND LEAF BORON CONTENT OF MANGO (MANGIFERA INDICA L.) CVS. KHIEO SAWOEI AND NAM DOK MAI

Boron (B) deficiency is a widespread and common nutrient disorder of agricultural and forestry crops in Thailand. To correct B deficiency in mangoes (Mangifera indica L.), a study was conducted to evaluate the effectiveness of soil application in raising leaf B concentrations in two cvs, Khieo Sawoei and Nam Dok Mai. Borax (11% B) was applied to the soil beneath trees at 0, 15, 20, 25 g m-1 of canopy diameter (split into three applications). When B applications were given in the summer (wet season) the response was rapid, with leaf concentrations increasing by 93.6 and 75.6% at the highest rate for ‘Khieo Sawoei’ and ‘Nam Dok Mai’, respectively, 5 months after treatment. There was no evidence of phytotoxicity. Gummosis, a condition of resin ooze from splits that develop on major limbs and trunks in B deficient trees, decreased in ‘Khieo Sawoei’ trees that received borax at either 20 or 25 g m-1 of canopy diameter

Effect of temperature on seed and fruit development in three mango (Mangifera indica L.) cultivars

The effect of diurnal maximum/minimum (20/10 or 25/15 °C) temperatures on seed and fruit development of ‘Irwin’, ‘Kensington’ and ‘Nam Dok Mai’ mangoes (Mangifera indica L.) was studied in a controlled-environment glasshouse. Exposure to low temperatures (20/10 °C day/night) 3 days after hand pollination significantly increased the percentage of stenospermocarpic fruit (nubbins), in which embryos were aborted at some stage during early fruit development. There were significant differences between cultivars in the percentage of nubbins produced out of the total fruit set following overnight exposure to 10 °C with 21% for ‘Nam Dok Mai’, 11% for ‘Kensington’ and 3% for ‘Irwin’. At 45 days after pollination, nubbin fruits were much smaller in size and weighed ca. 50% less than normal fruits. The lower percentage of nubbin fruits in ‘Irwin’ implies a greater adaptation to cool temperatures by this cultivar during fruit set and early embryo development

Effects of leaf to fruit ratios on mango (Mangifera indica L. 'Kensington') fruit growth, nutrition and quality

During the 1995/96 season, 30 uniform mango trees (Mangifera indica L. ‘Kensington’) in Ayr, North Queensland (20°S) had branches thinned to a single fruit and girdled to provide 30, 60 or 120 leaves per fruit above the girdle. Control branches were not thinned or girdled and had approximately 40 leaves per fruit. Increasing the leaf to fruit ratio to 120 leaves per fruit increased fruit size, reduced fruit pulp Ca concentrations, advanced maturity (as indicated by % dry matter and pulp colour) and reduced the number of days to ripen at 22°C (shelf life). Decreasing the leaf to fruit ratio to 30 decreased fruit size, increased pulp Ca concentrations, delayed maturity, increased shelf life and improved storage potential by reducing chilling injury. Increasing leaf to fruit ratios to 60 increased fruit size without reducing fruit Ca concentrations, and increased fruit shelf life. Fruit from girdled branches had 20% less green skin when ripe and increased incidence of blush and disease. Increases in fruit size were associated with increased lenticel spotting

Seasonal effects of foliar application of phosphonate on phosphonate translocation, in vitro pollen viability and pollen germination in `Hass' avocado (Persea americana Mill.)

Phosphonate fungicides are used widely in the control of diseases caused by Phytophthora cinnamomi Rands. For the most part phosphonate is seen as a safe to use on crops with phytotoxicity rare. However, recent research has shown that phosphonate has detrimental effects on the floral biology of some indigenous Australian plants. Since phosphonate fungicides are regularly used for the control of Phytophthora root rot in avocados, research was carried out to study the translocation of phosphonate fungicide in 'Hass' trees and any effects on their floral biology. Field-grown trees were sprayed with 0, 0.06 or 0.12 M mono-dipotassium phosphonate (pH 7.2) at summer flush maturity, floral bud break or anthesis. Following treatment, phosphonic acid concentrations were determined in leaves, roots, inflorescence rachi and flowers and in vitro pollen germination and pollen tube growth studied. Phosphonic acid concentration in the roots and floral parts was related to their sink strength at the respective times of application with concentration in roots highest (36.9.mg g±1) after treatment at summer flush maturity and in flowers (234.7 mg g±1) after treatment during early anthesis. Phosphonate at >0.03 M was found to be significantly phytotoxic to in vitro pollen germination and pollen tube growth. However, this rate gave a concentration far in excess of that measured in plant tissues following standard commercial applications of mono-dipotassium phosphonate fungicide. There was a small effect on pollen germination and pollen tube growth when 0.06 and 0.12 M mono-dipotassium phosphonate was applied during early anthesis. However, under favourable pollination and fruit set conditions it is not expected to have commercial impact on tree yield. However, there may be detrimental commercial implications from phosphonate sprays at early anthesis if unfavourable climatic conditions for pollination and fruit set subsequently occur. A commercial implication from this study is that phosphonic acid root concentrations can be elevated and maintained with strategic foliar applications of phosphonate fungicide timed to coincide with peaks in root sink strength. These occur at the end of the spring and summer flushes when shoot growth is relatively quiescent. Additional foliar applications may be advantageous in under high disease-pressure situations but where possible should be timed to minimize overlap with other significant growth events in the tree such as rapid inflorescence, and fruit development and major vegetative flushing

The influence of planting density on the production of 'Goldfinger' (Musa spp., AAAB) in the subtropics

'Goldfinger', a tetraploid banana produced from the Fundación Hondureña de Investigación Agrícola (FHIA) breeding program, was released to the Australian industry in 1995. It was promoted as an apple-flavoured dessert banana with resistance to Fusarium wilt race 1 and subtropical race 4, as well as resistance to black and yellow Sigatoka (Mycosphaerella fijiensis and M. musicola, respectively). This study was initiated to provide agronomic information to the banana industry, which was under threat from Fusarium wilt, on a new cultivar which could replace 'Williams' (AAA, Cavendish subgroup) or 'Lady Finger' (AAB, Pome subgroup) in those areas affected by Fusarium wilt. Also few studies had reported on the production characteristics of the new tetraploid hybrids, especially from subtropical areas, and therefore two field sites, one a steep-land farm and the other a level, more productive site, were selected for planting density and spatial arrangement treatments. The optimum density in terms of commercial production, taking into account bunch weight, finger size, length of the production cycle, plant height and ease of management, was 1680 plants/ha on the steep-land site where plants were planted in single rows with 2.5 m × 2.5 m spacings. However on the level site a double-row triangular layout with inter-row distances of 4.5 m to allow vehicular access (1724 plants/ha) gave the best results. With this arrangement plants were in an alternate, triangular arrangement along a row and a spacing of 1.5 m between plants at the points of each triangle and between each block of triangles