A. H. Wearing

Rootstocks influence ‘Hass’ avocado fruit quality and fruit minerals

Summary To determine the potential for rootstocks to improve avocado quality, ‘Hass’ fruit from trees on seedling ‘Velvick’ (SV), clonal ‘Velvick’ (CV) or clonal ‘Duke 7’ (CD) rootstocks were harvested in 1999 and 2000, ripened at 20°C immediately after harvest (non-stored) or stored at 5°C for four weeks (stored), and then fruit quality and mineral concentrations determined. In 2000, non-stored CV fruit had lower severity of body rots (5% of flesh volume affected, caused mainly by Colletotrichum gloeosporioides) than CD fruit (20%), while body rots severity in stored CV fruit was 20% compared with 38% in stored CD fruit. Stored CV fruit had less severe diffuse discoloration (3% and 9% in 1999 and 2000, respectively) than CD fruit (7% and 20%, respectively). Stored CV fruit also had less severe vascular browning (19%) than CD fruit (33%) in 2000. In both seasons, CV fruit had 15–19% higher flesh calcium concentrations, 17–22% higher flesh boron concentrations, and 14–15% lower flesh nitrogen concentrations than CD fruit. In 2000, CV fruit also had 27% more skin calcium and 14% less skin nitrogen than CD fruit. There were no effects of rootstock on yield, canopy volume, measured length of non-suberized roots, average fruit weight, fruit length: width ratio, skin thickness, dry matter, proportion of the fruit skin purple-black when ripe, or seed mass: fruit weight ratio. These results suggest that there is potential to improve ‘Hass’ avocado quality through rootstock selection, and that tree and fruit mineral concentrations have a role in this relationship.

Treatment with Cu2+ ions extends the longevity of cut Acacia holosericea foliage stems

Summary Wilting associated with diminishing post-harvest water uptake is a major constraint on longevity for cut stems of many Acacia spp. Cu2+ ions have known biocidal activity and may also act as inhibitors of plant wound reaction enzymes. A series of experiments were conducted using Cu2+ ions as a pulse, or as a vase-solution additive to evaluate their potential to extend the vase-life of cut A. holosericea foliage stems. Provision of Cu2+ ions in the vase-water at 0.5 mM, or as a 5 h pulse at 1.0 mM, resulted in 1.4- to 2.8-fold, and 1.7- to 2.1-fold extensions of vase-life, respectively, compared to de-ionised water controls. The pulse treatment was optimised, and a 5 h pulse with 2.2 mM Cu2+ ions resulted in a 2.6-fold extension of vase-life. Thus, Cu2+-pulsing can effectively delay wilting in cut A. holosericea foliage stems.

Freeze-induced reduction of broccoli yield and quality

Summary. Sub-zero temperatures can result in freezing injury of broccoli (Brassica oleracea L. var. italica Plenck) plants and thereby reduce head yield and quality. In order to predict effects of frosts, it is desirable to know the stages of development at which broccoli plants are most susceptible to freezing injury. In this study, the effect of a range of sub-zero temperatures for a short period at different stages of crop development were assessed and quantified in terms of mortality, yield and quality of broccoli. Whole plants in pots or in the field were subjected to sub-zero temperature regimes from –1 to –19°C. Extracellular ice formation was achieved by reducing temperatures slowly, at –2°C per hour. The floral initiation stage was most sensitive to freezing injury, as yields (fresh and dry head weights) were significantly reduced at –1 and –3°C, and the shoot apices were killed at –5°C. There was no significant yield reduction when the inflorescence buttoning stage was treated at –1 and –3°C. Although shoot apices survived the –5°C treatment at buttoning, very poor quality heads of uneven bud size were produced as a result of arrested development. The lethal temperature for pot-grown broccoli was between –3 and –5°C, whereas the lethal temperature for field-grown broccoli was between –7 and –9°C. The difference was presumably due to variation in cold acclimation. Freezing injury can reduce broccoli head yield and quality and retard plant growth. With regard to yield and maturity prediction, crop development models based only on simple thermal time without restrictions will not apply if broccoli crops are frost damaged.

Saprophytic microorganisms with potential for biological control of Botrytis cinerea on Geraldton waxflower flowers

Saprophytic bacteria, yeasts and filamentous fungi were isolated from Geraldton waxflower flowers and screened to identify potential antagonism towards Botrytis cinerea. Isolates from other sources (e.g. avocado) were also tested. Isolates were initially screened in vitro for inhibition of B. cinerea conidial germination, germ tube elongation and mycelial growth. The most antagonistic bacteria, yeasts and fungi were selected for further testing on detached waxflower flowers. Conidia of the pathogen were mixed with conidia or cells of the selected antagonists, co-inoculated onto waxflower flowers, and the flowers were sealed in glass jars and incubated at 20 degreesC. The number of days required for the pathogen to cause flower abscission was determined. The most antagonistic bacterial isolate, Pseudomonas sp. 677, significantly reduced conidial germination and retarded germ tube elongation of B. cinerea. None of the yeast or fungal isolates tested was found to significantly reduce conidial germination or retard germ tube elongation, but several significantly inhibited growth of B. cinerea. Fusarium sp., Epicoccum sp. and Trichoderma spp. were the most antagonistic of these isolates. Of the isolates tested on waxflower, Pseudomonas sp. 677 was highly antagonistic towards B. cinerea and delayed waxflower abscission by about 3 days. Trichoderma harzianum also significantly delayed flower abscission. However, as with most of the fungal antagonists used, inoculation of waxflower flowers with this isolate resulted in unsightly mycelial growth.

Field applications of three different classes of known host plant defence elicitors did not suppress infection of Geraldton waxflower by Botrytis cinerea

Ethylene-mediated flower abscission caused by Botrytis infection afflicts cut Geraldton waxflower stems. Preharvest spray applications of three known host plant defence elicitors, benzothiadiazole (BTH), methyl jasmonate (MeJA) or silicon (Si), were applied to Geraldton waxflower cvv. Mullering Brook and My Sweet Sixteen. Their individual efficacy in postharvest suppression of Botrytis disease developmentwas assessed. Field applications of BTH or Si generally had no significant (P>0.05) effect on Botrytis disease severity on either cultivar. MeJA sprays did not significantly (P>0.05) reduce disease severity on cv. Mullering Brook, but slightly and significantly (P<0.05) suppressed Botrytis on cv. My Sweet Sixteen at concentrations of 500 and 750 μM MeJA. One Si treatment, 1500 mg SiO2/L, significantly (P<0.05) reduced floral abscission on cv. Mullering Brook. Overall, field applications of these three host plant defence elicitor chemicals as spray treatments had little effect on vase life, water uptake and relative fresh weight of the cut flower sprigs. Moreover, they did not suppress Botrytis or associated postharvest floral abscission in cut Geraldton waxflower flowers.

Between-tree variation in fruit quality and fruit mineral concentrations of Hass avocados

Inconsistent internal fruit quality in Hass avocados affects consumer confidence. To determine the influence of individual trees on fruit quality, Hass avocado fruit were harvested from adjacent trees of similar external appearance in 3 commercial orchards in 1998 and 1 orchard in 1999. The trees in each orchard were grown with similar commercial practices and in similar soil types. Within each location, there were significant (P<0.05) differences in the mean ripe fruit quality between trees with respect to fruit body rot severity (mainly anthracnose) with and without cold storage, internal disorders severity due to diffuse discolouration and vascular browning (after cold storage), days to ripen, percentage dry matter, and the percentage of the skin area with purple-black colour when ripe. These effects were also noted in the same orchard in 1999. There were significant (P<0.05) differences in fruit flesh calcium, magnesium, potassium, boron and zinc concentrations between trees. Significant (P<0.05) correlations were observed between average fruit mineral concentrations in each tree (particularly calcium, magnesium and potassium) and body rot severity, percentage dry matter and fruit mass. There was little conclusive evidence that characteristics such as the growth of the non-suberised roots or the degree of scion under- or overgrowth was involved in these tree effects; however, differences between trees with respect to other rootstock characteristics may be involved. The inconsistency of the correlations across sites and years suggested that other factors apart from tree influences could also affect the relationship between fruit minerals and fruit quality.

Calcium treatment of harvested Geraldton waxflower does not enhance postharvest quality

Postharvest flower abscission from cut Geraldton waxflower (Chamelaucium uncinatum) is mostly caused by fungal invasion. Elevated plant tissue calcium concentrations through postharvest application reduces fungal disease severity in various crops. Such results may be explained by strengthening of plant cell walls by calcium. Strengthening provides a structural barrier to fungal hyphae, thereby restricting invasion of plant cells. Postharvest pulsing with calcium solution substantially increased calcium concentrations in waxflower tissues. Ca-45 tracer revealed calcium distribution throughout flowering sprigs, including infection sites such as stylar tissue. However, pulsing waxflower sprigs with calcium did not suppress either disease or flower abscission, nor did it enhance vase life.

Pythium irregulare can cause root rot of Platysace lanceolata

Cultivated Platysace lanceolata cv. Valentine Lace plants in south-east Queensland are affected by root rot that reduces shoot growth and flower production, and leads to plant death. Pythium irregulare, Rhizoctonia solani and Cylindrocarpon lichenicola were isolated from diseased roots using selective media and their pathogenicities tested under glasshouse conditions. Parameters used to assess disease severity included the proportion of the root system visibly rotting, the weight and size of the root system, and the proportional increase in total shoot length. Glasshouse plants inoculated and colonised with R. solani and C. lichenicola did not develop the symptoms of the field disease. Inoculation and colonisation with P. irregulare caused a detectable increase in root rot only when roots were wounded at the time of inoculation. However, root size, root weight and shoot length increases of plants inoculated with P. irregulare were all lower than those of non-inoculated plants, either with or without wounding. The combination of wounding and inoculation with P. irregulare resulted in rotting of ∼11% of the root length.

Iterative germination and innovative techniques for the production and inoculation of secondary conidia of sorghum ergot (Claviceps africana)

Abstract Five methods of harvesting pure secondary conidia were attempted, but only one was successful. The only successful method involved moving air across the surface of water agar plates or moist soil on which secondary sporulation was occurring at 0.2 – 3 m/s. Among the inoculation techniques, brushing the secondary conidia on to the wet stigma gave maximum ergot infection (80%) followed by brushing the secondary conidia on to the dry stigma (77%). Conidia of Claviceps africana produced up to seven generations of sibling conidia. The length and width of secondary conidia showed a reduction in size from 10.92 – 10.13 µm and 5.49 – 5.13 µm, respectively, across generations. However no definite trend in decline in size of secondary conidia was evident. The optimum temperature range for germination of secondary conidia of Australian isolates of C. africana is between 14.6 and 20°C.