Shoot growth and carbohydrates
As discussed, there is a relationship between flowering intensity and fruit set, and an implied relationship between shoot growth and flowering potential. It therefore suggests that by increasing shoot growth you should be able to increase potential for fruit set. But what is controlling the level of shoot growth from year to year and can this be manipulated?
Stored carbohydrate levels in the major limbs of the avocado tree just prior to flowering are considered to be an important indicator of flowering potential. Cyclic patterns of accumulation and depletion of carbohydrates have been recorded by Scholefield et al. (1985), Whiley et al. (1996) and Dixon et al. (2005). Low starch levels are recorded at the end of the autumn flush and highest levels just before flowering and shoot flush in spring.
Whiley et al. demonstrated a reasonable relationship between increasing trunk starch concentration in July and increasing yield for the next season. Therefore it is possible the starch concentration had an effect on the developing flowers and fruit set during winter and spring.
Scholefield et al. (1985) demonstrated an apparent relationship between crop load and the depletion and accumulation of starch, with a deeper depletion of starch following a light crop year and a lower accumulation of starch after a heavy crop year. This was for Fuerte grown in the Murray Valley Irrigation Area.
Whiley et al. (1996) did not find any significant relationship between heavy and light cropping years and changes in the cyclic pattern of starch accumulation on Hass in subtropical Queensland. Scholefield et al. (1985) recorded very high levels of starch accumulation of up to about 18%, while Whiley et al. (1996) and Dixon et al. (2005) recorded much lower maximum levels of 6 to 7%. These high differences could be due to differing extraction methods.
Dixon et al. (2008b) found limited effect of light or heavy flowering on the timing or duration of the subsequent vegetative spring flush. However, they did report a reduced overall level of growth of shoots and roots after heavy flowering as compared to that after a light flowering. Interestingly, they also noted an even more extreme drop in level of shoot and root growth in one year following a light flowering accompanied by a period of moisture stress. Dixon et al. (2008b) also demonstrated the link, albeit not consistently, between a heavy crop and following light flowering and vice versa.
While a relationship has been drawn between carbohydrate levels in the trunk and subsequent yield, equally it is likely that a relationship can be drawn between carbohydrate levels and the strength of the vegetative flush. Other conditions equal, a strong vegetative flush, with a higher leaf area, could be expected to result in higher accumulation of starch than a weaker vegetative flush. As a result, a strong vegetative flush should lead to a higher accumulation of starch.
Is it the heavy flowering and/or subsequent heavy crop that are potentially drawing too heavily on the plant's starch reserves to allow adequate shoot growth? The shoot growth is required for the development of sufficient inflorescences to allow for a follow up good flowering. Or is it that low carbohydrate reserves following a heavy cropping year limit the final development of the inflorescences?
Whiley et al. (1996) demonstrated that delaying the harvest of mature fruit until flowering exacerbated alternate bearing. Garner and Lovatt (2008) found that there was no relationship between crop load of the previous season and subsequent abscission of the reproductive structures during flowering. This tends to suggest that crop load is likely affecting the development of the shoots and accompanying inflorescences during the growth season rather than impacting on the actual flowering and fruit set stage. If it is the energy draw of the flowering, can this be overcome with different management strategies?
Irrigation and nutrition
Dixon et al. (2008b) noted the potential of increased water stress on reducing the vegetative flush. Therefore, ensuring adequate moisture levels are maintained during spring and summer would be one step to improving vegetative flush.
Lovatt (2001) demonstrated that selective timing of nitrogen fertiliser application can modify yields and the severity of alternate bearing. Applications during autumn, after irreversible commitment to flowering resulted in increased fruit size at harvest. While applications in spring after anthesis assisted in reducing the severity of alternate bearing.
Spring flush timing
The spring flush is also in competition for carbohydrates at flowering. The spring flush normally will start around the same time as flowering and fruit set. This competition is often considered to be one of the triggers of low fruit set. Salazar-Garcia and Lovatt (1998) have suggested that modifying the timing of the vegetative growth may offer an opportunity to reduce the level of competition at critical times.
Observations in the South-West of Western Australia have hinted that earlier flushing of vegetative growth in spring results in a stronger spring flush after a heavy flowering than if the shoot-flush is later. This could assist in a better return flowering. Salazar-Garcia and Lovatt (1998) demonstrated that application of GA3 could be used to advance the development of the vegetative flush of indeterminate inflorescences.
Small fruitlet abscission
Abscission of immature fruit immediately after flowering continues to concern growers. Growers believe this to be an even greater problem for fruit set from late-season flowers. Garner and Lovatt (2008) found that peak abscission of immature fruit occurred about one month after completion of flower abscission. But there was no indication as to what flowers set this fruit. What causes this immature fruit to abscise is still to be determined, but does not appear related to the size of the previous crop.
Genetics
It may be that the variety Hass does not have the capacity to consistently set large crops and in cooler climatic zones this leads to significant alternate bearing and that future efforts may need to consider variety improvements rather than management improvements.