Avocados trees have characteristic vegetative flushes in spring, summer and autumn. These flushes combined can create up to a metre or more of vegetative growth in one growing season. Avocados are also characterised by poor fruit set which has been partly attributed to a relationship between flowering time and the growth of these substantial vegetative flushes. Shortly after flowering and fruit set the first spring flush begins growing. Competition between the newly set fruit and the developing leaf flush was first suggested as being involved in poor fruit set in 1979 and again in 1983.
Research has found that partially limiting the spring flush is beneficial in maximising fruit set through techniques such as delaying or reducing nitrogen application during the flowering/growth flush stage, nipping off the shoot apical bud, girdling, and finally by the application of plant growth regulators. There are two growth regulators which can be used by Western Australian avocado growers; Paclobutrazol and Uniconazole.
Research in the plant growth regulator paclobutrazol started in the late 80s and early 90’s with efforts on the varieties ’Hass’ and ‘Fuerte’. Research only began after good methods had been developed to manage phytophthora root rot. Paclobutrazol caused a significant reduction in the growth of the spring flush and also significantly increased the proportion of dry matter going into the fruits whilst reducing dry matter going to the leaves and stems. Fruit retention was also improved by the application of paclobutrazol as a spray or as an injection (Kremer-Kohne, 1987). The improved retention of fruit in that particular piece of research was permanent and was not nullified by summer fruit drop. Some other research found that in high yielding trees summer fruit drop nullified the effect of increased fruit retention (Wolstenholme, Whiley and Saranah, 1990).
Interestingly while some research has not shown yield increases due to paclobutrazol use in a one off season the benefit appears to come in higher yields in off years resulting in higher cumulative yield compared to the control. In on-years a carbohydrate stress was suggested to explain how the tree is limited by resources and thus unable to support the larger fruit set caused by paclobutrazol. In off-years however the tree does not have this stress and thus can carry more fruit (Wolstenholme, Whiley and Saranah, 1990). Research in Israel provides extra evidence to support this hypothesis with trees with higher yields responding less to paclobutrazol (Adato and Gazit, 1974). Smaller applications of paclobutrazol (1.25 g/l and 0.62 g/l) by Whiley et al. (1992) saw the greatest cumulative yield of 350-400 kilograms per tree compared to approximately 250kg for the control tree (Whiley, Saranah and Wolstenholme, 1992). Current label rates are in the range of the above values and need to be followed to have the best effect.
Aside from slight improvements in yield the greatest benefit from using growth regulators such as paclobutrazol are their ability to manage tree vigour and effectively induce a dwarfing effect on an otherwise very vigorous tree. A South African trial used Paclobutrazol to minimise tree size in an 800 tree per hectare high density planting with success (Kohne and Kremer-Kohne, 1992).
Some later research also measured the impact paclobutrazol had on calcium levels in the fruit; calcium in the fruit is important for post-harvest purposes with higher calcium levels being beneficial for reducing post-harvest damage to the fruit and improving shelf life. The use of paclobutrazol was found to increase fruit calcium levels for eight weeks after fruit set but returned to the same level as the control by the 10th week (Whiley, Saranah and Wolstenholme, 1992).
Research into Uniconazole in avocados began in 1998 and resulted in increases in fruit size, which was described by the authors at the time as being conducive to a more valuable crop, despite there not being a yield increase. Average fruit size of hass was increased from 221g to 275g by the application of uniconazole at the mid-bloom. Application of uniconazole to regrowth following pruning was also able to increase flowering. The research also described how the fruit shape was changed to a more round shape (H.D. Erasmus, 1998).
The ‘rounding’ effect of growth regulator is well known and reported in literature (Brogio et al., 2018). Some people do not prefer the round shape of treated avocados over the natural pear shape. Anecdotally some growers have reported that the effect can be managed by using the correct rate of growth regulator. Either way, the positive benefits of growth regulator use-greater cumulative yield, larger fruit, and particularly the reduction in vegetative vigour-account for the change in fruit shape and justify their use in a productive avocado orchard.
There is an important disclaimer that needs to be added to the research reviewed above. All of the above research has been done in climates that are more tropical than the climate of South West Western Australia. The colder winters in Western Australia mean that during spring checking the growth of that first spring flush could potentially be damaging to the trees ability to collect valuable carbohydrates to contribute to the trees survival. It was described earlier how paclobutrazol applied to heavily loaded trees did not lead to yield increases as the tree was already resource limited. After a cool winter local experience appears to have shown that trees are short on resources and therefore the growth regulator may not have the desired effect. Unfortunately in California, which has a similar climate, there has not been sufficient or notable research into paclobutrazol or uniconazole with efforts instead going towards gibberellic acid (Lovatt, 2005). In fact some unpublished research by Carol Lovatt in California found that uniconazole had a detrimental effect on crop yield.
To balance that out, in Chile/Peru-which also has a similar climate- both uniconazole and paclobutrazol are both very popular. Some research presented at the 8th world avocado congress in Peru in 2015 saw slightly better yields from paclobutrazol use (26.21 t ha-1) to the control (23.34 t ha-1) with a reduction in shoot length from 46.5 cm to 34.5 cm. it is also important to note that the highest yield was observed with the lowest dose of paclobutrazol, higher rates saw large yield reductions with the highest rate resulting in 17.13 t ha-1 being produced. The higher rates likely had to extreme an effect on the plant and badly reduced its ability to produce carbohydrates to grow the crop. Therefore the rate needs to be correct for a good effect.
The use of the above growth regulators is regulated by the Australian Pesticides and Veterinary Medicine Association (APVMA). As of the time of writing Uniconazole is registered for use in avocado while paclobutrazol can only be used with a minor use permit. Information about registration and permits can be found on PUBCRIS.
References
Adato, I. and Gazit, S. (1974) ‘Water-deficit Stress, Ethylene Production, and Ripening in Avocado Fruits’, Plant Physiology, 53(1), pp. 45–46. doi: 10.1104/pp.53.1.45.
Brogio, B. do A. et al. (2018) ‘Influence of gibberellin inhibitors applied during flowering of nonirrigated “Hass” avocado trees’, Pesquisa Agropecuária Brasileira, 53(8), pp. 918–923. doi: 10.1590/s0100-204x2018000800006.
H.D. Erasmus, W. H. B. (1998) ‘Foliar Application of Uniconazole (Sunny) to Avocado Trees to Improve Fruit Size and Yield and to Change Fruit Shape’, 21, pp. 52–53.
Kohne, J. S. and Kremer-Kohne, S. (1992) ‘Yield Advantages and Control of Vegetative Growth in a High-Density Avocado Orchard Treated with Paclobutrazol’, (1990), pp. 233–235.
Kremer-Kohne, S. (1987) ‘Vegetative growth and fruit retention in avocado as affected by a new plant growth regulator ( Paclobutrazol )’, South African Avocado Grower’s Association Yearbook, 10, pp. 64–66.
Lovatt, C. J. (2005) ‘Plant Growth Regulators for Avocado Production’, California Avocado Society Yearbook, 88, pp. 81–91.
Whiley, A. W., Saranah, J. B. and Wolstenholme, B. N. (1992) ‘Effect of Paclobutrazol Bloom Sprays on Fruit Yield and Quality of cv . Hass Avocado Growing in Subtropical Climates’, Proceedings of Second World Avocado Congress.
Wolstenholme, B. N., Whiley, A. W. and Saranah, J. B. (1990) ‘Manipulating vegetative: Reproductive growth in avocado (Persea americana Mill.) with paclobutrazol foliar sprays’, Scientia Horticulturae, 41(4), pp. 315–327. doi: 10.1016/0304-4238(90)90112-R.