U.S. researchers have found a treatment that reduces the negative effects of alternate bearing on Hass avocades while increasing yields in on-crop years.

University of California Riverside (UCR) plant physiologist Carol Lovatt, told www.freshfruitportal.com the treatment was not yet commercially available, but avocado-producing countries have formed a consortium to send out information to growers and conduct tests.

Californian Avocado Commission

"The goal of that consortium is to get this information to all the avocado producing countries and to test what’s true about alternate bearing in California. Is it true in Chile? Is it true in New Zealand, or because of the difference of the climate and the phenology of the tree, is there something else going on?" she said.

"The treatment is not commercially available, at least it’s not commercially available in California because of the strict regulations we have for using plant growth regulators, but there is a group that is working with this technology to get it to the growers.

"To speed up the process of getting it to the growers we’ve also been testing naturally occurring cytokinins and naturally occurring auxin-transport inhibitors, and we have a couple of really good candidates we are now focusing on."

Test results

Speaking from the Global Avocado Congress in Australia last week, Lovatt said her research team were surprised to find the trunk injections improved yields during the crop year, with uncertainty over what that would mean for the off-crop this August.

"We were concerned because that treatment also increases yields in the on-crop year, and so we thought we might be creating a situation where we have a lot of very small fruit," she said

"But when you look at the yield of the fruit that are of a commercially valuable size in California, that treatment actually increases the yield of commercially valuable size fruit for the two year cumulative yield - some of it’s in the on-crop year and the bulk of it’s in the off-crop year.

"We got the results for the on-crop because that’s the year we applied the treatments, and we thought  ‘oh no, this is going to be bad’, but it turned out to be very good."

She said one of the key drivers of the treatment's success is it's capacity to boost floral shoot numbers, particularly with what are known as 'determinant' floral shoots and those that are borne out of summer vegetative shoots.

The reason for this is that in off-crop years there is a greater absence of determinant shoots, so solving this problem helps mitigates the 'strongest negative effect' of alternate bearing fruit in an orchard.

While the tests to date have involved trunk injections, Lovatt is open to the idea of other treatment methods and forms of application further afield.

"For the research we’ve done so far we’ve only done trunk injection to test our hypothesis, because we knew we’d get a true result. It wouldn’t run the risk of being compromised and not work just because the hormones didn’t get into the tree.

Carol Lovatt

"Now we have to scale this up to work as a fuller application and also so that for the California avocado industry, because of our steep hills, we’re testing the possibility that the treatment could be applied through irrigation.

"A lot of growers in California are ready to do trunk injections because they’re used to injecting phosphorous acid, and in fact somebody asked me could they be mixed, and in fact as long as you keep the pH OK there’s no reason why they couldn’t."

In terms of Chile's issues surrounding pollination on its avocado farms, Lovatt said the Californian experiment showed the results were the same for farms with and without bees.

"In California not all growers bring in bee hives, so we did some of our research in an orchard that has bee hives, and in orchards that don’t bring in bees, and we’re getting very similar results.

"You can make it worse or less worse by having the bees in the orchard, but we need to do this overriding treatment to mitigate the alternate bearing."

Fertilizer awareness

While the treatment methods under testing in California could radically change the global industry if they become widespread, Lovatt emphasized that basic fertilization principles still need to be upheld to keep avocado farms in good shape.

"If the grower doesn’t supply adequate fertilizer to support the crops that he’s producing each year, then he will eventually run into a problem.

"This is one of the things that we’ve been trying to do in California is to educate the grower to fertilize to match the demand of his trees, not to just say, ‘oh, I have an off-crop, I’m not going to fertilizer’.

"You still need to support mature fruit that are on the tree, and you have to also have enough to support vegetative shoot growth and the flowering progress."

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