Too much flower inducer spraying bad for mangoes

BATAC, Ilocos Norte – Fruit experts warn that the continued spraying of mango trees with overdoes rates of potassium nitrate to force them to flower may lead to the collapse of the Philippine mango industry.

Dr. Ramon Barba, the scientist who discovered potassium nitrate as a mango flower inducer, and Rolando G. Bugayong, former president of the Philippine Fruit Association (PFA), made this warning during the 9th Philippine fruits symposium at the Mariano Marcos State University here.

Bugayong said many mango farmers now force their trees to flower and bear fruits three times a year through the use of potassium nitrate. These farmers would even spray the chemical before the fruits are harvested. Thus, the trees would be flowering again right after harvesting.

In other instances, farmers would make a second application of potassium nitrate if trees sprayed with the chemical do not flower. They would even double or triple the rate of application. The normal recommendation is one percent potassium nitrate solution.

Barba pointed out that if the tree does not flower after potassium nitrate application, it means that it is not physiologically ready to flower. A mature shoot is physiologically ready to flower eight to 10 months after flushing or the appearance of new shoots and leaves.

Still, other farmers force their trees to flower during the off-season. But Barba stressed that it is difficult to induce the trees to flower during the rainy season because they normally respond to potassium nitrate spray during the rainy season because they normally respond to potassium nitrate spray during the dry season.

What’s worse, Barba and Bugayong said, many farmers would even spray as much as 10-percent potassium nitrate to make sure that their trees flower. This form of abuse produces flowers even on the steams and trunks of the trees, but the flowers would not produce fruit sets.

"These farmers are trying to get more than what is expected from the trees," Bugayong said.

With the indiscriminate application of potassium nitrate, the yield of mango trees has been continuously declining, the two experts bewailed. They said the average yield now has gone down to only 200 fruits per tree.

According to Bugayong, the production of a plantation in Bataan with 25,000 mature trees that are continuously sprayed with potassium nitrate is only equivalent to the yield of 5,000 trees given the proper cultural management.

Research results of Barba showed earlier that new shoot production and leaf sizes were reduced significantly with the annual application of the one percent potassium nitrate solution for five years. Trees sprayed with the chemical produced more fruits than unsprayed trees, but the average size of the fruits became smaller.

In the 7th year, all the trees in the study were induced to flower with potassium nitrate. The previously unsprayed trees produced 50 to 60 percent more fruits than those previously sprayed with the chemical for five years.

The experts advise mango growers not to deviate too far from the normal cropping cycle of the trees. The vegetative stage of mango trees starts from flushing (new leaves) until flower induction, which is about six to 10 months or an average of eight months. The reproductive state, about four months, is from flower emergence to harvest. Thus, mango trees can be induced to produce fruits only once a year if these flush immediately after harvest.

Since the normal cropping cycle for mango is 16 to 18 months, farmers would do well to have two crops in three years to maintain the productivity of their trees.

Bugayong added that the new leaves must be allowed to mature in six to 10 months before flower induction so that flower buds would develop and carbohydrates (food) accumulate through photosynthesis.

Moreover, farmers must use one percent potassium nitrate solution during the regular season and two to 2 .5 percent during off-season.