Helping Citrus Trees Resist Postbloom Fruit Drop

With fungicides on the wane, look to the plants’ own defenses for control.

Citrus groves have been so devastated by Huanglongbing (HLB), it’s easy to forget that there are other diseases to manage. Postbloom fruit drop (PFD) is one of them. With more frequent rains during the winter months, conditions are ideal for Colletotrichum acutatum, the fungus that causes PFD.

postbloom fruit drop

The “buttons” left when the fruitlets drop prematurely are a sure sign of postbloom fruit drop infection.

Because PFD can cause 20% to 80% fruit drop, depending on its prevalence in a given season, it’s not to be taken lightly. Spores survive between bloom periods on the leaves, twigs and buttons (left over calyces from infected flowers). When flowers open, the spores move onto the petals when it rains, infect them and produce more spores. With each rainfall, the spores continue to spread and infect new flowers. The more often it rains, the more the spores are spread. When the infected flower begins to form fruit, the fungus causes it to drop off almost immediately, leaving just the buttons. Once the flower is infected, there is no way to save the fruit.

The disease is even more of a problem on citrus cultivars that have repeat bloom or longer bloom periods, such as navel oranges. The longer flowers are on the tree, the more chance the fungus has to multiply and spread. Without preventive measures, the disease can quickly spread out of control in a rainy season and cause major crop loss.


Fewer Tools for Control

Fungicides are fairly effective if applied very early in the bloom period and several times thereafter if rainy conditions prevail during flowering. However, fungicides can have a detrimental effect on pollinators if applied during flowering – exactly when PFD control needs to happen. In addition, consumers are demanding fewer chemicals on their produce, government regulations are limiting or removing some fungicides from the market, and the disease itself is developing resistance to some of the more commonly used products.

“There are fewer fungicides available today, and some of them are being overused, which leads to development of resistance,” says Gerald O’Connor, CEO of Keyplex. “Growers are also hitting the upper limits of what they can spray due to maximum residue levels, and regulating organizations are looking for softer chemistries. The ability of Keyplex nutritional products to stimulate the tree’s natural defense systems makes it an effective tool in managing PFD.”


Triggering the Plant’s Own Immune System

It has long been established that when a plant is attacked by a pathogen or pest, it is triggered to form specific defensive proteins that help to protect the rest of the plant. Known as systemic acquired resistance, or SAR, the process is triggered within just a few hours of the injury by the invading organism. It was trickier to figure out the exact mechanism and then how to use that knowledge for crop protection.

In 1994 Keyplex partnered with USDA researchers under a cooperative research and development agreement (CRADA) to understand this process in citrus trees.

“Keyplex nutritionals, while they are first and foremost, foliar fertilizers, contain elicitors of the SAR response,” O’Connor says. “Our research with the USDA CRADA project showed that Keyplex 350 is able to trigger this response, causing the plants to produce the defensive proteins. Incidence of PFD dropped by 48% compared to the untreated control. Keyplex 350 OR, which is formulated for organic growers, had the same result. A new product under development, the 1000 series, is showing even better results, reducing disease incidence by 70%.”

postbloom fruit dropTiming is Key to Prevention

For control of PFD,  Keyplex products should be applied right at bloom time and 3 to 5 more times throughout the spring months. Regular applications will build up the plant’s immune system and reduce damage from the fungus. At the same time, the plant’s nutritional needs will be met, as well.

“The highest time of stress in the plant is when the fruit is developing,” O’Connor says. “We’re able to get the maximum amount of calcium, boron, magnesium, manganese, iron, , molybdenum, and zinc – all the minors – into the plant. Because of the glucoheptonate and humic acid we use, we’re able to generate more efficacy and more absorption of our product into the plant. Glucoheptonate is an excellent chelating agent, and grabs and pulls the metal ions into the plants. Additionally, if you do use fungicides in your tank mix with Keyplex, it enhances the  uptake.”

Keyplex products are pollinator friendly with a short, four-hour worker re-entry interval (REI).