Fire blight is an apple grower’s most worrisome threat. At its worse, this disease can result in the loss of whole trees and even orchards. But as the old saying goes, forewarned is forearmed. David Booty, technical development manager with OMEX® Agrifluids, explains.
It’s somewhat ironic that the process of pollination, on which apple growers depend for their livelihoods, can also deliver such a potentially devastating disease. But Erwinia amylovora, the bacteria that causes fire blight, hitches its ride on pollinating insects. The honey-bee’s industrious efforts, collecting pollen and its nectar reward, are one of the primary ways in which the bacteria — and the disease it causes — moves through an orchard.
Few growers will be unaware of fire blight’s threat. Native to North America, we now know far more about the disease than when it was first spotted in New York’s Hudson Valley more than 200 years ago. That allows us to exert greater control over its effects, but in many ways it remains a mystery. The size and severity of outbreaks, whether in orchards that have previously suffered or in ‘virgin’ orchards, often follow no discernible pattern, although in recent years we have begun to understand some of the events that favor its development.
How can we better manage this disease? Let’s look at the basics. First, remember that any outbreak of a plant disease isn’t an isolated event. The interaction of three factors should be considered: the host plant, the pathogen and the environment.
For the disease to take hold, it must find its ‘sweet spot’. The host must be in a susceptible condition; the pathogen must be present in sufficient quantities; and environmental conditions (temperature, humidity, wind speed, rainfall, etc.) suitable.
Our approach to controlling any disease, from the COVID-19 pandemic through plant or animal disease, is to upset this sweet spot by disrupting the equilibrium between host, pathogen and environment by artificially adjusting one or more.
So it is with fire blight. The disease first appears in spring as environmental changes create the sweet spot: the temperature rises above 65 degrees Fahrenheit and moisture from rain, dew and humidity favor infection. Bacteria overwinter in cankers on the tree and while relatively few cankers become active, just a single canker can produce millions of bacteria that transfer to open flowers by insects, rain or even cultural practices such as pruning.
From the flower, the bacteria enter the tree’s vascular system, prompting death of flowers, leaves and fruit and — worst case — the entire tree, especially if it reaches the rootstock.
We can’t change the weather. But we can reduce the host’s susceptibility and control the pathogen.
There’s only really one means to control the pathogen: antibiotics. Streptomycin-based products are available and licensed for Erwinia control, but they should be treated as a last resort. There is growing concern around the use of antibiotics outside human health, because of the very real risk that overuse favors development of deadly, resistant bacteria such as MRSA.
Thankfully, several cultural practices can help reduce both the incidence of the pathogen and the susceptibility of the tree. Variety choice is important: some (such as Red Delicious or Jonafree) are more resistant, while certain rootstocks (M.26 or M.9) are known to be less resistant. Meanwhile, dormant-season pruning — and proper disposal through burning — of obviously blighted twigs and cankers means there’ll be no caches of bacteria to trigger further infection. It’s also important to be careful when pruning infected shoots during summer and follow best practice: only prune during dry weather, only cut into disease-free wood, and sanitize the pruning tool between every cut.
Avoid excessive applications of nitrogen. This promotes growth of soft, succulent tissue, very susceptible to Erwinia infection.
Copper-based fungicides have displayed some success in controlling outbreaks. But there’s growing concern about the application rates of such products: copper is relatively immobile, yet phytotoxic in excess. It’s also known to cause russeting, and residues on finished fruit are another cause for concern.
However, copper is an effective bactericide and it’s also implicated in plants’ immune systems. At OMEX®, we looked for a way to deliver copper in a more mobile form, permitting lower application rates, and identified organic acids for further research. Commonly found in healthy soils with good organic content, these ‘complex’ the metal ions and increase their bioavailability to plants.
In trials, an application of Cell Power® Zynergy™ — a fertilizer coformulation of copper and zinc with organic acids — delivered an 84.45 per cent control of fire blight in apple blossoms, comparing extremely favorably with the control of 84.85 per cent provided by the default streptomycin treatment.
For best results in the field, this translates into making a first application of Cell Power® Zynergy™ at bud formation, ahead of infection, and then repeated applications during the flowering period.
With the potential consequences of failure to control fire blight so severe, we believe it’s a risk few growers should take. Prevention is always better than cure; Cell Power® Zynergy™, with its benefit to the plant even in the absence of disease, should be something to consider and evaluate for every apple grower this season.
Learn more at www.OMEXusa.com.
The product names and brands referenced here are registered and trademarks of OMEX® Agrifluids, Inc.
© OMEX® Agrifluids, Inc. 2021.