Methods and Techniques for Managing Rust on the Farm
My my hey hey, Blueberry rust is here to stay, Is it better to burn it out, or let it fade away, my my hey hey…
Ok, that’s not how the song went was it? “Rust never sleeps” is also a phrase familiar to the vintage auto enthusiasts among us. Rust can rot out fenders and frame rails of cherished cars from the inside-out. I won’t name any marques known for this… but if you own one, you know who you are, and you worry about what part of your car is rusting away at any given time! Blueberry growers in Florida know that this phrase also seems to apply to southern highbush blueberries lately. Central Florida growers (I-4 corridor) and those attempting evergreen production have had the most issues with rust as far as I can tell, and this year— out of the blue— we saw more of the disease on fruit than in recent years prior. Of course, iron oxide rust has nothing to do with the rust disease on blueberry other than being called the same “four-letter” word.
The disease we’re talking about is called leaf rust on blueberry and is caused in Florida by the fungus Pucciniastrum vaccinii. The name is somewhat misleading because symptoms and fungal fruiting bodies do occasionally occur on fruit and stems as we saw this past season, but leaves are where the most common and obvious symptoms occur. The disease causes small somewhat round lesions visible on the upper surface of leaves (Fig. 1). Multiple black to red lesions can occur on the same leaf prior to the whole leaf turning yellow, red, and falling off the plant (Fig. 2). Leaf rust gets its name from the color of spores that the pathogen produces on the underside of the leaves, below those black lesions (Fig. 3). Different parts of the plant are more and less suitable for growth of the pathogen. Leaves are the best food for rust fungus, but when spores fall on blossoms, green fruit, or young tender stems, successful infection can sometimes occur. On fruit, small blister-like raised lesions can occur, usually near the calyx end and on the calyx itself (Fig. 4). Bright orange spores confirming that the disease is leaf rust on fruit may form on the calyx if fruit are incubated in seal-top bags with moist paper towel (Fig. 5). In other cases, the pathogen will cause a small lesion that never results in additional sporulation.
The spores are microscopic individually, but when hundreds to thousands of them are produced on an infected leaf, they take on a bright orange color that will wipe or wash off the leaf easily. When leaves dry, spores become powdery and can be lifted into the air, floating on a breeze short to long distances before settling out, or being washed out by rain or irrigation. When those spores happen to land on blueberry leaves, or in some cases fruit, you now have the potential for a new infection to occur. That infection won’t happen if there is a fungicide residue on or in that plant to stop the spore from penetrating, or if the environment is too dry to allow the spore to absorb moisture and germinate. Humid, wet conditions for extended periods of time give the fungus a chance to grow, form structures, and get into the leaf before drying up and dying.
Once in the plant, moisture, carbohydrates and everything the fungus needs is present. After a few days of growing in the leaf, the plant cells die as the fungus is making the fruiting structures necessary to produce hundreds to thousands of new spores that will continue the disease cycle. While in the plant, moisture is not usually limiting, so disease continues to develop, even during dry weather; however, new infections require another cycle of humid, wet conditions, just like the first. Systemic fungicides have the potential to move into infected plant parts and stop the fungus development. Most of these products will reduce the amount of sporulation that results, or may delay that sporulation, but usually cannot kill the fungus completely inside the leaf. Fungicides do a much better job protecting against new infection compared to stopping those that have already occurred, so making repeat applications and maintaining a protective residue on the leaves and fruit, are key to preventing leaf rust.
But we already established that rust never sleeps, and it isn’t economically or environmentally responsible to keep fungicide on the plants 24/7/365—so when is it important to manage rust with fungicides, and what else can we do to besides spray?
Okay, we know that some cultivars of blueberry tend to get more rust than others. ‘Jewel’ is a good example of a really good cultivar that is susceptible to leaf rust. Breeding for resistance is an attractive long-term option that is being pursued. Cultivar selection for new plantings should take rust susceptibility (if known) into account where rust is likely to be an issue. Short-term options include reducing the “carryover” of fungus on infected leaves over the winter, or where possible, modifying the environment to limit the amount and duration of moisture on leaves and fruit. Drip irrigation can help reduce the amount of time leaves are wet compared to overhead irrigation and will help reduce several foliar disease threats. Timing overhead irrigation to avoid applications that will result in extended periods of leaf wetness also may help reduce our overall disease pressure. An example would be to time irrigation for after dew formation rather than early evening. This way, the water is being applied when leaves are wet from dew anyway, as opposed to wetting leaves earlier in the evening when more continuous hours of leaf wetness are likely to result.
So when should fungicides be used for rust management? Research done by Dr. Williamson here at UF and at other institutions has shown that defoliation prior to flower bud initiation can reduce the number of flower buds and the next season’s yield. Flower buds are initiated sometime between late August and late October for most cultivars. Managing rust to prevent defoliation through late October on susceptible cultivars makes good sense. Post-harvest applications of products not used-up to protect berries pre-harvest is a good strategy. Bravo WeatherStik applied at the label rate of three pints per acre allows for four applications to blueberry after harvest with a reapplication interval not before 10 days. Applications to ‘Jewel’ and other susceptible cultivars should begin when disease occurs on the new flush after summer pruning and continue until disease subsides or mid-October. Systemic fungicides in the products Orbit and Indar offer rotation partners for Bravo WeatherStik. Orbit and Indar both have 30 day pre-harvest intervals, so using these apps for rust control post-harvest would make sense. Abound is in another class of fungicide with rust activity as well. Any allowable applications not used for fruit rot control, would also serve as rotation options for leaf rust management post-harvest.
The time-frame when fungicides can help to specifically reduce fruit infection by rust fungus is bloom through harvest. These applications will be important in evergreen production or where large amounts of rust fungus have survived through the winter period, despite hydrogen cyanamide application. In these cases, rust should be added to the list of berry rots to treat for preventatively. Products used for ripe rot, alternaria rot, and gray mold that include Abound, Cabrio, Captan, Pristine, Switch, and Quash will also have some rust efficacy. With the exception of Quash (seven-day pre-harvest interval), those products have zero-day pre-harvest intervals. Elevate use for blossom blight and gray mold control would not have rust efficacy, so applying with Captan, or applying Captevate as an alternative would be recommended where rust is a concern. Where Captan is prohibited by your marketer, then an early application of Omega (30 day pre-harvest interval), Ferbam (40 day PHI) or Ziram (apply at loose bud scale) may help reduce pre-bloom disease severity if allowed, but I don’t know of any research that has tested this.
A few folks have asked about the use of products containing Copper, elemental sulfur, lime sulfur, phosphorous acid, and/or oil sprays as dormant or semi-dormant applications. I don’t have enough data to be able to recommend any of these for this use, and feel that the potential for crop damage, especially where hydrogen cyanamide also is applied, is too great given we have other safer and effective alternatives. Any growers thinking about or currently using these should use caution!
Use pesticides safely; always read and follow manufacturer’s labels carefully. Trade names were used in the interest of providing specific information but does not imply guarantee or warranty, and does not signify any approval or endorsement to the exclusion of other similar products.
by PHILIP HARMON, Associate Professor and Extension Specialist, UF/IFAS Department of Plant Pathology