Detail of chrysanthemum white rust.
Of all the greenhouse maladies that plague growers of floral crops, perhaps fungal rust is the most insidious of all.
While aphids and whitefly eggs are difficult to spot on plants, fungus spores are impossible to see — at least with the naked eye. Compounding the problem is the uncanny ability that fungus pathogens possess to spread between greenhouses and travel on propagation material, including material entering the United States.
Rusts are varied and host-specific, so we at least know which plants are susceptible to different types of rust. These include aster, carnation, fuchsia, florist’s geranium (Pelargonium X hortorum), gladiolus, lilium, marigold, poinsettia, snapdragon, statice, rose (Rosa species), chrysanthemum and viola (including pansies).
The first symptom that will actually be visible is often a chlorosis on the upper surface of the leaf. All infected plants produce powdery masses of postules (typically on leaf undersides) that are orange, yellow, purple, brown and black. Further penetration of spores will result in lesions coalescing into large areas of necrosis. Leaf distortion and defoliation will often follow.
Managing rust diseases
Here are some steps you can take to manage rust diseases:
One of the best defenses against rust is choosing rust-resistant plants, according to James Buck, associate professor at the University of Georgia. However, he says, choosing disease-resistant plants is often in conflict with the desire to grow varieties that are popular with consumers.
There are several effective fungicides available to growers, including new triazole fungicides that are being screened for efficacy, says Cristi Palmer, ornamental horticulture manager at Rutgers University.
Palmer suggests that growers check to make sure that the targeted rusts are on the label and that these new products are permissible on their particular crops. It’s also important to remember that not all products are registered in all states.
“Unless a crop is listed specifically on a label or [growers] have previous experience with applying it on that crop, a small test on a few flats or pots with a waiting period before applying the product on the whole crop is a wise idea,” says Palmer.
Here is a list of rust diseases that affect specific crops and what to look for:
Aster rust. There are several rust diseases affecting asters, including Coleosporium campanulae, Puccinia asteris, P. campanulae, and Puccinia species. Look for orange-red postules on the leaf undersides. Heavy infections can cause yellowing and necrosis.
Carnation rust. Look for small pustules of powdery, brown urediospores caused by Uromyces diantha. Resistant varieties are available.
Chrysanthemum rust. Less common in greenhouse plants is chrysanthemum rust. Two species of Puccinia may appear on non-resistant varieties in the field later in the summer. Watch for dirty brown pustules and yellowish-green spots on upper leaf surfaces.
P. horiana is responsible for chrysanthemum white rust, a disease that not only afflicts greenhouse plants but is subject to federal quarantine and eradication. This dreaded disease will decimate a crop of chrysanthemum, as the leaves become distorted and discolored and the plant eventually dies. Contact state and federal agricultural officials if you suspect your crop has been infected with this fungal disease.
Fuchsia rust. Fuchsia rust (Pucciastrum epilobi) will normally be noticed on incoming plant propagation material and can be avoided by purchasing propagation material from a reputable grower.
Geranium rust. Caused by Pucinia pelargoni-zonalis, geranium rust is most common on florist’s geranium (Pelargonium X hortuorum), though it has been reported on zonal and seed geraniums as well. Geranium rust is widespread throughout the United States due to its ability to spread on infected cuttings. Unfortunately, symptoms may not be observed until after the crop has been in the greenhouse, says Buck.
Nevertheless, watch for small, circular, yellow spots on the top of leaves opposite the postules on the lower leaf surface, which will eventually enlarge to blister-like postules of rust to cinnamon brown spores that often develop in concentric rings.
Poinsettia rust. Growers of this holiday favorite should purchase stock from a reputable supplier while keeping an eye out for the unsavory Uromyces euphorbiae, which will manifest itself as a cinnamon brown spot on the top and bottom of the leaves.
Rose (Rosa species) rust. Roses are plagued by different rust diseases, though disease-resistant varieties are available. Vigilant growers often provide preventive treatments to roses that they know are susceptible to particular rust diseases. Should a rust disease establish itself in the greenhouse, remove infected material immediately and burn it.
Viola species rust. More common in the northeast United States is Puccinia viola. The symptoms first appear as small, pale green spots on the upper leaf surface. As the fungus develops, expect to see corky spots, blisters or pustules containing rusty brown spores that develop on leaf undersides.
Neil Moran is a horticulturist and regular contributor to Greenhouse Management, a sister publication. Visit his website at www.neilmoran.com.