Battling rust diseases

Rusts are among the most destructive of plant diseases and affect a wide variety of crops and greenhouse ornamentals are no exception. The fungi that cause rust diseases are fascinating, with many requiring more than one host to survive (called alternate hosts). All produce, in succession, two or more different types of spores during the growing season.

These organisms are often difficult to detect until rust pustules become evident on infected tissues. Pustules appear on leaves, stems, needles, and fruit. Some rust fungi have a unique life history, requiring more than one host plant to grow and reproduce. These different hosts are called alternative hosts, and without them the fungus cannot survive.

An example of a “heteroecious” rust disease in a commonly grown greenhouse ornamental is Fuchsia rust. The alternative hosts of this fungus are fir (Abies) and fireweed (Epilobium) and close relatives such as fuchsia. Alternatively, other rust fungi do not require more than one host plant to survive and do not have an alternative host in their life cycle. An example of an “autoecious rust” is geranium rust, which affects zonal geraniums.

All rust fungi produce two types of spores: basidiospores, which result from genetic recombination, and teliospores, spores that support the development of basidiospores. Many rust fungi also produce additional spore types such as pycniospores, aeciospores, or urediniospores. Each of these spore types are found in a specialized pustule (fruiting structure) that develops on a given host during a certain point in the disease cycle. Rusts are obligate pathogens that cannot complete their life cycle in absence of a living host.

Two of the most troublesome rusts in greenhouse ornamentals include fuchsia rust and geranium rust.
 

Fuchsia rust (a.k.a. fir-fireweed rust)

Fuchsia rust, caused by Pucciniastrum epilobii f. sp. palustris, is a disease of Fuchsia x hybrida that occurs in greenhouses throughout the United States, most often during winter months. Symptoms of fuchsia rust include large, circular regions of chlorosis on the upper surface of older leaves. As the disease progresses, yellow-orange urediniospores first form in pustules on the corresponding lower leaf surface (Figure 1 above) and then on the upper surface as well. Spore pustules may also form along the veins. Leaves on severely affected plants turn yellow and drop prematurely. Losses due to fuchsia rust are most severe during propagation, but plants affected at any time lose vigor and aesthetic value and are not marketable.

Fuchsia rust becomes troublesome in greenhouses when aeciospores, released from infected fir (the alternate host), penetrate the leaves of fireweed and fuchsia. The fungus then produces urediniospores (known as the “repeating stage”) in the yellow-orange pustules described above. Urediniospores are easily dispersed within the greenhouse, spreading the disease as long as susceptible hosts are present. When it is time for the fungus to survive in absence of a host, the urediniospore pustules on fireweed produce thick-walled teliospores instead of urediniospores (teliospores are not known to form on fuchsia). In spring, these teliospores germinate to form basidiospores that then infect the needles of fir. Infected needles give rise to pustules containing aeciospores, which are released to infect fuchsia and fireweed, thus renewing the disease cycle.
 

Geranium rust

The disease cycle of geranium rust is less complicated. The source of inoculum for this rust disease is simple: other geraniums, especially from infected cuttings. Geranium rust, caused by Puccinia pelargonii-zonalis, causes small, pale yellow spots to form on the upper leaf surface (Figure 2). Pustules containing rust-colored urediniospores erupt in concentric rings that form a distinct “target” on the lower leaf surface (Figure 3). These spores are splashed or carried by air currents to new geraniums. Heavily infected leaves are disfigured, yellow, and drop prematurely.

Susceptibility to geranium rust varies with species and cultivar. Zonal (or common or bedding) (Pelargonium x hortorum) geraniums have many susceptible cultivars but ivy geranium (P. peltatum), regal geraniums (P. x domesticum), wild geraniums, and the scented leaf types are not known hosts of the fungus. Temperatures of 70 degrees F are optimal for disease development, and a thin film of water must be present on the leaves for infection to occur. Spores may remain viable (infective) in the greenhouse for three months or longer.
 

Managing rust diseases

To manage rust diseases, first remove sources of fungal inoculum. Keep in mind that the spores that infect your crop may be spores from other plants both inside and outside the greenhouse.

Sources of inoculum:

• Fuchsia rust: aeciospores from fir or urediniospores from fireweed or other fuchsia plants. Plants grown outdoors may be infected by spores in the air. Remove fireweed inside and outside the greenhouse.
• Geranium rust: urediniospores from other infected geraniums, especially cuttings. Carried-over stock plants may be another source of inoculum.
   

Purchase plants and cuttings that are free of disease. Consider resistant varieties where rust disease is troublesome. Carefully inspect incoming stock for symptoms; keep new plants isolated to allow time for rust pustules, if present, to develop. Scout the crop regularly for symptoms and signs of disease. Look for chlorosis on the upper leaf surface and fungal pustules on the lower surface.

Practice good sanitation: carefully remove rust-infected leaves and other debris, and discard or isolate affected plants. Place diseased plant material in plastic bags to remove them from the greenhouse and destroy the refuse by burning, burying, or rapid-composting. At the end of the production cycle, clean up debris, and thoroughly clean and surface-disinfest greenhouse benches and propagation areas using a commercially available product for this purpose. Do not carry over plants exposed to rust to the next production cycle.

Manage the moisture in the greenhouse to discourage disease, reducing leaf wetness and thus the opportunity for fungal spores to penetrate susceptible tissues. To improve air circulation and reduce humidity, space plants accordingly, use fans, and during cold periods, heat and ventilate the greenhouse at sunset to avoid condensation. Avoid wetting the foliage when watering; if overhead irrigation is necessary, water in the early morning hours to give plants a chance to dry quickly.

As part of the disease management tool kit, fungicides may be used on a preventive basis where there is history of disease. Remember that fungicides must go hand-in-hand with cultural management strategies to minimize disease development.

Compounds labeled for control of rust on fuchsia include: chlorothalonil (Exotherm Termil), fludioxonil, imazalil, myclobutanil, and tebuconazole. Compounds labeled for geranium rust include: azoxystrobin, chlorothalonil, copper sulfate (pentahydrate), fludioxonil, fluoxastrobin, flutolanil, imazalil, kresoxim-methyl, mancozeb, myclobutanil, pyraclostrobin, thiophanate-methyl, triadimefon, or trifloxystrobin. Check publications from your local extension service for compounds labeled in your state. Follow all label directions carefully when applying pesticides: the label specifies host, disease, timing, and rate of application. Be sure to rotate chemical classes or FRAC groups to avoid development of fungicide resistance, and follow cautions on some labels to avoid phytotoxicity.

Ann Brooks Gould an associate extension specialist in ornamental pathology at Rutgers University.

Peeling the Scab of Poinsettia

Poinsettia growing season is in full-swing. Make sure you’re protected from scab of poinsettia.

Scab, or spot anthracnose, of poinsettia (Euphorbia pulcherrima), is caused by the fungus Sphaceloma poinsettiae. This disease occurs on Euphorbia (cultivated and wild species) in tropical regions of the Americas and the South Pacific. Scab was detected in Florida in 1941 and limited poinsettia production there until disease-free cuttings were more freely available in the 1960s. Scab now occurs sporadically in greenhouses throughout North America. Wet conditions and poor sanitation encourage disease development.
 

Symptoms and signs
Scab of poinsettia appears as characteristic leaf spots and stem lesions. As the name suggests, lesions that occur on stems, petioles, and leaf midribs are “scabby”: corky, slightly raised, circular to elongate, and rimmed with purple or red. These lesions may coalesce and girdle the stem, causing tissue distal to lesions to die. Affected stems may also be abnormally leggy or elongate, with small leaves.

On leaves, lesions appear as tan, scabby, or puckered spots (up to 4 mm in diameter). The spots appear rimmed by a red or purple border and are accompanied by a yellow halo. Lesions may coalesce to form large areas of necrosis at leaf margins and tips and affected leaves may drop prematurely.

S. poinsettiae produces spores (conidia) in a grayish, velvety layer on the surface of stem lesions. The fungus prefers warm temperatures and wet conditions. In the greenhouse, these minute conidia are easily dispersed with moving water. Younger leaves are more susceptible than older ones, and new spores may be produced within a week after inoculation.
 

Management
Scab of poinsettia can be well-controlled in greenhouses with proper sanitation and moisture management. Use culture-indexed plants, and carefully inspect and monitor all existing and incoming stock, especially at the base of plants, for symptoms of the disease. Scout the existing crop weekly because affected plants with elongate stems are easy to spot. Since scab spreads through moving water, use cultural practices that minimize leaf wetness, splashing, and high humidity, avoiding overhead watering when possible. Keep the greenhouse and propagation areas clean, wash hands and disinfest tools immediately after handling diseased plants. Remove debris from the greenhouse because Botrytis, another disease common on poinsettias, readily colonizes plant debris and, like Sphaceloma, spreads easily through the greenhouse.

If scab is found, then immediately remove and destroy all symptomatic plants as well as the plants adjacent to them. Several fungicides may be used to protect the remainder of the crop. Those labeled for control of scab include copper-based products, mancozeb, myclobutanil, thiophanate-methyl, trifloxystrobin, and combination products that contain boscalid + pyraclostrobin or chlorothalonil + thiophanate-methyl. Include in your management plan a regular re-application schedule, especially if moisture in the greenhouse is high. Ensure thorough coverage of upper and lower leaf surfaces and stems.

Check publications from your local Extension service for compounds labeled in your state. Follow all label directions carefully when applying pesticides; the label specifies host, disease, timing, and rate of application. Be sure to rotate chemical classes or FRAC groups to avoid development of fungicide resistance, and follow cautions on some labels to avoid phytotoxicity or problems with residues.

Ann Brooks Gould is an associate extension specialist in ornamental pathology at Rutgers University.