Identification and Management of Diseases of Perennials in the Landscape

PP061 (3/03R)

By Dr. Sharon M. Douglas
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106

Telephone: (203) 974-8601 Fax: (203) 974-8502
Email: Sharon.Douglas@po.state.ct.us

Use of perennials in the landscape continues to increase dramatically, particularly here in the Northeast. The demand for perennials has been estimated to be growing at a rate of 8-15% per year, especially as long as producers continue to make new plants available. As a group, perennials are fortunately relatively problem-free in the landscape. In part, this is attributed to the diversity of plant species in perennial gardens that helps to minimize chances for widespread disease outbreaks. However, we still need to be able to accurately identify disease problems when they do occur. Some diseases tend to be fairly host-specific and are limited to plants within a genus or family, whereas other diseases can be widespread and have broad ranges of hosts. It is also important to recognize which diseases are aesthetic rather than life-threatening so appropriate control strategies can be implemented.

The focus of this fact sheet is recognition and management of diseases of perennials in the landscape. Since it is not feasible to know all of the specific diseases on all perennials (a daunting task considering the number of plants and diseases out in the landscape) learning to recognize and manage key diseases will allow you to make informed decisions for effective control of most problems that are encountered. Before discussing the key diseases, it is important to consider disease prevention, a fundamental component of management and one of the most effective ways to deal with diseases in the landscape.

DISEASE PREVENTION

By following a few basic measures, many diseases of perennials can be avoided or minimized in the landscape.

1. Planting Practices
Spacing: use the correct spacing since crowded plants do not do well and too-close spacing can promote disease by inhibiting drying and air circulation.
Planting Depth: plant at the depth recommended for each species; this is especially important for winter-hardiness and, in some cases, for flowering.

2. Cultural Care
Fertilizing: appropriate applications to maximize plant growth and vigor and to avoid plant stress due to deficiencies or toxicities.
Mulching: properly applied mulch helps with weed control, soil temperature moderation, and soil moisture retention; summer mulches should not be applied too thick or too close to the stems or crowns of plants.
Watering: maintain adequate soil moisture for the plant species; this usually translates to approximately one inch of water per week; in the absence of natural rainfall, irrigation should be used and, depending on soil type, this is best delivered as a deep soaking; avoid overhead irrigation or water plants early in the day to allow foliage to dry before nighttime.
Winter Protection: winter mulches can be effective to protect plants from heaving during freeze-thaw cycles; mulches should be applied AFTER the ground has frozen and removed BEFORE or WHEN new growth starts in the spring.

3. Plant Selection
Hardiness: often an overlooked aspect of disease prevention; most of Connecticut is in USDA Zone 6 (some Zone 5); usually not a problem with most perennials, but a factor for consideration when trying new species.
Plant Requirements vs. Site Characteristics: match these as closely as possible; special attention to soil type and pH, drainage, and light level.
Genetic Resistance: if resistant or tolerant cultivars or species are available, they should be selected for use.
Plant Health: purchase healthy, vigorous plants and carefully inspect the root system before planting.
4. Sanitation
Remove Infected Plants and Debris: symptomatic plants or infected plant parts should be removed promptly to minimize disease spread; after tops are killed by frost in autumn, all plant debris should be removed to reduce or minimize overwintering inoculum.
Groom Plants: remove spent flowers and leaf debris during the growing season to minimize inoculum buildup and spread.

KEY DISEASES

Key diseases are diseases that are problems on many types of plants each year. These are best categorized by the part of the plant that is affected. This section provides a brief description of the symptoms, causal agents, and additional strategies for disease management and control that are necessary when preventative methods are not effective. Examples of available pesticides are given for each key disease although the lists are not all-inclusive. Pesticide regulations vary from state to state and many pesticides are not labeled for use on perennials or may only be labeled for use on specific plants. Although many companies have recently broadened their fungicide labels for use on perennials, care should be taken when using a product for the first time on a particular species to avoid phytotoxicity problems. Additionally, it is also important to carefully read the pesticide label since some compounds may have labels for greenhouse and/or nursery use but not landscape use.

A. FOLIAR DISEASES:

1. Leaf Spots
Leaf spots are the most prevalent of all plant diseases. They appear as dead areas scattered over the leaf surface and often have defined margins. Since the fungi and bacteria that cause these diseases tend to be fairly host-specific, widespread outbreaks on different species of plants usually do not occur. Most fungi associated with leaf-spot diseases require free water on leaf surfaces in order to infect so these diseases are often most serious during wet, humid weather.

Fungicide applications can supplement preventative control measures. Among the compounds registered for use are chlorothalonil, thiophanate methyl, copper compounds, and mancozeb. Most of these fungicides are protectants and must be applied to developing foliage before symptoms appear. The effectiveness and the number of sprays required for control will vary with weather conditions.

Key Hosts: Aster, Phlox, Veronica, Coreopsis, Rudbeckia, Heuchera, Hosta, Iris, Hemerocallis

Key Pathogens: Septoria, Cercospora, Didymellina, Phyllosticta, Pseudomonas, Xanthomonas

2. Blights
Blights are characterized by sudden and conspicuous leaf and growing tip symptoms that are more severe than leaf spots since entire leaves, stems, and flowers may be killed. Infected tissues may also appear blackened or wilted. Blights are often more severe during wet weather or on plants that have been stressed by other factors. Although a wide range of fungi and bacteria are known to cause blights, one of the most notorious and ubiquitous is Botrytis. Botrytis cinerea and several other related species commonly infect a very broad range of perennials.

Fungicides are helpful in some host-pathogen combinations, but many are protectants and therefore need to be applied before symptoms appear. The effectiveness and number of sprays required for control will vary with weather conditions. Among the compounds registered for use are chlorothalonil, copper compounds, mancozeb, thiophanate methyl, and iprodione.

Key Hosts: Paeonia, Geranium, Dahlia, Iris, Rudbeckia, Saliva

Key Pathogens: Alternaria, Botrytis, Rhizoctonia, Erwinia, Xanthomonas

3. Rusts
Early symptoms of rust infections appear as pale spots on the upper surface of the leaf. More diagnostic symptoms are present on the undersurface of the leaf as blisters or pustules that break open to reveal the rusty-orange to brown spores that give these diseases their names. Rusts are specialized fungi that are host-specific and do not casually spread from species to species in the landscape. They typically infect only one or several closely related hosts. However, there are also rusts that require more than one type of host in order to complete their life cycles.

Fungicide applications can supplement preventative measures for control. Among the compounds registered are chlorothalonil, mancozeb, propiconazole, and triadimefon.

Key Hosts: Alcea, Potentilla, Coreopsis, Aconitum, Delphinium, Iris

Key Pathogens: Puccinia, Coleosporium, Phragmidium, Uromyces

4. Powdery Mildews
By far one of the most common diseases of perennials in the landscape, powdery mildews are recognized by their distinctive symptoms, which are similar on essentially all hosts. Symptoms first appear as chlorotic or pale green, irregular patches on the upper leaf surface that gradually develop into diagnostic white to gray, powdery patches. Symptoms are usually most evident on the upper surface of the leaf and often first appear on the lower foliage where humidity levels stay higher for longer periods of time. In some cases, severely infected leaves quickly turn brown and drop. Although the disease looks the same on every host, in reality the fungi are fairly host-specific. For example, the powdery mildew fungus that infects aster is not able to infect lupine and vice versa.

While preventative measures often keep these diseases at manageable levels in the landscape, fungicides can be important for control in areas of high visibility. However, in most cases they need to be applied when disease is first detected since powdery mildew is difficult to effectively control once it is severe or has reached epidemic status. Among the compounds available are horticultural oils, neem oil, mancozeb, propiconazole, triadimefon, and sulfur.

Key Hosts: Aster, Coreopsis, Dahlia, Monarda, Phlox, Delphinium, Rudbeckia, Lupinus, Veronica

Key Pathogens: Erysiphe, Sphaerotheca, Uncinula, Phyllactinia

5. Downy Mildews
Downy mildews can be troublesome in the landscape and are often misdiagnosed or overlooked because of the symptoms they produce. Pale green or yellow areas appear on the upper surface of the leaf during the early stages of infection. Diagnostic symptoms gradually develop on the undersurface as the fungus grows out of the infected leaf and appears as a fuzzy, white to gray-purple growth. These symptoms often go unnoticed until the infection is severe and heavily infected leaves turn brown and shrivel, often in a very short period of time.

Fungicides can help to keep the disease in check as long as severely infected plants are removed from the planting bed. Among those available are fosetyl-Al, mancozeb, and mancozeb plus thiophanate methyl.

Key Hosts: Aster, Veronica, Artemisia, Geum, Coreopsis, Potentilla, Rudbeckia

Key Pathogens: Peronospora, Plasmopara

B. ROOT AND STEM DISEASES:

1. Root, Stem, and Crown Rots
These rots often produce fairly nonspecific aboveground symptoms and plants turn yellow, wilt, and droop. Infected plants can also appear stunted and have poor vigor. In some cases, stems and roots are discolored, soft, and mushy. In others, sunken, discolored areas or cankers develop along the stem or on individual shoots that can result in dieback. In extreme cases, entire plants die. Plants that are under stress (especially in sites with poor drainage) are most susceptible. A number of fungi and bacteria are associated with root and crown rots. The majority of these pathogens are soil contaminants that are capable of infecting an extremely broad range of plants.

Fungicides can be effective in supplementing preventative strategies. However, accurate diagnosis of the pathogen is necessary in order to select the appropriate fungicide since they vary in their efficacy. Among those available are etridiazole, etridiazole plus thiophanate methyl, fosetyl-Al, iprodione, mefenoxam, metalaxyl, and PCNB.

Key Hosts: most perennials (Chrysanthemum, Dianthus, Papaver)

Key Pathogens: Pythium, Phytophthora, Rhizoctonia, Sclerotinia, Erwinia

2. Vascular Wilts
Symptoms of vascular wilts include loss of rigidity, wilting, yellowing of foliage, drooping, gradual dieback and, in extreme cases, plant death. In many cases, the symptoms are very similar to those caused by root rots. All of these symptoms indicate water stress since the fungi and bacteria that cause these diseases systemically invade the plant’s vascular system (most commonly the xylem, but occasionally the phloem). A diagnostic symptom can sometimes be seen if the stem is cut to reveal a discoloration or browning of the vascular system. Since the infection is systemic and the entire plant is involved, little can be done once a plant is infected. Several fungi (and a bacterium in warmer areas) are responsible for vascular wilts. They are soilborne and have extremely broad host ranges.

Fungicides are not usually effective for control. Use of resistant species or varieties and rotation are often the only effective means for control.

Key Hosts: Aster, Coreopsis, Chrysanthemum, Dahlia, Dicentra, Callistephus, Dianthus, Phlox, Paeonia

Key Pathogens: Fusarium, Verticillium (southern states: Pseudomonas)

C. MISCELLANEOUS DISEASES:

1. Viral Diseases
Although symptoms of virus infection are often distinctly different from those caused by other types of pathogens, they can also be very difficult to diagnose or detect. Among the distinctive symptoms are mosaics, mottles, ring spots, and distorted growth habits. These can develop on leaves or shoots. More subtle symptoms involve the entire plant and include stunting and poor vigor. Virus infections are often systemic in the plant and, once infected, little can be done. Most viruses that are encountered in the landscape have very broad host ranges, although there are some that can be fairly host-specific. Viruses are most often spread through vegetative propagation or by insect vectors (including whiteflies, aphids, thrips, and leafhoppers).

Pesticides are ineffective for control once infection has occurred. However, insecticides aimed at the vectors can reduce the spread of the disease in the landscape. Symptomatic plants should be promptly removed to eliminate sources of the pathogen.

Key Hosts: Aquilegia, Aster, Delphinium, Gaillardia, Dicentra, Dahlia, Paeonia

Key Pathogens: Cucumber mosaic virus, Impatiens necrotic spot virus, Peony ringspot

2. Phytoplasmal Diseases
Symptoms of phytoplasmal infection can be quite variable and include yellowing of foliage, greening of flowers (called virescence), distortion of growth, witches’-brooming, and stunting. Aster yellows is by far the most common and widespread disease caused by this category of pathogens. Phytoplasmas are typically spread by vegetative propagation or by insect vectors (most often leafhoppers). As with viral infections, phytoplasmal infections are usually systemic in the host and cannot be eliminated once infection has occurred.

Pesticides are ineffective for control once infection has occurred. However, insecticides aimed at insect vectors can reduce spread of disease. Symptomatic plants should be promptly removed to eliminate sources of the pathogen.

Key Hosts: Delphinium, Dahlia, Gaillardia, Salvia, Rudbeckia, Campanula, Coreopsis

Key Pathogens: Aster yellows

3. Nematode Diseases

Foliar Nematodes
Several species of nematodes infect the foliage and shoots of a wide range of perennials. The foliar nematode, Aphelenchoides spp., is very common on many perennials.

Symptoms include foliar discoloration, twisting and curling of leaves, necrosis, and blighting of affected leaves by mid to late summer. Lesions are often delineated by the venation pattern of the leaf, so they have an angular appearance.

Key Hosts: Hosta, Baptisia, Hypericum, Phlox, Heuchera, Geranium

Root-Knot Nematode
The northern root-knot nematode, Meloidogyne hapla, has a wide range of perennial hosts. This nematode infects roots and causes small galls to develop on feeder roots. The galls interfere with root function. Symptoms of infected plants include stunting, poor vigor, and poor color.

Key Hosts: Achillea, Chelone, Liatris, Phlox, Rudbeckia, Monarda

D. PLANT DISEASE LOOK-ALIKES:
1. Insect Problems (cyclamen mite, broad mite, four-lined plant bug)
2. Nutritional Deficiencies and Toxicities
3. Misapplied Pesticides and Phytotoxicity

Summary

Interest in perennials has increased dramatically in the past few years, especially in Connecticut. While perennials as a group are relatively problem-free, this fact sheet discusses strategies for disease prevention and the identification and management of the key diseases of perennials in the landscape.