LEAF AND CROWN ROT OF LIRIOPE

 

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James Strandberg

University of Florida

Mid-Florida Research and Education Center

 

Plant Pathology Research Report #2001-3

 

Table of contents:

  • Introduction
  • Symptoms
  • Key diagnostic features
  • The pathogen
  • Disease development
  • Factors that favor the spread of the disease
  • Management of leaf and crown rot during nursery production
  • Management of leaf and crown rot in the landscape
  • Replacement of diseased plants in the landscape
  •  

    INTRODUCTION - The popularity of Liriope muscari (Decne.) Bailey for use in Southern landscapes has been accompanied by a large apparent increase in the incidence of a leaf and crown rot disease which attacks Liriope during nursery production and in landscape plantings. Based upon symptoms and damage, this disease has been named Liriope leaf and crown rot. Apparently, this disease is widespread in the Southeastern states and can probably occur wherever Liriope is grown. Our research has shown that all Liriope species and cultivars are susceptible to the disease, but the widely-grown cultivar 'Evergreen Giant' is more susceptible than others. Leaf and crown rot is probably not a new disease, but one that has become more common with the increased production and use of 'Evergreen Giant'. Leaf and crown rot is caused by a strain of the common plant pathogen, Phytophthora palmivora.

     

    SYMPTOMS - Liriope reproduces from rhizomes, so several plants develop in a compact cluster. Leaf and crown rot usually affects and kills only one, or a few plants, within the cluster at any given time. Initially, one or more mature leaves associated with a diseased plant rapidly develop a pale- to bright-yellow color which extends upward from the leaf base toward the tip. When immature leaves (pale green, not fully expanded) close to the growing point become diseased, they become tan or light brown and water soaked. Eventually, entire leaves of affected plants become bright yellow before they turn brown and die (Figs. 1, 2, and 3). In advanced stages of disease, all the leaves are discolored. Death normally occurs within 2 to 4 weeks of infection).

    The disease damage is very noticeable in landscapes (Fig. 1). Dead leaves are unsightly and persist on plants for several weeks. Infrequently, leaf and crown rot can spread to additional plants within the cluster, but more often, the rest of the cluster remains temporarily unaffected. Eventually, additional plants in the group can, but do not always, become diseased.

    The rotted portion of diseased leaves extends downward into the crown and is delimited from the upper, yellowed portion by a small zone of dark-brown tissue. Rotted leaf bases are not particularly soft, because Liriope leaves are fibrous. Even in advanced stages of decomposition, leaf bases remain mostly dry and fibrous. In advanced stages of disease development, affected leaves are easily pulled free from the crown or rhizome. Root systems of diseased plants can have numerous brown to brick-red lesions. Root systems of affected plants are often much-reduced and severely damaged.

     

    FIGURE 1. Typical disease damage caused by leaf and crown rot on Liriope, cv 'Evergreen Giant'.

     

     

    FIGURE 2. Closer view of Liriope leaf and crown rot damage caused by Phytophthora palmivora.

    FIGURE 3. Patterns of leaf discoloration produced by Liriope leaf and crown rot.

     

    KEY DIAGNOSTIC FEATURES - Diseased leaves are discolored (yellow) and appear water-soaked, discolored, and rotted at the base. Near the base, affected leaves become chocolate-brown. Closer to the crown, rotted leaf bases are light brown. Early in disease development, affected leaves develop a pale green or yellowish cast. Later, the bright yellow discoloration appears just above the rotted portion and extends slowly upward. Initially the upper-portion of the leaf remains green, but eventually, the entire leaf turns yellow and dies. Affected leaves, particularly those in advanced stages of disease development are easily pulled free from the rhizome or crown. Leaves that are brown or yellowed at the tip but remain green down to the point of attachment are probably affected by other leaf diseases, nutritional problems, or physiological disorders.

     

    THE PATHOGEN - Leaf and crown rot of Liriope is incited by the fungus-like organism, Phytophthora palmivora. In the past, leaf and crown rot has been attributed to several causes including fungal pathogens, insects, and mites, but our research has demonstrated that insects, mites, or nematodes, which are frequently abundant in diseased crowns, are unlikely to be involved and probably do not pre-dispose plants to the disease. Several fungi, especially Fusarium species can be routinely isolated from diseased leaves, roots, and rhizomes, but we have demonstrated that leaf and crown rot is incited by Phytophthora palmivora. Inoculation with other fungi isolated from disease Liriope plants has not resulted in diseased plants, even if leaves and roots were wounded before inoculation. Under some conditions, other pathogens might cause symptoms similar to leaf and crown rot. So far, our surveys indicate that the wide-spread disease presently affecting Liriope is caused by P. palmivora, but we are continuing to investigate other fungi which might be involved or capable of producing similar disease damage.

    The causal organism has been difficult to isolate and identify without special laboratory methods, but once efficient isolation and detection methods were developed, Phytophthora palmivora was consistently isolated from diseased plants and from soil surrounding the root system of diseased plants. Inoculation of healthy Liriope plants with zoospores of P. palmivora produced in pure culture consistently resulted in the infection of roots, the crown area, and leaves and eventually, typical, leaf and crown rot damage.

    Phytophthora palmivora is frequently found in nurseries and greenhouses and affects numerous other hosts among foliage and landscape plants such as several types of palms, Ivy, and some bedding plants, so unless appropriate precautions are taken, inoculum could already be present in some nurseries and landscapes (see: Plant Pathology Research Report 2001-4E.)

    DISEASE DEVELOPMENT - At present, only a moderate amount of detailed research has been completed on the ecology of Liriope leaf and crown rot. However, just establishing the identity of the pathogen provides much information about the biology of the disease, because the biology of some other diseases incited by P. palmivora has been extensively investigated. For example, this fungus-like organism causes major disease problems on Cacao, palms, and rubber crops in the Tropics. Thus, our present knowledge about leaf and crown rot is being pieced together from our specific research on the Liriope problem and from what others have discovered regarding similar diseases of other plants caused by P. palmivora and other Phytophthora species.

         

    Figure 4. Disease cycle of Liriope leaf and crown rot and important points about the biology of Phytophthora palmivora

     

    As for most Phytophthora species which asexually reproduce by abundant sporangia and zoospores, water provided by irrigation and rain is of great importance in the reproduction and secondary spread of P. palmivora. For example, sporangia and zoospores of P. palmivora can be easily spread between densely packed container-grown plants produced under overhead sprinkler irrigation or during rain storms. Both splashing water and surface run-off transports zoospores and sporangia from diseased to non-diseased plants and between plant containers (See Figure 4).

    To reproduce and disperse, the pathogen requires abundant water in the soil. When plants are frequently kept at, or near, soil moisture capacity, conditions are especially favorable for infection. Secondary inoculum is provided by sporangia which are lemon-shaped, sack-like structures which contain several zoospores (swimming spores). Sporangia are produced in abundance near the surface of diseased roots and in the soil nearby. Moreover, they are produced under normal Liriope production conditions when the soil is not saturated nor excessively wet. Under disease-favorable conditions (moderate temperatures, abundant soil moisture), sporangia germinate and release their zoospores which soon disperse within the film of water that surrounds soil particles or the water that fills the capillary space between soil particles. Both sporangia and zoospores are tiny and can only be observed with a microscope.

    Zoospores swim about for several minutes, then attach themselves to root surfaces, and soon infect new root tissues. Zoospores require at least a thin film of water and normally disperse over relatively short distances (inches). However, mature sporangia, which contain about 18 to 40 zoospores, are easily detached as a unit. Detached sporangia can be transported for many feet or yards by wind or splashing and running water such as surface run-off, then release their zoospores. Zoospores can also be transported for relatively long distances by the same methods.

    Following infection and colonization of the roots, the pathogen invades rhizomes, leaf bases (crown) and leaf tissue. During this process, numerous additional sporangia are produced on the diseased root tissue and in the soil nearby. This information is based upon greenhouse and laboratory experiments, observations in nurseries, or inferred from studies of other, similar, diseases caused by the same Phytopthora species. For example, the specific importance or extent of sporangia dispersal in Liriope leaf and crown rot has not yet been fully documented, but sporangia function similarly in many Phytophthora diseases where dispersal has been studied, so they would be expected to serve the same purpose in Liriope leaf and crown rot. However, it is known that zoospores are needed to infect Liriope roots which eventually leads to crown and leaf rot damage. Chlamydospores, which are thick-walled, spherical spores, are produced in abundance by P. palmivora; they are resistant to decay and can persist in the soil for long periods. Chlamydospores can germinate and infect roots directly or they can function as sporangia. So far, our experiments have indicated that they do not appear to be effective in pathogen survival or in the infection of Liriope roots, but this research is difficult and more work is needed to resolve this issue.

    Following root infection, affected leaves turn yellow within a week or two as the basal portions become rotted. Typically, within 2 to 4 weeks after the first symptoms appear, the crown and bud become rotted and the older leaves turn yellow and die.

     

    FACTORS THAT FAVOR AND SPREAD THIS DISEASE - Our observations and weekly monitoring of P. palmivora populations in nurseries indicate that the impact of leaf and crown rot is seasonal. That is, initial disease damage and new infections are more evident in late spring and early summer. This is probably a result of the appearance of more frequent rains that thoroughly saturate the growth medium rather than a direct temperature effect. On the other hand, cool winter and fall temperatures clearly reduce the incidence and development of leaf and crown rot. Research to resolve these issues presently underway.

    Observations of leaf and crown rot problems in nursery production systems indicate the disease can be easily transmitted during propagation. This is significant, because Liriope is usually propagated by division of rhizome-plant clusters which results in extensive root and rhizome wounding. During this process, diseased roots and infested soil are effectively transferred to new pots. There is also evidence that wounding might be important in promoting the establishment disease.

    Once present in a nursery, the disease is easily spread between densely packed container-grown plants produced with overhead sprinkler irrigation. Over-watering, and particularly, periodic flooding provide ideal conditions for infection and dispersal in both nurseries and landscapes. Sporangia are produced and accumulate over time in soils that have a normal moisture content. A sudden episode of soil saturation or flooding induces the sporangia to release their zoospores which disperse in the excessive water and infect new roots. Sporangia can release their zoospores in place or can be easily detached as a unit, dispersed in water by splashing and running water such as surface run-off, and release their zoospores in another place. Flooding also promotes the infection of roots that emerge from drain holes in pots; such roots are directly exposed to flood or run-off water containing sporangia and zoospores.

     

    MANAGEMENT OF LEAF AND CROWN ROT DURING NURSERY PRODUCTION - The best approach to managing Liriope leaf and crown rot is to exclude or avoid the disease. Attempts to control leaf and crown rot in the nursery with fungicides are costly and, with presently-available fungicides, have been only moderately effective. Routine treatment of plant material at propagation and during early phases of Liriope growth thereafter with fungicides (or other materials known to have good activity against Phytophthora and related pathogens) has been moderately effective in suppressing or reducing the incidence of disease during nursery production. However, fungicide applications probably have limited usefulness, because they only help to prevent, but do not cure, leaf and crown rot. Fungicides such as mefenoxam, fosetyl-aluminum, and certain phosphorous acid-based materials can temporarily prevent disease establishment by inducing or activating natural defense systems within the Liriope plant, but these fungicides have little effect on residual inoculum (sporangia) in the soil. When such fungicides decompose or are leached from the soil (often within 4 to 6 weeks), disease usually re-appears.

    During nursery production, fungicides are most effective when used in conjunction with comprehensive disease management programs that avoid introducing inoculum and minimize opportunities for the production and dispersal of secondary inoculum. In general, presently-available fungicides have not been very effective in controlling disease in Liriope that are already infected with P. palmivora.

    Appropriate cultural practices can reduce disease losses. For example, we have obtained evidence that the type of growth medium affects the incidence of leaf and crown rot. Among the growth media tested, builder's sand, was least conducive to infection and disease development. Disease development increased with increases in peat moss content of the media. Several experiments are underway to evaluate growth medium ingredients including composted hardwood bark and other bio-control materials.

    Note: Check this web site occasionally for results of fungicide and cultural control experiments now underway at MREC.

    For nurserymen, a much better approach has been to establish and maintain a source disease-free propagation stock that remains appropriately isolated from production sites and to develop strategies and procedures which prevent the secondary spread of the pathogen between production units. Routine access of workers and visitors to propagation stock areas should be restricted. Isolation, restricted access, and appropriate dis-infestation and sanitary practices are essential to and maintain pathogen-free propagation material and to produce disease-free plants. If good practices are used, fungicide applications might be unnecessary. During production, fungicides are often helpful, but cannot make up for production and cultural practices conducive to disease development and spread.

     

    MANAGEMENT OF LEAF AND CROWN ROT IN LANDSCAPES - Landscapers should obtain and install disease-free plants. Inspect and obtain plants from a nursery that is successfully managing or avoiding the leaf and crown rot disease and can consistently provide disease-free Liriope plants. Avoid plant crowding, planting too deep, and over-watering. Do not plant Liriope where there is a likelihood of frequent flooding or collection of excessive surface runoff. Where leaf and crown rot is already present, planting Liriope cultivars that are not highly susceptible to the disease is one possible solution. Presently-available fungicides probably have limited use in the landscape because they are costly and temporarily suppress, but do not cure, leaf and crown rot (see section on nursery production). Avoid soils with high organic and peat moss content (see section on nursery production).

     

    REPLACEMENT OF DISEASED PLANTS IN THE LANDSCAPE - Experiments and observations have demonstrated that when diseased or dead plants are removed from a landscape planting and a new plant is re-planted in the same location, there is a good chance that the new plant will eventually become diseased. The reason for this is that there is likely to be residual inoculum (probably sporangia) of P. palmivora in the soil that was in contact with the root system of the original, diseased plant. Often, infection and disease development in the new plant requires from 2 to 6 months to occur, but is likely to happen sooner during the summer rainy season. Sometimes, for reasons unknown, the new plant does not become diseased. Plantings free of leaf and crown rot are likely to remain unaffected unless the disease (or inoculum) is somehow introduced. Treatment of replacement locations with fungicides before replacing the plant is unlikely to be completely effective, but such treatments might reduce the number of new plants that become diseased. Experiments to determine the efficacy of fungicides for this purpose are underway.

    Revised 02/05/02

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