Cylindrocladium scoparium

By: Allan Howard

PP 728 Soilborne Plant Pathogens Class Project

Introduction:

This fungus was first described by A.P. Morgan in 1892 and has been reported as a plant pathogen on plants in 66 genera of 31 families from many places in the world. It causes problems such as pre- and postemergence damping off, root rots, stem lesions, leaf spots/blights, and post-harvest fruit decay. The sexual phase of this fungus is known as Calonectria morganii and is less often encountered than the Cylindrocladium anamorph (3).

Host Range and Distribution:

The presence of Cylindrocladium scoparium has been confirmed in many parts of North and South America, with about 20 mostly eastern states in the U.S. Cylindrocladium scoparium has a very wide host range, mainly on woody plants. It is of particular importance as a pathogen of young eucalyptus and pine seedlings. Other hosts include, but are not limited to: azalea, rhododendron,many different hardwood trees, peanut, potato, soybean, beet, strawberry, watermelon and many other ornamental plants (3).

Isolation:

Thies and Patton(4) developed a technique for isolation of the pathogen called the “spot-plate” method. This is performed by growing alfalfa seedlings in soil samples as a bait for C. scoparium. The seedlings are then removed from the soil and placed on an agar medium to culture the fungus. Maintaining the soil samples at field moisture and temperature levels is critical to survival of the microsclerotial propagules. C. scoparium can be successfully grown on standard potato dextrose agar. Optimal temperature for growth is 25-30 degrees Celsius with a minimum limit of 5 degrees C and a maximum limit of 35 degrees C (4).

Cultures of C. scoparium and C. floridanum

courtesy of Bugwood.org

Identification:

Cylindrocladium scoparium is an ascomycete in the Nectriaceae family, one of five families within the Hypocreales order. Conidiophores develop scattered over the leaf surface or over the surface of agar in culture, and are about 0.5mm in height and from 5-8mm in width at the base. The main axis of the conidiophore extends beyond the sporogenous zone to form the long sterile appendage known as a stipe extension, which is characteristic for the genus. The width of the stipe extension becomes narrower just below the swollen, apical vesicle, which is hyaline and ellipsoidal in shape. This vesicle is about 27mm in length and about 12mm in diameter. The sporagenous portion of the conidiophore is composed of two or more bifurnicate lateral branches to the main stipe and are 22-44 x 5-7mm. The primary branches give rise to secondary and sometimes tertiary branches with progressively smaller cells, each branch ending in two or more ovoid to doliiform phialides measuring 7-12 x 3-4mm. Conidia are produced that are straight, cylindrical or slightly swollen in the upper cell, 1 septate, measuring 50-60 x 4.5-6mm. Growth on potato dextrose agar is reddish-brown from below with an irregular white margin, aerial hyphae that are radially striate, white, and becoming reddish-brown and bearing abundant conidiophores. Chlamydospores develop in abundance in older cultures, forming in chains or clumps, causing the culture to appear almost black from below. These chlamydospores are referred to as microsclerotia (3).

Conidia, conidiophores and vesicles

courtesy of Bugwood.org

Symptoms and Signs:

Symptoms of root rot on conifer seedlings and hardwood seedlings differ. On conifer seedlings look for necrosis of lateral and primary roots, often accompanied by blackening and slipping of the root cortex when the disease is further advanced. On hardwood seedlings, look for an obvious blackening of the root cortex, often accompanied by longitudinal cracking. Severe cases of root infection can lead to high mortality levels in both coniferous and hardwood seedlings. Stem infections found on eucalyptus are generally centered around leaf petioles. On conifers, such as eastern white pine, foliage blight is characterized by yellow or brown needle discoloration, necrosis, defoliation, and subsequent seedling mortality when disease pressure is high enough. Leaf spots of azaleas usually occupy less than 1/3 of the leaf area, and are usually accompanied by roots rotted in the manner previously mentioned for hardwood species. Under favorable environmental conditions, abundant spores(conidia) may be produced upon infected plant tissue. These appear as white powdery coverings. Conidia are cylindrical with rounded ends, measuring 50-60 x 4.5-6mm, and are 1-septate (1).

Leaf spot on azalea	conidia on eucalyptus stem
Blackened hardwood roots	Nursery beds of spruce infected with C. scoparium

All images courtesy of Bugwood.org

Ecology and Life Cycle:

Cylindrocladium scoparium overwinters as microsclerotia in soil and infected plant tissues. Break down of plant debris in the soil releases the microsclerotia. Intercellular penetration of the root cortex occurs within 24 hours of germination, germination being triggered by exudates encountered when the root comes into contact with the propagule. Subsequent hyphae then begin to produce microsclerotia within several days. During periods of high humidity levels and rainfall, orange colored pertithecia may develop on the upper tap root and crown of the infected plant. These perithecia will produce asci and ascospores that are then disseminated by splashing water and run-off to serve as secondary inoculum. Ascospores may be discharged forcibly or in a viscous droplet, forcible discharge being triggered by a drop in humidity levels at daybreak. Conidia and ascospores may be disseminated short distances via wind-blown plant debris and tillage practices. Both ascospores and conidia are extremely sensitive to dessication, and survival of either is less than 10% after two minutes of normal daytime field temperatures and humidity. C. scoparium, along with other Cylindrocladium species have a unique ability to tolerate a wide range of pH levels (3).

Management:

Early detection and diagnosis is critical to success in controlling this pathogen. Determine and avoid infested soils as much as possible. Avoid spread of the disease by limiting movement of plant or soil material between nurseries as much as possible. For cover crops, favor non-host crops such as corn and other grasses, rather than soybeans or other leguminous crops. Avoid high plant-bed densities that will not allow adequate air movement through the seedlings. Cull, remove and destroy infected seedlings from the bed. Cull any plants with discolored or wilted foliage, and those seedlings with 25% or more visible root rot damage to the taproot before out-planting. Minimize storage periods and maintain storage temperatures at 1.6 - 4.4 degrees Fahrenheit. This is especially important for hardwood seedlings. Fumigate beds of infested nurseries immediately before sowing. One very effective fumigant is a formulation of 67% methyl bromide with 33% chloropicrin. Deep soil fumigation(at least 12 inches) should be utilized before planting of highly susceptible and/or deep rooted hardwoods such as black walnut, yellow-poplar, and sweetgum. Foliar fungicides such as benomyl or chlorothalonil can be used to prevent foliar and shoot diseases on susceptible conifer and hardwood seedlings. Roots of hardwood and conifer seedlings can also be dipped in a thiophanate methyl solution prior to transplanting, which provides a good measure of control (2).

Links to other sites:

USDA ARS Systemic Botany and Mycology

http://www.mycobank.org

http://www.mycokey.com

Key References:

1. Barnard E. L. 1984. Occurrence, impact, and fungicidal control of girdling stem cankers caused by Cylindrocladium scoparium on eucalyptus seedlings in a south Florida nursery. Plant Dis. 68 6:471-473.

2. Cordell C.E., Anderson R.L., Hoffard W.H., Landis T.D., Smith R.S. Jr., Toko H.V. 1989. Forest Nursery Pests. USDA Forest Service, Agriculture Handbook No. 680, 184 pp.

3. Crous P. W. 2002. Taxonomy and pathology of Cylindrocladium (Calonectria) and allied genera. APS Press, St. Paul, Minn. 294 pp.

4. Thies, W.G., & R.F. Patton. 1966. Spot-plate technique for the bioassay of Cylindrocladium scoparium. Phytopathology 56:1116-1117.