PP728 Pathogen Profile

Monosporascus Root Rot and Vine Decline

Pathogen profile created by Susan J. Colucci
Requirement for PP 728 Soilborne Plant Pathogens, Spring 2007
Department of Plant Pathology, North Carolina State University

Introduction

Monosporascus cannonballus is a soilborne, root infecting Ascomycete. M. cannonballus causes root rot and vine decline in melons and watermelons and less commonly on other members of the Cucurbitaceae (Gourd) family. This pathogen is highly adapted to hot and dry areas and is often evenly distributed in fields resulting in devastating yield losses. M. cannonballus has emerged only recently as an important problem in melon production, but is thought to have been present for many years. Proper identification of the pathogen and shifts in cultural production of melons, such as changing from overhead to drip irrigation and the planting of hybrid cultivars, has resulted in elevated importance of this disease (3).
Losses in melon producing areas infested with M. cannonballus fluctuate from year to year from 10 to 25% of the crop, though individual fields may suffer from 100% loss (4).

Host Range and Distribution

M. cannonballus has been reported only members of Cucurbitaceae in arid, hot areas. The most important hosts in the field are melon (Cucumis melo) and watermelon (Citrullus lanatus). By 1997 the disease had been reported from Texas, Arizona, and California in the US and from other countries including: Mexico, Guatemala, Honduras, Spain, Israel, Iran, Libya, Tunisia, Pakistan, India, Saudi Arabia, Italy, Brazil, Japan, and Taiwan (3).
Map of areas where M. cannonballus has been reported.
In 1993, Mertely et al. (6) conducted a pathogenicity test of nine cucurbit and eight non-cucurbit crops. Though M. cannonballus appeared pathogenic on bean, corn, sorghum and sugar beet, fungal structures (perithecia) were rarely found on the roots.

Symptoms and Signs

Aboveground and Field Symptoms

Field symptoms first reveal themselves as stunted plants. However, this may go unnoticed if an entire field is uniformly affected.
In general, older crown leaves begin to turn chlorotic and die within weeks of harvest. This yellowing and death of the leaves will advance rapidly toward the end of the vines, resulting in collapse of the vine. Within 5 to 10 days of the first foliar symptoms, most of the canopy may be killed.

Melon field wiped out by M. cannonballus
(Photo courtesy of G.J. Holmes, NC State University)

Fruit of diseased plants are smaller, may abscise from the pedicle before ripening and have reduced sugar content. Fruit may also become sunburned due to lack of foliage.
Stem lesions are generally lacking and above ground symptoms may be confused with other vine declines caused by Macrophomina phaseolina (charcoal rot), Didymella bryoniae (gummy stem blight), Lasiodiplodia theobromae (Lasiodiplodia decline), and Myrothecium roridum (Myrothecium canker) (4, 5, 6).

Belowground Symptoms

Root lesions, root rot, loss of feeder roots and, in severely dry conditions, death of taproot are results of Monosporascus root rot and vine decline.
Lesions first develop as small areas of necrosis at the joints between secondary and tertiary roots or at the tips of young roots. These lesions are typically dry, however in the event of abundant soil moisture they may appear as a wet rot. Lesions are tan to red-brown.

Melon roots infected with M. cannonballus showing
lesions and loss of feeder roots
(Photos courtesy of R. D. Martyn, Purdue University (right) and M. E. Stanghellini, Univ. Calif. Riverside (left))

In severe cases of M. cannonballus infection, most of the root system may become necrotic and result in death of the plant. Large, black perithecia form on dead roots and are visible to the naked eye. The perithecia first appear on smaller feeder roots in the first few centimeters of soil and typically appear late in the season.

M. cannonballus infected root with many perithecia
(Photo courtesy of G.J. Holmes, NC State University)

Shiny, black, round ascospores are readily released from the perithecia and are visible with a hand lens (2, 5).

Ruptured perithecia exuding ascospores
(Photo courtesy of M.E. Stanghellini, Univ. Calif. Riverside)

Causal Organism

Monosporascus cannonballus Pollack & Uecker is a Pyrenomycete, a perithecia-forming Ascomycete. The unique feature of the fungus is that the asci produce only one ascospore, rather than the typical eight, hence the name Monosporascus, however occasionally they bear two.

Immature Ascospores Inside Asci of M. cannonballus
(Photo courtesy of M. E. Stanghellini, Univ. Calif. Riverside)

Ascospores are one-celled, globose and jet-black. Upon maturity they are opaque and are shiny when discharged and resemble cannonballs. The asci are clavate, thick-walled and are on short stalks (seen above). Smooth, black, globose, thick-walled fruiting structures called perithecia are scattered on roots and in culture. The perithecia may reach 500μm in diameter. Paraphyses are present, thick-walled and filamentous (4, 5, 7).

Disease Cycle and Epidemiology

Infection of the roots can occur via germinating ascospores or active mycelium in the soil. This initial infection is believed to occur early in the season, however tissue colonization is encouraged as the soil temperature rises during the production season. This rise in the soil temperature encourages perithecia formation in the roots. When disturbed, the perithecia will release ascospores. Ascospores are thought to be the primary inoculum, however their germination is rare and the role in infection is unknown. Ascospores are believed to be the long-term survival structures of the fungus. It is assumed that Monosporascus root rot and vine decline is a monocyclic disease since no known asexual (anamorph) stage has been identified (2, 3, 5).

Dissemination of M. cannonballus is unknown. It is likely that it is spread by movement of infested soil or infected plant material. Ascospores may also be moved via furrow water or heavy rains. Airborne spread is unlikely due to the large ascospores. Vegetative mycelium is effective at inhabiting decaying tissue, however mycelium will not survive even moderate desiccation (2, 3, 5)

Isolation

From the Soil

If M. cannonballus is expected in an area, collect soil samples. Take 20 grams of soil and place in a flask with 200 mL of sterile distilled water. With a magnetic stir bar, stir the soil and water for 5 minutes. Pass the contents through nested 75 μm and 38 μm sieves. Recover the material from the 38 μm sieve and wash into a centrifuge tube. Pellet the material for 4 minutes at 900 g. Discard supernatant and resuspend in a 50% sucrose solution for 2 minutes at 900 g. The supernatant will now contain the ascospores if present. Decant the supernatant on a 38 μm sieve and wash the ascospores into a Petri dish for identification and counting (if necessary). Resuspend the remaining pellet in 50% sucrose solution and repeat the latter procedure (8).

From Infected Tissue

Excise areas of upper taproot, cortical lesions, and necrotic margins of small, lateral roots dying back from Monosporascus root rot and vine decline. These tissues are then surface-disinfected for 30-90 seconds in 0.5% NaOCl and rinsed with sterile water. Tissue can then be transferred to media (see below) (6).

Media and Conditions for Culture

Vegetative mycelial growth is vast in the range of 25 to 35 C with the optimal temp. for perithecia formation at 25 to 30 C (6). Mycelial growth occurs over a pH range of 5 to 9, with and optimal range from pH 6 to 7. Growth is stopped under highly acidic conditions (pH 4 and below). The fungus grows without difficulty on potato dextrose agar, V-8 juice agar, and water agar. Fertile black perithecia usually develop within 2 to 3 weeks. Perithecia are readily visible against the light gray or dirty white mycelium(3, 4).

Management

Management of M. cannonballus has proven to be difficult do to its heat tolerance, thick-walled resting structures (ascospores), growing list of host plants and the lack of genetic resistance in melons and common cultural practices that favor the pathogen and disease development such as drip irrigation and black plastic mulch (3).

Fumigation

Soil fumigation with methyl bromide has been the standard for control of M. cannonballus. However with the phase-out of methyl bromide, alternatives have been examined (1, 2, 3).

Breeding and Grafting

Alternatives to soil fumigation include breeding for resistance and grafting melons on Cucurbita spp. rootstock (1).

Chemical Control

Fungicide treatments are often effective and less expensive compared to fumigation.
Post-plant chemigation with fluidoxonil or thiophanate-methyl applied through buried drip irrigation (if used) beginning at plant emergence has shown to be effective at controlling Monosporascus root rot and vine decline. (1, 2, 3, 4, 5).

Post-plant chemigation
(Photo courtesy of M. E. Stanghellini, Univ. Calif. Riverside)

Crop rotation with non-susceptible hosts should be a standard for control (1, 2, 3, 4, 5).

Integrated Pathogen Management

IPM is a great option for Monosporascus root rot and vine decline because there is no one “silver-bullet”. These options include manipulating the structure of the root system to promote a larger, more prolific system that can help the plant overcome wilting. This can be achieved through forms of irrigation that encourage wide root systems, such as overhead or furrow irrigation (however this may encourage spread of ascospores), as well as with direct seeding.
Traditional soil solarization is not effective for this pathogen because of its extreme heat tolerance. However, if it is combined with reduced rates of fumigation there is potential for control.
Also, reduce build up of inoculum (ascospores) in the soil by pulling roots out of the ground directly after final harvest or destroying the roots with a fumigant, like metam sodium, applied through the drip line (1, 3, 4, 5).

Acknowledgements

The author gratefully acknowledges the use of images from the collections of Dr. G. Holmes, NC State University, Dr. R. Martyn, Purdue University and Dr. M. Stanghellini, UC Riverside.

References

1. Cohen, R., S. Pivonia, Y. Burger, M. Edelstein, A. Gamliel, and J. Katan, J. 2000. Toward integrated management of Monosporascus wilt of melons in Israel. Plant Dis. 84:496-505.

2. Koike, S. T., Gladders, P., and Paulus, A. O. 2007. Vegetable Diseases. A Color Handbook. Elsevier, Boston, MA.

3. Martyn, R.D. 2002. Monosporascus root rot and vine decline of melons. The Plant Health Instructor. DOI: 10.1094/PHI-I-2002-0612-01.

4. Martyn, R. D. and Miller, M. E. 1996. Monosporascus root rot and vine decline. Plant Disease. 80(7):716-725.

5. Martyn, R. D. and M.E. Miller. 1996. Monosporascus root rot/vine decline of muskmelon and watermelon. Pages 18-19. in: T. A. Zitter, D. A. Hopkins, and C. E. Thomas (eds.), Compendium of Cucurbit Diseases, APS Press, St. Paul, MN.

6. Mertely, J. C., Martyn, R. D., Miller, M. E., and Bruton, B. D. 1991. Role of Monosporascus cannonballus and other fungi in a root rot/vine decline disease of muskmelon. Plant Disease 75:1133-1137.

7. Sivanesan, A. 1991. IMI Descriptions of Fungi and Bacteria No. 1035. Monosporascus cannonballus. Mycopathologia 114:53-54.

8. Stanghellini, M. E., and Rasmussen, S. L. 1992. A quantitative method for recovery of ascospores of Monosporascus cannonballus from field soil (Abstr.) Phytopathology 82:1115.

Return to Pathogen Profiles Homepage