2012. Plant Management Network. This article is in the public domain.
Myrothecium roridum Leaf Spot and Stem Canker on Watermelon in the Southern Great Plains: Possible Factors for Its Outbreak
Benny D. Bruton, Research Plant Pathologist, and Wayne W. Fish, Research Chemist, Wes Watkins Agricultural Research Laboratory, USDA-ARS, P.O. Box 159, Lane, OK 74555
Corresponding author: Benny D. Bruton. firstname.lastname@example.org
Bruton, B. D., and Fish, W. W. 2012. Myrothecium roridum leaf spot and stem canker on watermelon in the southern Great Plains: Possible factors for its outbreak. Online. Plant Health Progress doi:10.1094/PHP-2012-0130-01-BR.
The first known record of Myrothecium roridum Tode ex Fr. on cucurbits appears to be from Mexican cantaloupe (Cucumis melo) intercepted at the Texas border in 1950 and later in the Rio Grande Valley of Texas in 1961 (4). Although watermelon (Citrullus lanatus) has been reported as a host for M. roridum (1), the first reports of M. roridum causing leaf spot in watermelon under field conditions were from Korea in 2003 (3) and Georgia (USA) in 2005 (5).
In July of 2010, at the Wes Watkins Agricultural Research Laboratory, Lane, OK, an outbreak of leaf spot and stem canker occurred in a 0.5-hectare experimental field containing twenty different watermelon cultivars with fruit 1 to 2 weeks from harvest. In the early stage of the leaf spot phase, lesions appeared round, oblong, and/or irregular in shape, were dark brown to black in color with a light interior, and possessed a yellow halo around the perimeter of the lesion (Fig. 1A). Frequently, the necrotic leaf tissue broke away leaving the appearance of shot holes in the leaf (Fig. 1B). On some leaves, a majority of lesions were at the leaf margin suggesting fungal entry through the hydathodes or that guttation provided a more favorable environment for infection (Fig. 1C). Older leaf spots frequently exhibited concentric rings along with sporodochia arranged in a somewhat concentric pattern (Fig. 2). Disease incidence was uniform within each cultivar, but disease severity varied among cultivars. A disease rating using an interval scale of 0 to 4 with 0 being healthy, 1 = 1 to 25% of the leaf or cotyledon exhibiting leaf spot, 2 = 26 to 50%, 3 = 51 to 75%, and 4 = 76 to 100% was employed to describe disease severity. Seven cultivars exhibited a disease rating less than 1.7, eight cultivars scored between 2.0 and 2.5, and five cultivars scored between 2.8 and 3.8. Infrequent crown and stem lesions were present, but no fruit exhibited disease symptoms (Fig. 3).
Diseased tissue was plated on Potato Dextrose Agar (PDA). The resulting fungal colonies reached 40-60 mm in diameter after 14 days at 25°C and were white, floccose, wrinkled, and somewhat raised in the center. Sporulation occurred throughout the colony in concentric greenish-black zones (sporodochia) (Fig. 4). Conida were rod-shaped (1.5-2 μm × 5-10 μm). Characteristics of the fungus were consistent with those reported for M. roridum (2). Pathogenicity tests, using M. roridum isolates from watermelon were carried out on healthy seedlings of various cucurbits producing symptoms similar to those observed in the field. When re-plated, sections of symptomatic tissue from test seedlings yielded only M. roridum colonies, thereby fulfilling Koch's Postulates.
Cultural practices were reviewed in an effort to provide possible clues as to why this field of watermelon sustained a high incidence of M. roridum leaf spot while adjacent fields of other cucurbits (cantaloupe, honeydew, and watermelon) exhibited few or no symptoms.
After transplants were set out on 18 May, 7.26 cm of rain fell on 20 May, and 0.79 cm fell on 31 May. Between 12 June and 20 July, from 1.25 cm to 7.42 cm of rain fell each week. Ten moisture potentiometers placed across the field yielded an average daily reading of 21 ± 11 kPa during this period. Average daily humidity between 15 June and 15 July was 76 ± 9%, and average high and low temperatures for this period were 32.3 ± 2.7°C and 22.5 ± 1.3°C, respectively.
Cropping history of the M. roridum-infected watermelon field was perhaps the most conspicuous reason for the disease outbreak. The field had been in continuous cantaloupe cultivation between 1994 and 2009 except for three years when mixed crops of watermelon, squash, pumpkin, and gourd were grown. In 2008, crater rot was documented on cantaloupe grown in the field.
It is likely that a combination of three factors facilitated this outbreak of M. roridum on watermelon under field conditions. First, the field soil was probably heavily infested with M. roridum as a result of the continuous cropping of cantaloupe, a susceptible host. Second, weather conditions were warm and wet during the four weeks leading up to the disease outbreak. Third, frequent rains during this four week period prevented the regular weekly spraying of fungicides targeted at foliar diseases on the watermelon, allowing M. roridum-induced leaf spot and crown/stem canker to gain a substantial foothold.
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