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Peer Reviewed

© 2000 Plant Health Progress.
Accepted for publication 20 December 2000. Published 27 December 2000.

Mature Watermelon Vine Decline: A Disease of Unknown Etiology in Southwestern Indiana

Daniel S. Egel, Extension Plant Pathologist, Southwest Purdue Agricultural Center, Vincennes, IN 47591; Karen Rane, Plant Disease Diagnostician, Richard Latin, Professor, and Ray D. Martyn, Professor, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907

Corresponding author: Daniel S. Egel.

Egel, D. S., Rane, K., Latin, R., and Martyn, R. D. 2000. Mature watermelon vine decline: A disease of unknown etiology in southwestern Indiana. Online. Plant Health Progress doi:10.1094/PHP-2000-1227-01-HN.

Fig. 1. An entire field of watermelon plants affected by mature watermelon vine decline. Note the presence of black plastic mulch. Mature watermelon vine decline has been observed only in the presence of plastic mulch (click image for larger view).

Fig. 2. A field of watermelon affected by mature watermelon vine decline (right) immediately adjacent to a field of healthy muskmelon (left). Note the presence of watermelon fruit that did not sufficiently ripen due to MWVD (click image for larger view).

Mature watermelon vine decline (MWVD), a disease of unknown etiology, has occurred sporadically in southwestern Indiana since the mid 1980s (Fig. 1). In 2000 the disease was especially severe, affecting more than 50% of the area planted to watermelon and resulting in yield losses of 20%. MWVD has been observed only in association with plastic mulch culture and no other cucurbit crops appear to be affected (Fig. 2). Cropping history and length of rotation period do not appear to be reliable predictors of MWVD incidence or severity. Watermelon plants (Citrullus lanatus (Thunb.) Matsum. & Nakai) affected with MWVD have been observed in fields fumigated with methyl bromide/chloropicrin as well as in non-fumigated fields. In some cases, watermelon vines begin to wilt as early as mid-June just as fruit begin to gain size. In other cases, decline and collapse occur later, shortly before harvest. Wilting vines are often observed first in low, poorly drained areas of the field and the incidence of symptomatic plants increases over time to include plants on higher ground. Affected plants have few secondary roots. The primary roots of affected plants exhibit a variety of symptoms including a wet rot or crater-like dry rot. Symptom development does not generally include stem cankers or vascular discoloration. No single fungal species has been consistently associated with the roots of plants affected with MWVD; however, Rhizoctonia solani, Pythium species and Fusarium species are routinely isolated. Preliminary greenhouse seedling inoculations using isolated fungi or dried, ground roots from symptomatic plants have failed to reproduce MWVD symptoms.

A field experiment was conducted during the 2000-growing season to compare the severity of MWVD in plots treated with soil-applied fungicides and a biological control agent. Mefenoxam-PCNB (Ridomil Gold PC , Novartis) was applied at 4.72 g/m. BioYield 213 (Gustafson LLC) was mixed at the rate of 1:40 v:v with the soilless transplant mix before seeding. Each plot measured 15.24 m in length and was planted with the watermelon hybrid 'Desert Storm' at 1.52 m intervals on May 16. Experimental plots treated with Mefenoxam-PCNB, BioYield 213 or untreated were completely randomized and each treatment was replicated 16 times. Each plot was covered with 2 ft wide x 2 mil black plastic mulch (Visqueen 4020). Experimental plots were rated for disease severity by noting the percentage of foliage within each plot that had wilted past the point of recovery. Treatments were rated visually for disease severity on 29 June, 11 July, and 17 July using the Horsfall-Barratt rating scale. The Horsfall-Barratt rating scale is a log10-based scale used to assign percent foliage affected into one of 11 severity classes. No significant differences occurred in disease severity ratings among treatments on any date. On 17 July, 50 to 75% of the foliage was affected by MWVD. Fruit were picked and weighed on 21 July. There was no significant difference in the number or weight of fruit in any treatment. The mefenoxam-PCNB treatment was included to suppress diseases caused by Pythium and Rhizoctonia species. BioYield 213, a combination of Paenobacillus macerans and Bacillus amyloliquefaciens, is designed to promote growth and increase general disease tolerance by colonizing roots. That neither treatment significantly suppressed MWVD severity, combined with the observation that severe MWVD has been observed in fields where methyl bromide/chloropicrin was used suggests that the cause of MWVD may be primarily noninfectious. Alternatively, infectious agent(s) may be involved in recolonizing the fumigated area and/or in infecting roots growing outside the fumigated or treated area, but such agents have not been identified. Environmental or cultural conditions may be involved as primary or secondary factors and may include high temperatures and excess moisture in combination with the stress associated with watermelon culture and fruit load.