Department of Plant Pathology, University of California, Davis, CA 95616, USA

Background and objectives
Approximately 35,000 hectares of cantaloupe and honeydew (Cucumis melo var. reticulatus and var. inodorus) were planted in California in 1997, representing 65% of production in the USA. A root rot and associated vine decline has been a continuing problem for California production. Above-ground symptoms include yellowing and death of the crown leaves, which may be followed by collapse of the vines. Below-ground symptoms include a rot of secondary and feeder roots and reddish or corky lesions on the taproot. Vine collapse typically occurs just prior to harvest, resulting in poor fruit quality. The objective of this work was to survey melon fields in California for the occurrence of root pathogenic fungi and to test their pathogenicity under field and greenhouse conditions.

Materials and methods
Plants were inoculated by direct-seeding in soil infested with a colonized sand and oat hull medium. In greenhouse trials, honeydew, watermelon, squash, and two cantaloupe varieties were evaluated at 50 ;days after planting for root disease, dry weight, and shoot length. In field microplot trials, the cantaloupe variety Magnum .45 was evaluated at horticultural maturity for root disease and root length density.

Results and conclusions
The fungi most frequently isolated from symptomatic roots from 75 commercial fields were Acremonium cucurbitacearum, Rhizopycnis vagum, Monosporascus cannonballus, Fusarium solani, Macrophomina phaeseolina, Pythium spp., and Verticillium dahliae. The frequency of isolation of the various fungi was found to vary with root symptomology and geographical location. In microplots, M. cannonballus and V. dahliae caused vine collapse, while A. cucurbitacearum and R. vagum caused root disease but no foliar symptoms. M. cannonballus caused the most severe root rot, reducing root length density by 93%. Pathogenicity of California isolates of M. cannonballus, R. vagum and A. cucurbitacearum was also demonstrated in greenhouse tests. M. ;cannonballus reduced dry weight of cantaloupe by 39.5%, while R. ;vagum and A. ;cucurbitacearum reduced dry weight by 40.5% and 23.2%, respectively. Root symptoms caused by these three fungi resembled those associated with declining plants in our survey of commercial fields. This is the first report of R. ;vagum in California.

While decline caused by M. ;cannonballus was reproduced under simulated field conditions, this fungus was isolated from only a few fields in our survey. Symptoms and/or signs of Monosporascus root rot were observed frequently in the Imperial Valley and lower San Joaquin Valley, rarely in the upper San Joaquin Valley, and never in the Sacramento Valley. Ascospores of M. ;cannonballus were recovered from soils in each of these areas.

In field microplots, collapse was only associated with vascular colonization by V. ;dahliae or severe root rot caused by M. ;cannonballus. In commercial fields, collapse is often associated with only slight to moderate root rot symptoms and no vascular discoloration. This indicates that the pathogenicity of fungi alone may not explain the phenomenon of vine decline. Root colonization by fungi may be a predisposing factor that makes the plants more responsive to environmental stresses that could result in vine decline.