B. J. Aegerter

Occurrence and Pathogenicity of Fungi Associated with Melon Root Rot and Vine Decline in California

The occurrence of fungi associated with root rot and vine decline of melon (Cucumis melo) in commercial fields in California was surveyed over 3 years. The fungi most frequently isolated from discolored vascular tissue or root rot were Acremonium cucurbitacearum, Rhizopycnis vagum, Monosporascus cannonballus, Fusarium solani, Macrophomina phaseolina, Pythium spp., and Verticillium dahliae. The frequency of isolation of the various fungi varied with root symptomology. Pythium spp., and M. phaseolina were frequently associated with a wet, brownish root rot, while A. cucurbitacearum, R. vagum, and Rhizoctonia solani were generally associated with a dry, corky root rot. Presence of Monosporascus cannonballus was associated both with a wet, brownish rot as well as with discrete, reddish, corky lesions. The frequency of isolation of a given pathogen varied with geographic location, with M. cannonballus present only in the southern production areas, while A. cucurbitacearum and Rhizopycnis vagum were most common in the northern production areas. In pathogenicity tests in field microplots, M. cannonballus caused vine collapse and severe root rot of cantaloupe, reducing root length density by 93%. California isolates of R. vagum and A. cucurbitacearum, although only weakly pathogenic in field microplots, caused root discoloration and reduced vine growth in greenhouse tests. Reduction in dry weight of greenhouse-grown cantaloupe was 40, 23, and 39% for R. vagum, A. cucurbitacearum, and M. cannonballus, respectively.

Environmental Factors Affecting Rose Downy Mildew and Development of a Forecasting Model for a Nursery Production System

The effect of various environmental parameters on rose downy mildew caused by Peronospora sparsa was determined under controlled conditions and in the field. In growth chambers, optimal temperatures for infection and colonization of rose leaves were 15 to 20°C and 20 to 25°C, respectively. At optimal temperatures, infection required only 2 h of leaf wetness, although disease severity increased significantly with an increasing duration of leaf wetness up to 10 h. Infection of leaves occurred at temperatures as low as 5°C with 8 h of leaf wetness. The latent period of infection varied from 4 to 7 days. Weather and disease incidence data collected from natural field epidemics were used in the development of a predictive model of rose downy mildew. Logistic regression was used to identify those weather variables that explained the largest portion of the variation in disease incidence. The optimum regression model incorporated three weather variables calculated as cumulative totals over the previous 10 days: (i) hours of leaf wetness when temperatures were less than 20°C (positive correlation); (ii) hours between 15 and 20°C (negative correlation); and (iii) hours when temperatures exceeded 30°C (negative correlation). The simplest model, which was also a good fit, included only the 10-day cumulative number of hours of leaf wetness. The critical number of hours of leaf wetness for disease development was an average of 8.4 h per day over 10 days.

Detection and Management of Downy Mildew in Rose Rootstock

A technique utilizing the polymerase chain reaction (PCR) was developed to investigate the occurrence and location of Peronospora sparsain dormant, woody rose tissues. PCR primers were designed to amplify the internal transcribed spacer region of the ribosomal DNA of the pathogen. Inhibition of the reaction by plant compounds was minimized by optimizing the reagents used in the extraction of DNA from roses and in the amplification reaction. The PCR assay was capable of detecting as little as 2 pg of DNA from P. sparsa against a background of 4 ng of DNA from rose cane cortex. With this method, DNA of P. sparsa was detected in the cortex of stem and root tissues of symptomatic plants. Pathogen DNA also was detected in the cortex of crown tissues of asymptomatic mother plants used as a source of propagation materials. Epifluorescent and differential interference contrast microscopy were used to confirm the presence of abundant hyphae and oospores within the stem cortex of infected canes. Preplant treatments of dormant rootstock cuttings in fungicides or hot water were evaluated during natural outbreaks of the disease in commercial rose nurseries. In three trials conducted over 2 years, a 10-min preplant dip in the systemic fungicides metalaxyl or mefenoxam at rates of 100 to 10,000 mg a.i./liter reduced the area under the disease progress curve by 63 to 76% relative to nontreated plots. The evidence from PCR assays, microscopy, and fungicide trials all support the occurrence of perennating infections of P. sparsa within rose. A technique utilizing the polymerase chain reaction (PCR) was developed to investigate the occurrence and location of Peronospora sparsain dormant, woody rose tissues. PCR primers were designed to amplify the internal transcribed spacer region of the ribosomal DNA of the pathogen. Inhibition of the reaction by plant compounds was minimized by optimizing the reagents used in the extraction of DNA from roses and in the amplification reaction. The PCR assay was capable of detecting as little as 2 pg of DNA from P. sparsa against a background of 4 ng of DNA from rose cane cortex. With this method, DNA of P. sparsa was detected in the cortex of stem and root tissues of symptomatic plants. Pathogen DNA also was detected in the cortex of crown tissues of asymptomatic mother plants used as a source of propagation materials. Epifluorescent and differential interference contrast microscopy were used to confirm the presence of abundant hyphae and oospores within the stem cortex of infected canes. Preplant treatments of dormant rootstock cuttings in fungicides or hot water were evaluated during natural outbreaks of the disease in commercial rose nurseries. In three trials conducted over 2 years, a 10-min preplant dip in the systemic fungicides metalaxyl or mefenoxam at rates of 100 to 10,000 mg a.i./liter reduced the area under the disease progress curve by 63 to 76% relative to nontreated plots. The evidence from PCR assays, microscopy, and fungicide trials all support the occurrence of perennating infections of P. sparsa within rose.

Mefenoxam-Resistant Isolates of Pythium irregulare in an Ornamental Greenhouse in California

Root rot, caused by Pythium species, is a common malady in ornamental greenhouses in Monterey County, CA. In 2001, a root rot of Gerbera daisy (Gerbera jamesonii) and lisianthus (Eustoma grandiflora) caused plant losses in excess of 15 and 75%, respectively, in one greenhouse. Some plantings were total losses. Although mefenoxam was used repeatedly, no disease control was reported. P. irregulare was identified based on morphological structures produced on grass blades in water (1) and on the sequence of the rDNA internal transcribed spacer (ITS) region. Four isolates from each host were tested for sensitivity to mefenoxam in a laboratory bioassay. Using corn meal agar amended with mefenoxam at 0, 0.1, 1, 10, 50, or 100 μg a.i./ml, no inhibition of growth of any isolate occurred at concentrations of 10 μg/ml or less. At 50 and 100 μg/ml, radial growth of colonies was inhibited by approximately 20%. In contrast, 26 isolates of P. ultimum from various agricultural soils in California were completely inhibited by mefenoxam at 100 μg a.i./ml. At 0.1, 1, 10, and 50 μg/ml, growth of these isolates was inhibited by 33, 61, 78, and 96%, respectively. Each treatment was replicated three times, and the experiment was repeated with similar results. Mefenoxam was introduced in 1996 to replace metalaxyl. While metalaxyl contains a mixture of active and inactive enantiomers, mefenoxam contains only the active enantiomer. The intensive use of metalaxyl and mefenoxam in this greenhouse may have resulted in selection for resistant isolates. A resistant isolate (8-007 from Gerbera) has been submitted to the American Type Culture Collection: Manassas, VA. Reference: (1) A. J. van der Plaats-Niterink. Monograph of the Genus Pythium. Stud. Mycol. No. 21. Centraalbureau voor Schimmelcultures, Baarn, the Netherlands, 1981.