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Title: Evaluation of Commercial Watermelon Rootstocks for Tolerance to Crown Rot and Powdery Mildew, 2007

item Kousik, Chandrasekar - Shaker

Submitted to: Plant Disease Management Reports
Publication Type: Research Notes
Publication Acceptance Date: 6/24/2009
Publication Date: 8/18/2009
Citation: Kousik, C.S., Hassell, R.L. 2009. Evaluation of Commercial Watermelon Rootstocks for Tolerance to Crown Rot and Powdery Mildew, 2007. Plant Disease Management Reports. 3:V144.

Interpretive Summary:

Technical Abstract: Grafting watermelon onto rootstocks of other cucurbit genera to manage soil-borne diseases is a relatively new strategy being tested in the US. Grafting is being successfully used to manage various soil borne diseases in parts of Asia and Europe. Commercial rootstocks obtained from various seed companies were evaluated for tolerance to Phytophthora capsici in a greenhouse in Charleston SC. Plants of the various rootstocks were grown in 3.5-in pots filled with metro-mix. Each rootstock was replicated four times with five plants per replication. The experiment was arranged as a randomized block design with replications being the block and rootstocks arranged randomly within each replication. After 24 days of growth a single leaf disk was cut out from one of the cotyledons of each plant and was placed with the adaxial side up in a petri-plate lined with a wet blotter paper. Five disks from each replication for a given rootstock were placed in a single petri-plate. Each leaf disk was inoculated by brushing powdery mildew conidia (approximately 1205 conidia/leaf disk) of a race 1 isolate maintained on early prolific straight neck squash plants. The plates were incubated at 18±1º C in an incubator with artificial lighting (12 h day/night). Twelve days after inoculation, data on extent of powdery mildew on the leaf disks were recorded on a 1-5 rating scale, where; 1= trace amounts of powdery mildew with 0-5% leaf disk covered with conidia and 5=>75%. The same 24-day old plants were used for evaluation of Phytophthora crown rot. Each plant was inoculated with 1 ml zoospore suspension (25,000 zoospores/ml) at the base of the plant. The plants were kept under water saturated condition for 3 days after inoculation. Data on crown rot development was recorded a month after inoculation on a 1-9 scale where 1=no visible disease symptoms on the hypocotyl, and 9= plant dead. Data from the powdery mildew and crown rot ratings were converted to percentages based on the mid-percentage point for each rating, then arcsine-transformed and analyzed using PROC GLM procedures of SAS. Mean separation was performed using Fisher’s protected LSD (P=0.05). Significant differences (P=0.0001) were observed among the rootstocks tested in this study with respect to Crown rot caused by P. capsici and powdery mildew. All the Cucurbita inter-specific hybrids were very susceptible to crown rot and powdery mildew. Similarly the wild watermelon Ojakkyo was also very susceptible to both the diseases. The three watermelon checks also were very susceptible to powdery mildew. The Lagenaria rootstocks Macis and WMXP-3945 appeared to be the most tolerant to the cucurbit powdery mildew race 1. However, the Lagenaria rootstock WMXP-3944 was very susceptible to powdery mildew. All the Lagenaria rootstocks were significantly more tolerant to Phytophthora crown rot compared to the watermelon checks and the inter-specific Cucurbita hybrids. There were no significant differences among the Lagenaria hybrids for their tolerance to Phytophthora crown rot. These results suggest that watermelon growers should use tolerant Lagenaria rootstocks in areas where there is history of P. capsici infections.