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- Author or Editor: David Wolff x
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We conducted a field screen of 130 melon cultigens to identify potential sources of host-plant resistance to Monosporascus cannonballus. Seed were sown in Speedling trays with inoculated or non-inoculated media. Plants were transplanted into a field known to be highly infested with Monosporascus cannonballus. Noninoculated plots were planted in rows that were fumigated with Telone II. Cultigens were arranged in a randomized complete block with three replications in each treatment (fumigated, nonfumigated). A disease symptom rating (1 = complete death to 5 = no symptoms) was taken at 78 and 90 days post-transplanting. Disease symptoms were most severe and occurred earliest in the inoculated, nonfumigated plots. Natural infection by Monosporascus occurred in the fumigated plots as over 95% of root samples collected contained perithecia. At the second rating date, 108 of the 130 cultigens tested were classified as moderately to highly susceptible (rating < 2.5). The four most resistant genotypes had a second rating equal or close to 4.0 (`Galia', `Deltex', `Rocky Sweet', and `Charlynne'). A group of 14 genotypes showed moderate resistance with a second rating of 3.0. Included in this group were `Morning Ice', `Doublon', `Israeli', `MR-1', `Santa Clause', and `Primo'. The physiological stress of a concentrated fruit set increases severity of vine decline symptoms.
The fungus Monosporascus cannonballus Pollock and Uecker infects melon (Cucumis melo L.) roots and causes root rot/vine decline disease, which has reduced productivity of commercial muskmelon and honeydew cultivars in South Texas. To assess the impact of the fungus on several root traits, two greenhouse experiments were carried out over two seasons. A comparison of inoculated vs. control root systems was carried out with four melon cultivars representing both susceptible (`Magnum 45' and `Caravelle') and tolerant types (`Deltex' and `Doublon'). The sand medium was inoculated with 50–60 colony forming units (CFUs) per gram of the severe Monosporascus strain, TX90-25. After a 30-day growth period, the control and inoculated root systems were carefully cleaned and evaluated. Roots were scanned by a computer and the data were analyzed by the Rhizo Pro 3.8 program. The traits of interest included total root length, average root diameter, number of root tips, number of fine roots (0–0.5 mm), and number of small roots (0.5–1 mm). Significant differences existed between the two tolerant cultivars and the two susceptible ones for four of the traits. Total root length, fine and small root length, and root tip number were greater for `Deltex' than for both susceptible cultivars and greater for `Doublon' than for `Caravelle'. The results suggest that tolerance to this pathogen is closely linked to the integrity of the root structure. The potential for improving root vigor to combat root rot/vine decline merits further investigation.
Three randomly amplified polymorphic DNA (RAPD) markers (E07, G17, and 596) linked to the Fom-2 gene, which confers resistance to race 0 and 1 of Fusarium oxysporum f. sp. melonis, were evaluated by RAPD-polymerase chain reaction for their linkage to Fusarium wilt resistance/susceptibility in diverse melon cultigens (48 resistant, 41 susceptible). Primer 596 was identified in the multiple disease-resistant breeding line MR-1, whereas E07 and G17 were identified in the susceptible `Vedrantais'. The RAPD markers E07 (1.25 kb) and G17 (1.05 kb) correctly matched phenotypes in 88% and 81% of the cultigens. The validity of the RAPD scores was verified by Southern hybridization analysis for sequence homology and bulked segregant analysis of a selected cross population for the linkage. These results will facilitate the introgression of resistance genes into susceptible lines from multiple sources in marker-assisted selection.
We conducted a field screening of melon (Cucumis melo L.) cultigens to identify potential sources of host-plant resistance to Monosporascus cannonballus Pollack & Uecker. Seed were sown in Speedling trays with inoculated or noninoculated media. Plants were transplanted into a field known to be highly infested with M. cannonballus. Cultigens were arranged in a randomized complete block with three replications in each treatment (fumigated/noninoculated, nonfumigated/inoculated). Vine decline (1 = no symptoms to 5 = dead vines) was rated at 78 and 90 days post-transplanting. Disease symptoms were most severe and occurred earliest in the inoculated, nonfumigated plots. However, natural infection by M. cannonballus occurred in the fumigated plots, as all root samples collected contained perithecia. The majority of cultigens (85%) showed moderate to severe vine decline symptoms. Mean vine decline ratings of melon types from the spring test, ranked from most tolerant to least tolerant, were: Charentais = Ananas = Galia > Misc. = Honeydew = Mixed > US cantaloupe. A Fall test of the most tolerant cultigens showed similar results, in that Ananas and Charentais types had the highest tolerance; however, Galia types performed poorly, which may be due to the different environmental conditions, or differing fruit loads on the vines in the two tests. `Deltex', an Ananas hybrid, showed the highest tolerance based on vine decline ratings in the two tests.
Monosporascus root rot/vine decline (MRR/VD), caused by Monosporascus cannonballus, is a serious disease of the major melon production areas of Texas, California, and Arizona. We have previously identified differing levels of tolerance in melon germplasm based on vine disease symptoms. This study was conducted to evaluate the yield response of commercial and experimental cantaloupe and honeydew hybrids subjected to MRR/VD. Thirty-nine and six cantaloupe and honeydew hybrids, respectively, were transplanted into a field highly infested with M. cannonballus in March 1995 in a randomized, complete block with 4 replications. The field was highly infested with Monosporascus cannonballus. `Caravelle' (very susceptible) and `Deltex' (tolerant) were included as control entries. Fruit were harvested at maturity and sized. Any fruit that did not mature completely due to vine death were counted as culls (unmarketable). Marketable yield of the cantaloupe entries ranged from 26.74% to 67.35%. The most tolerant hybrids were `SR103654', `Don Carlos', `Explorer', and `Ovation'. Marketable yield of the honeydews ranged from 8.43% to 41.46%, with `Morning Ice' and `Creme de Menthe' showing the most tolerance. The best performing hybrids were evaluated again the Fall 1995 and Spring 1996 seasons. In general, genotypes which matured later, and had a more dispersed fruit set, were more tolerant to MRR/VD. This supports previous data showing that high physiological stress (heavy, concentrated fruit load) leads to more severe and rapid vine collapse.
The nature and magnitude of genotype × environment interactions will determine the extent of testing required (locations, years) to accurately evaluate a genotype's performance. Data from yearly T-AES muskmelon variety trials were analyzed to determine the level of variety (V) × year (Y), V × location (L), and V × Y × L interactions for yield and fruit size. Data analyzed were of nine hybrids grown at three commercial farms over two years. Fruits were harvested similar to grower practices, and were sorted into size classes (9 - 30) or culls. V × Y and V × L interactions for marketable yield and total yield were not significant. V × Y × L interaction was significant for marketable yield, but not for total yield. V × Y × L interactions were highly significant for percentage culls and percentage of fruit in each size class. V × L interactions were also significant for percentage of fruit in most size classes. Data indicate that specific location-year combinations differentially affect a genotype's fruit size, most likely due to weather, planting time, and stress factors. Multiple year and location testing of genotypes is therefore critical, particularly for evaluation of fruit size.
The effect of zero, one, and two fruits per vine on plant growth and reaction to Monosporascus root rot/vine decline were investigated. In the first study, four cultivars with differing levels of tolerance were evaluated (`Primo', `Deltex', `Caravelle', `Magnum 45'). Vine decline ratings were taken weekly during the harvest period for 4 weeks. Treatments with no fruit showed delayed and less-severe vine decline symptoms. Temperature also effected vine decline symptom expression. In a Fall test, with lower temperatures during fruit maturity, symptoms were delayed in all treatments and often absent in treatments with no fruit load. Vine decline symptom expression is greatly effected by physiological (fruit load) and temperature stress. A subsequent study was conducted to more precisely quantify the effect of various fruit loads on shoot/root partitioning and vine decline symptoms. In addition to growth parameters root disease ratings were taken. `Caravelle', the most-susceptible genotype, was grown under differing fruit loads as mentioned above in Weslaco and Uvalde, Texas. As fruit load increased, root size decreased. Increased vine decline symptoms were observed under higher fruit loads. The implications on germplasm screening and breeding for resistance will be discussed.