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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.

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Gummy stem blight [Didymella bryoniae (Auersw.) Rehm] is the second most important pathogen of field-grown cucumbers (Cucumis sativus L.) in North Carolina and a severe problem for greenhouse-grown cucumbers worldwide. To determine whether resistance exists under North Carolina field conditions, 83 cultigens [cultivars, breeding lines, and plant introduction (PI) accessions] were evaluated in the field for 4 years for their resistance to a mixture of D. bryoniae isolates. Plants were inoculated at the vine tip-over stage and rated for foliar lesion size and number. Cultigens identified as resistant in Wisconsin and The Netherlands were not resistant in North Carolina. When averaged over years and locations, the most resistant C. sativus cultigens were PI 164433, `Slice', PI 390264, M 17, and M 12. Several accessions of related Cucumis species were highly resistant: PI 299568 (C. myriocarpus Naud.), PI 282450 (C. zeyheri Sond.), PI 299572 (C. myriocarpus), and PI 233646 (C. anguria L.). The most susceptible cultivars were `Colet', `Meresto', `Supergreen', `Dura', `Pioneer', `Marketmore 76', `Pickmore', and `Addis'. `Calypso' and `Dasher II', popular cultivars in North Carolina, were moderately susceptible.

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Field and detached-fruit screening tests were developed for evaluating cucumber (Cucumis sativus L.) plant introduction accessions, breeding lines, and cultivars (hereafter collectively referred to as cultigens) for resistance to fruit rot caused by Rhizoctonia solani Kuhn. The factors that were examined for developing a detached-fruit test were five inoculum levels of R. solani AG-4 and two cover treatments. The major influence on disease reaction was inoculum level. The best method used 50-mm-diameter fruit placed on sterile soil inoculated with 6400 oat grains/m2 colonized with R. solani and watered initially, then misted every 3 days for 1 minute, left uncovered, and rated after 10 days for the percentage of the fruit surface covered with lesions. Field and detached-fruit tests were conducted using two different inoculum levels of R. solani (3200 and 4800 oat grains/m2). The four most resistant cultigens, based on the results of field and detached-fruit tests, were PI 163216, PI 197088, PI 357852, and PI 280096. One field and detached-fruit test pair was not significantly correlated (detached-fruit screening test one vs. field screening test one), but the other (detached-fruit test two vs. field screening test two) was correlated (r = 0.50). The detached-fruit test could be used for general classification of resistance or susceptibility. Resistant cultigens could be identified with either method, but the field test had slightly lower coefficients of variation.

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Salt tolerance of 59 cultigens of tomato (Lycopersicon esculentum Mill.), seven wild Lycopersicon accessions (acc.), and one interspecific hybrid was studied under arid field conditions. Evaluation of salt tolerance was based on relative total dry matter (RD) and relative total yield (RY), calculated as the ratio between performances of salinetreated and control plants. The tomato cultigens were irrigated with water having electrical conductivities (ECi) of 1.5 (control), 5, 10, or 15 dS·m−1. Considerable variation in salt tolerance was found among the cultigens, but at 15 dS·m−1 all showed reduced RD and RY (<0.6). The cultivar M82-1-8 (M82), one accession of L. cheesmanii (Lc), three accessions of L. pennellii (Lpen), three of L. peruvianum (Lper), and an interspecific F1 hybrid (M82 × Lpen acc. LA-716) were examined for RD at three salinity levels, ECi = 1.5, 10, and 20 dS·m−1, in three annual trials. The salt tolerance of Lpen and Lper were higher than those of M82 and Lc; the interspecific F1 was the most tolerant and was usually unaffected by even the highest salinity level. The results of this study indicate the existence of a genetic potential for high salt tolerance in wild Lycopersicon germplasm.

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Abstract

Two netted and three nonnetted (casaba) muskmelon (Cucumis melo L.) cultigens and their hybrids were examined after harvest for ethylene production and for concentration of ethylene in the cavity. Whole fruit ethylene production was related to cavity concentrations. Netted muskmelon fruit produced appreciable amounts of ethylene at or near harvest while nonnetted fruit did not produce ethylene until as late as 20 days postharvest. Hybrids were generally intermediate to the parents in rate and time of production of ethylene, thus demonstrating that rates of ethylene production and cavity concentrations of ethylene in muskmelon fruit appear under genetic control.

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Seventeen U.S. honeydew-type cultivars of melon (Cucumis melo L.) and three control cultigens were evaluated in replicated, artificial inoculations under controlled conditions for resistance against downy mildew and Alternaria leaf blight. All cultivars tested were susceptible to downy mildew. However, all of the tested cultivars were significantly more resistant to Alternaria leaf blight than the susceptible control. Twelve of these cultivars were not significantly more susceptible to Alternaria leaf blight than the two resistant controls. These cultivars may provide useful sources of Alternaria leaf blight resistance for incorporation into other commercial melon types.

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A study was designed to determine whether temperature alone or temperature and relative humidity (RH) interactions affect the development of pillowy fruit disorder (PFD) in cucumber (Cucumis sativus L.). Fruit of `Calypso', `Flurry', `Carolina'? and inbred breeding line 39 were matured in four environments: cyclic and high (22 to 45C) and moderate (22 to 30C) temperatures at two RHs (35% and 75%). PFD symptoms were most severe at high temperature and RH; thus, both contribute to the development of this disorder. Line 39 had the highest PFD ratings, regardless of growing environment, a result indicating that cultigens respond differently to these imposed stresses.

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Gummy stem blight (Didymella blight), caused by Didymella bryoniae (Auersw.) Rehm and its anamorph Phoma cucurbitacearum (Fr.:Fr.) Sacc., is the second most important disease of cucumber (Cucumis sativus L.) in North Carolina after root knot nematodes Meloidogyne sp. Both Didymella blight and Phoma blight, caused by Phoma exigua Desm., have similar symptoms and control practices, and are generally referred to as gummy stem blight. In order to determine whether resistance existed to North Carolina isolates of D. bryoniae, 851 cultigens [cultivars, breeding lines, and plant introduction (PI) lines] were evaluated in the field. Plants were inoculated with one selected isolate (highly pathogenic in preliminary greenhouse tests) at the vine tip-over stage. They were rated for foliage lesion size and number on a 0 to 9 visual scale (0 = no disease, 9 = plant killed) and average ratings for 10 plants per plot were analyzed. The ratings ranged from 2.0 (highly resistant) to 8.5 (highly susceptible) with a mean of 6.2. The most resistant breeding lines and PI accessions were PI 200815, PI 390243, `LJ 90430', PI 279469, and PI 432855. The most resistant cultivars were `Homegreen #2', `Little John', `Transamerica', and `Poinsett 76'. The most susceptible cultigens in the study were PI 288238, PI 357843, PI 357865, and PI 167134. Two popular cultivars in North Carolina, `Calypso' and `Dasher II', were moderately resistant.

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The cucumber (Cucumis sativus L.) germplasm collection of 924 cultigens (accessions, breeding lines and cultivars) was evaluated for resistance to anthracnose (Colletotrichum orbiculare (Pass.) Ell. & Halst) in the field and greenhouse. The field test was run using 1 m plots grown in 4 environments (year-location combinations). The field was inoculated 3 weeks after planting using a backpack sprayer. A susceptible spreader cultivar (Wis. SMR 18) was planted every 5th row, and plots were overhead-irrigated 3 times/week. Plots were rated 1 and 2 weeks after inoculation. The greenhouse test was run using seedlings grown in flats of vermiculite, and inoculated with 104 spores/ml on one cotyledon. Plants were rated using the size of the chlorotic halo surrounding the lesion. There was no correlation (r=0.04 to 0.17) of seedling test with field test ratings, nor between any of the 4 field test environments. Correlations were significant among field tests when only cultivars and breeding lines were evaluated. We concluded that diversity within accessions resulted in the lack of correlation among tests. The cultigens that had high resistance in all tests were `Slice', NCSU M 21, Gy 14A, `Addis' and PI 164433 (India). Most susceptible were PI 175696 (Turkey) and PI 285606 (Poland).

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We examined the RAPD (random amplified polymorphic DNA) technique in our efforts toward mapping the watermelon genome. Three cultigens: Dixielee, G17AB and New Hampshire Midget (NHM), and the primitive watermelon PI 296341 and the hybrid NHM × PI 296341 were tested for polymorphic RAPD markers with 53 10-mer primers. Among total of 159 readable bands, 89 (62.3%) of the bands were polymorphic among the four accessions and 82 (51.6%) of the bands were polymorphic between NHM and PI 296341, but only 16 (10.1%) were polymorphic within the three cultigens. Results of cluster analysis based on the RAPD data were correlated with agronomic characters. The watermelon genome size was estimated by flow cytometer analysis to be relatively small, ca. 0.98 pg. The large number of polymorphic loci and the relatively small genome will enable us to develop a high density linkage map. Cosegregation analysis is under way to establish linkage relationships between the RAPD markers and estimate recombination distances between agronomic traits and molecular markers.

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