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Aref A. Abdul-Baki, Sanaa A. Haroon, and David J. Chitwood

Resistance to root-knot nematodes (Meloidogyne spp.) in tomato (Lycopersicon esculentum Mill.) plants has been reported to break down at soil temperatures >28C. We evaluated in vitro root explants of tomato heterozygous (Mimi), homozygous (MiMi) at the Mi locus, or lacking the Mi-1 gene for resistance to Meloidogyne incognita (Kofoid & White) Chitwood and Meloidogyne arenaria (Neal) Chitwood at 28, 31, 34, and 37C. Genotypes Ace-55 UF and Rutgers, lacking the dominant allele, were susceptible to M. incognita and M. arenaria at all temperatures. Genotypes possessing the dominant allele (heterozygous or homozygous) were equally resistant to both nematode species. The resistance level in these genotypes was maintained fully at 31C, partially maintained at 34C, and lost at 37C. Resistance in the heat-tolerant Mi-heterozygous accession CLN 475-BC1F2-265-4-19 was not different from that of the heat-sensitive genotypes. As temperature increased, the genotypes differed in their sensitivity to resistance conferred by the Mi-1 locus.

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Richard L. Fery, Philip D. Dukes, and Judy A. Thies

A series of greenhouse and field studies was conducted over 9 years to characterize three new sources of resistance in cowpea [Vigna unguiculata (L.) Walp.] to the southern root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] and to determine if the resistances are conditioned by genes allelic to the Rk root-knot nematode resistance gene in `Mississippi Silver'. Three plant introductions (PI), PI 441917, PI 441920, and PI 468104, were evaluated for reaction to M. incognita in four greenhouse tests, and in every test each PI exhibited less galling, egg mass formation, or egg production than `Mississippi Silver'. F2 populations of the crosses between `Mississippi Silver' and each of the three resistant PIs were also evaluated for root-knot nematode resistance in a greenhouse test. None of the F2 populations segregated for resistance, indicating that PI 441917, PI 441920, and PI 468104 each has a gene conditioning resistance that is allelic to the Rk gene in `Mississippi Silver'. Our observations on the superior levels of resistances exhibited by PI 441917, PI 441920, and PI 468104 suggest that the allele at the Rk locus in these lines may not be the Rk allele, but one or more alleles that condition a superior, dominant-type resistance. The availability of additional dominant alleles would broaden the genetic base for root-knot nematode resistance in cowpea.

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Judy A. Thies, Richard F. Davis, John D. Mueller, Richard L. Fery, David B. Langston, and Gilbert Miller

Root-knot nematode-resistant `Charleston Belle' bell pepper (Capsicum annuum L. var. annuum) and metam sodium treatment were evaluated for managing the southern root-knot nematode [Meloidogyne incognita (Chitwood) Kofoid and White] in fall-cropped cucumber (Cucumis sativus L.). `Charleston Belle' and its susceptible recurrent parent, `Keystone Resistant Giant', were planted as spring crops at Blackville, S.C., and Tifton, Ga. `Charleston Belle' exhibited high resistance and `Keystone Resistant Giant' was susceptible at both locations. After termination of the bell pepper crop, one-half of the plots were treated with metam sodium delivered through the drip irrigation system. Cucumber yields and numbers of fruit were highest for cucumber grown in plots treated with metam sodium following either `Charleston Belle' or `Keystone Resistant Giant'; however, root gall severity and numbers of M. incognita eggs in the roots were lowest for cucumber grown in plots treated with metam sodium following `Charleston Belle'. Conversely, root gall severity and nematode reproduction were highest for cucumber grown in plots following `Keystone Resistant Giant' without metam sodium treatment. Application of metam sodium through the drip irrigation system following a spring crop of root-knot nematode-resistant bell pepper should reduce severity of root galling and reproduction of M. incognita as well as increase fruit yield of fall-cropped cucumber.

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Anne M. Gillen and Fred A. Bliss

An F2 population from a single F1 plant from the cross of peach [Prunus persica (L.) Batsch] rootstock cultivars Harrow Blood (HB) × Okinawa (Oki) was used to locate the Mi locus, which conditions resistance to Meloidogyne incognita (race 1) (Kofoid and White) Chitwood. These data and comparison of common markers among published genetic linkage maps placed the Mi locus on Prunus L. linkage group 2. Two restriction fragment length polymorphisms (RFLPs) [linked at 4.8 and 6.8 centimorgan (cM), repulsion phase] and one random amplified polymorphic DNA (RAPD) marker (linked at 9.5 cM, coupling phase) were linked to Mi. The RAPD marker was cloned, sequenced, and converted to a polymerase chain reaction (PCR)-based cleaved amplified polymorphic sequence (CAPs) marker. Clones of resistance gene analogs (RGA) developed from Oki were highly polymorphic when used as RFLP probes. The RGA's mapped to four linkage groups but clustered on two of the four linkage groups, providing limited coverage of the genome. Even so, they may be useful as markers for disease resistance genes that occur in other populations. The linkage maps of the HB × Oki F2 population and a peach × almond (Prunus amygdalus Batsch) F2 population were colinear in certain regions, however, a significant number of markers mapped to different linkage groups among the two populations. The locus for the blood-flesh trait (red-violet mesocarp) mapped to the top of linkage group 4.

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Sindynara Ferreira, Luiz Antonio A. Gomes, Wilson Roberto Maluf, Vicente Paulo Campos, José Luiz S. de Carvalho Filho, and Daniela Costa Santos

beans are considered good hosts for both Meloidogyne incognita and M. javanica with losses that can reduce pod numbers and seed weight per plant by 65%. Several studies report the occurrence of these root-knot nematodes in P. vulgaris L. cultivars

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Susan L.F. Meyer

Two strains of the fungus Verticillium lecanii (A. Zimmermann) Viégas were studied as potential biocontrol agents for root-knot nematode (Meloidogyne incognita (Kofoid & White) Chitwood) on cantaloupe (Cucumis melo L.). For the study, pots were filled with soil that had been inoculated with M. incognita (inoculum was applied at two levels: 1000 and 5000 eggs/pot). Each fungus strain was applied individually by pouring an aqueous suspension (made from a wettable granule formulation) into the inoculated soil. Controls received water only. One cantaloupe seedling was then transplanted into each pot. Plants were grown for 55 days in the greenhouse, and then harvested and assessed for root and shoot growth and for nematode egg production. In pots inoculated with 1000 eggs/plant, neither fungus strain affected nematode egg numbers. At the 5000 eggs/plant inoculum level, both strains of the fungus suppressed egg numbers (counts were 28% and 31% less than water controls). Neither strain of V. lecanii affected the number of eggs embedded in root galls; the fungus suppressed nematode population numbers overall solely by affecting the number of eggs located outside of root tissues. Both fungus strains were also autoclaved and then applied to soil, to test for effects of nonviable fungus. In pots inoculated with 5000 eggs, application of one autoclaved strain resulted in a 35% suppression in egg numbers after 55 days, suggesting that the fungus produced a heat-stable substance deleterious to the nematode.

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Alexis K. Nagel, Guido Schnabel, Cesar Petri, and Ralph Scorza

inoculated seedlings and were plated on PARPH [PARP + 50 mg 5-methylisoxazol-3-ol (hymexazol)] selective medium to confirm the presence of the P. cinnamomi . Experimental plum plants were challenged with Meloidogyne incognita (Kofoid & White) Chitwood to

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Judy A. Thies, Don W. Dickson, and Richard L. Fery

The southern root-knot nematode, Meloidogyne incognita (Chitwood) Kofoid and White, causes severe yield losses to pepper production in sub-tropical climates throughout the world ( DiVito et al., 1985 , 1992 ; Sasser and Freckman, 1987 ). In

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Judy A. Thies and Amnon Levi

. citroides PIs that showed moderate resistance to M. arenaria race 1 for resistances to M. incognita race 3 and M. arenaria race 2. Materials and Methods Inocula. Meloidogyne incognita race 3 and M. arenaria race 2 were cultured on

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Jim C. Cervantes-Flores, G. Craig Yencho, Kenneth V. Pecota, Bryon Sosinski, and Robert O.M. Mwanga

, their extensive host ranges, and associations with fungi and bacteria in disease complexes rank RKN among the major pathogens affecting crops ( Sasser, 1980 ). Greater than 50 species of RKN have been described, but Meloidogyne incognita , M. javanica