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Judy A. Thies and Amnon Levi

Root-knot nematodes [Meloidogyne arenaria (Neal) Chitwood, Meloidogyne incognita (Kofoid & White) Chitwood, and Meloidogyne javanica (Treub) Chitwood] are serious pests of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus] in the southern United States and worldwide. Watermelon cultivars with resistance to any of these nematode pests are not available. Therefore, we evaluated all accessions of Citrullus colocynthis (L.) Schrad.(21) and Citrullus lanatus (Thunb.) Matsum. & Nakai var. citroides (L.H. Bailey) Mansf.(88), and about 10% of C. lanatus var. lanatus (156) accessions from the U.S. Plant Introduction (PI) Citrullus germplasm collection for resistance to M. arenaria race 1 in greenhouse tests. Only one C. lanatus var. lanatus accession exhibited very low resistance [root gall index (GI) = 4.9] and 155 C. lanatus var. lanatus accessions were susceptible (GI ranged from 5.0 to 9.0, where 1 = no galls and 9 = ≥81% root system covered with galls). All C. colocynthis accessions were highly susceptible (GI range = 8.5 to 9.0). However, 20 of 88 C. lanatus var. citroides accessions were moderately resistant with a GI range of 3.1 to 4.0; overall GI range for the C. lanatus var. citroides accessions was 3.1 to 9.0. Resistance to M. arenaria race 1 identified in the C. lanatus var. citroides accessions was confirmed on a subset of accessions in a replicated greenhouse test. The results of our evaluations demonstrated that there is significant genetic variability within the U.S. PI Citrullus germplasm collection for resistance to M. arenaria race 1 and also identified C. lanatus var. citroides accessions as potential sources of resistance.

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Judy A. Thies and Amnon Levi

Root-knot nematodes (Meloidogyne spp.) cause extensive damage to watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus], and resistance to root-knot nematodes has not been identified in any watermelon cultivar. Twenty-six U.S. Plant Introductions (PIs) of Citrullus lanatus (Thunb.) Matsum. & Nakai var. citroides (L. H. Bailey) Mansf., one PI of C. lanatus var. lanatus, and three PIs of Citrullus colocynthis (L.) Schrad. were evaluated in greenhouse tests for resistances to Meloidogyne incognita (Kofoid & White) Chitwood race 3 and Meloidogyne arenaria (Neal) Chitwood race 2. Twenty-three of the C. lanatus var. citroides PIs and the C. lanatus var. lanatus PIs were previously identified as moderately resistant to M. arenaria race 1. Overall, the C. lanatus var. citroides PIs exhibited low to moderate resistance, and the C. lanatus var. lanatus and C. colocynthis PIs were susceptible to both M. incognita race 3 and M. arenaria race 2. The C. lanatus var. citroides PI 482303 was the most resistant PI with gall index (GI) = 2.88 and reproductive index (RI) = 0.34 for M. incognita race 3 and GI = 3.46 and RI = 0.38 for M. arenaria race 2 (1 = no galling; 5 = 26% to 38% root system galled; 9 = 81% to 100% root system galled). These results demonstrate that there is significant genetic variability within C. lanatus var. citroides for reaction to M. incognita and M. arenaria race 2, and several C. lanatus var. citroides PIs may provide sources of resistance to root-knot nematodes.

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Richard L. Fery and Judy A. Thies

Greenhouse tests were conducted to compare the levels of resistance to the southern root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] exhibited by recently released Capsicum chinense Jacq. Scotch Bonnet-type germplasm lines PA-353, PA-398, and PA-426 to the levels of resistance exhibited by C. annuum L. `Carolina Cayenne' and `Mississippi Nemaheart'; to determine the inheritance of the resistance in C. chinense germplasm line PA-426; and to determine the genetic relationship between the resistances exhibited by C. chinense germplasm line PA-426 and C. annuum `Carolina Cayenne'. The results of a replicated test indicated that the level of resistances exhibited by the resistant released C. chinense germplasm lines is equal to the level of resistances exhibited by the resistant C. annuum cultivars. Evaluation of parental, F1, F2, and backcross populations of the cross PA-426 × PA-350 (a susceptible Habanero-type C. chinense cultigen) indicated that the resistance in C. chinense is conditioned by a single dominant gene. The results of an allelism test indicated that this dominant gene is allelic to the dominant gene that conditions much of the southern root-knot nematode resistance in the C. annuum `Carolina Cayenne'. The ease and reliability of evaluating plants for resistance to root-knot nematode and the availability of a simply inherited source of outstanding resistance makes breeding for southern root-knot nematode resistance a viable objective in C. chinense breeding programs.

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Judy A. Thies and Richard L. Fery

Several species of root-knot nematodes [Meloidogyne incognita (Kofoid & White) Chitwood, M. arenaria (Neal) Chitwood, M. javanica (Treub) Chitwood, and M. hapla Chitwood] are major pests of peppers (Capsicum spp.) in the United States and worldwide. Resistance to M. incognita, M. arenaria, and M. javanica has been identified in several Capsicum accessions, but there are few reports of resistance to M. hapla. Therefore, we selected a 10% core (440 accessions) of the 14 available Capsicum spp. in the Capsicum germplasm collection (3,731 accessions) maintained by the U.S. Dept. of Agriculture (USDA), and evaluated this core for resistance to M. hapla in unreplicated greenhouse tests. The 11 best (most resistant) and the 3 worst (most susceptible) accessions identified in these unreplicated tests were re-evaluated in a replicated greenhouse test. Seven of these 11 “best” accessions (PI 357613, PI 357503, PI 439381, PI 297493, PI 430490, PI 267729, and PI 441676) exhibited root gall severity indices <5.0 (1 = no galls; 9 = more than 80% of the root system covered with galls) in the replicated test, and each of these indices was significantly lower than the indices of the “worst” accessions and susceptible controls. Although a gall index <5.0 indicates a moderate level of resistance, more than 3000 M. hapla eggs were extracted per gram of fresh root tissue and the reproductive index was >1.0 for each of these accessions. These observations suggest that the most resistant accessions tested are somewhat susceptible to M. hapla. The results of our evaluation of a core of the USDA Capsicum germplasm collection demonstrates clearly that there is significant genetic variability within the overall collection for M. hapla resistance. Additionally, these results identify portions of the collection where future evaluations for M. hapla resistance should be focused. For example, the origin of the two most promising C. annuum accessions (PI 357613 and PI 357503) in the core was Yugoslavia. Thus, additional accessions from this temperate region of the world should receive priority attention in any effort to identify better sources of resistance in C. annuum to M. hapla.

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Judy A. Thies and Richard L. Fery

Expression of the N gene, which confers resistance to southern root-knot nematode (Meloidogyne incognita Kofoid and White) in bell pepper [(Capsicum annuum L. var. annuum (Grossum Group)], is modified at high temperatures (28 °C and 32 °C), but its expression in the heterozygous condition (Nn) has not been documented at moderate or high temperatures. Responses of the near-isogenic bell pepper cultivars, Charleston Belle and Keystone Resistant Giant (differing at the N locus), and the F1 and reciprocal F1 crosses between these cultivars to M. incognita race 3 were determined at 24, 28, and 32 °C in growth chamber experiments. `Keystone Resistant Giant' (nn) was susceptible at 24, 28, and 32 °C. `Charleston Belle' (NN) exhibited high resistance at 24 °C and resistance was partially lost at 28 and 32 °C. However, at 32 °C root gall and egg mass severity indices for `Charleston Belle' were still in the resistant range, and the number of M. incognita eggs per gram fresh root and reproductive index were 97% and 90% less, respectively, than for `Keystone Resistant Giant'. Responses of the F1 and F1 reciprocal hybrid populations to M. incognita were similar to the response of the resistant parent at all temperatures. Root fresh weights and top dry weights indicated that both hybrid populations tolerated M. incognita infections at least as well as `Charleston Belle'. These findings indicate that i) only one of the parental inbred lines needs to be converted to the NN genotype to produce F1 hybrid cultivars with fully functional N-type resistance to M. incognita; and ii) cytoplasmic factors are not involved in expression of N-type resistance and the resistant parental inbred can used to equal advantage as either the paternal or the maternal parent.

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Judy A. Thies and Richard L. Fery

Heat stability of the N gene that confers resistance to the southern root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood in pepper (Capsicum annuum L.), was determined at 24, 28, and 32°C. Responses of resistant bell pepper cultivars Charleston Belle and Carolina Wonder (homozygous for the N gene) and their respective susceptible recurrent backcross parents, `Keystone Resistant Giant' and `Yolo Wonder B', to M. incognita were compared. Numbers of eggs/g fresh root, reproductive factor of M. incognita, numbers of second-stage juveniles in soil, egg mass production, and root galling increased (P < 0.05) for all cultivars as temperature increased. The response of the resistant cultivars to temperature increase was less dramatic than the response of the susceptible cultivars. Both `Charleston Belle' and `Carolina Wonder' exhibited a partial loss of resistance at 28 and 32 °C. Reproduction of M. incognita was minimal on the resistant cultivars at 24 °C, but increased at higher temperatures. However, at 32 °C reproduction of M. incognita on the resistant cultivars was only 20% of that on the susceptible cultivars and root gall indices were within the range considered moderately resistant. Unlike the susceptible cultivars, the shoot dry weights of the resistant cultivars were not suppressed at 32 °C. This suggests that `Charleston Belle' and `Carolina Wonder' may be somewhat tolerant to M. incognita at high soil temperatures. Although results indicate a partial loss of resistance occurred in `Charleston Belle' and `Carolina Wonder' under high soil temperatures, resistant cultivars may be a useful component of cropping systems designed to manage M. incognita in hot climates.

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Richard L. Fery and Judy A. Thies

Scotch Bonnet and Habanero peppers, extremely pungent cultivar classes of Capsicum chinense Jacq., are increasing in popularity in the United States. Because the southern root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood, is a major pest of many C. annuum cultivars, a series of greenhouse and field experiments was conducted to determine if Scotch Bonnet and Habanero peppers from available commercial and private sources also are vulnerable to the pest. In an initial greenhouse test, a collection of 59 C. chinense cultigens was evaluated for reaction to M. incognita race 3. All cultigens obtained from commercial sources were moderately susceptible or susceptible. However, four accessions obtained through Seed Savers Exchange listings exhibited high levels of resistance. Three of these cultigens (PA-353, PA-398, and PA-426) were studied in subsequent greenhouse and field plantings, and each was confirmed to have a level of resistance similar to that available in C. annuum. All three of the resistant cultigens are well-adapted and each is potentially useful in commercial production without further development. None of the Habanero cultigens was resistant to the southern root-knot nematode. The resistant Scotch Bonnet cultigens may serve as sources of resistance for development of root-knot nematode—resistant Habanero peppers.