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Abstract

Although previous workers reported that root-knot nematode resistance was controlled by 2 pairs of genes, there were more than 1/16 resistant plants in an F₂ population from ‘Alabama No. 1’ (resistant) crossed with ‘Hawaiian Wonder’ (susceptible). The evaluation of F₃ families showed that the excess of resistant individuals was not due to escapes. The 2-gene hypothesis was insufficient to account for the segregation patterns in the F₃ families, but they could be explained on a 3-gene basis. The simplest genetic explanation is that there are at least 3 pairs of genes which are equal in their action, but a certain minimum number of genes for susceptibility are necessary before all resistance is lost.

The root-knot nematode (M. hapla Chitwood) poses a threat to carrot (Daucus carota L.) production in the United States. Little information is available concerning the genetic control of nematode resistance in carrot. Crosses between two inbreds, a resistant genotype (R₁) and susceptible genotype (S₁) identified in previous screening tests of carrot were studied in the F₂ and BC₁ generations to determine the heritability of resistance to the root-knot nematode. Seedlings of F₂ (R₁/S₁), BC₁S₁, and BC₁R₁ generations were evaluated for their responses to infestation of M. hapla Chitwood based on gall number per root, gall rating per root, and root rating per root in a greenhouse experiment conducted during 1994. Narrow-sense heritabilities were calculated according to the method of Warner (1952). Narrow-sense heritability was 0.16 for resistance based on gall number, 0.88 for resistance based on gall rating, and 0.78 for resistance based on root rating. This information may be of importance to geneticists and carrot breeders for the development of nematode-resistant carrot cultivars.

The primary focus of the stone fruit rootstock program at Byron, GA, has been the development of disease-resistant rootstocks for peach (Prunus persica L. Batsch). Historically peach tree short life (PTSL), aka bacterial canker complex, and Armillaria root rot (ARR) have been the two most important causes of premature mortality of commercial peach trees in the southeastern United States. Guardian^®, a seedling peach rootstock, was cooperatively released in 1993 by the U.S. Department of Agriculture (USDA)-Agricultural Research Service (ARS) and Clemson University. It has since been widely adopted by the southeastern peach industry. As a result, trees losses to PTSL have declined sharply. However, Guardian, like most other peach seedling rootstocks, is susceptible to ARR. ARR has now moved to the forefront as the primary cause of premature peach tree death in the Southeast. In response to this threat, the USDA-ARS in cooperation with the University of Florida, released ‘Sharpe’, a plum hybrid rootstock in 2007. Despite its broad disease resistance, ‘Sharpe’ proved unsuited for widespread commercial utilization due to its relatively poor cropping performance. In 2011, ‘MP-29’, a semidwarf, clonal, plum × peach hybrid, was released for commercial trial. ‘MP-29’s broad disease and nematode resistance, in combination with its dwarfing ability and excellent productivity, offered great promise for use in this production area and in others suffering from similar issues. Since its release, testing of ‘MP-29’ has continued both in researcher and grower trials. To date, performance has exceeded all expectations.

The genetics of resistance to root-knot nematode (M. hapla Chitwood) was studied in crosses of three carrot inbred genotypes, two resistant genotypes (R₁ and R₂) and one susceptible genotype (S₁) identified in previous screening tests. Seedlings of three parental genotypes, six F₁ crosses including three reciprocal crosses, two BC₁ populations, and three F₂ populations were evaluated for their resistance and susceptibility to infestation of M. hapla Chitwood based on gall number per root, gall rating per root, and root rating per root in a greenhouse experiment carried out in 1994. All six F₁ plants were susceptible, which indicated a lack of heterosis for resistance in these F₁s. The R₁ × S₁ cross segregated 3 susceptible: 1 resistant in the F₂, 1 susceptible: 1 resistant in the BC₁R₁, and did not segregate in the BC₁S₁. The R₁ × R₂ cross yielded 44 susceptible: 36 resistant seedlings in the F₂ (R₁R₂), and 48 susceptible: 32 resistant in the reciprocal cross of R₁ and R₂, both of which closely fit a 9: 7 ratio (P ≤ 0.001). These results indicate these two resistant genotypes carry two different homozygous recessive genes conditioning root-knot nematode resistance. We propose a model of duplicate recessive epistasis control the reactions of host plants and nematode in these crosses.

Two isogenic sets of bell pepper (Capsicum annuum L.) lines (differing at the N root-knot nematode resistance locus) were characterized for resistance to Meloidogyne arenaria (Neal) Chitwood races 1 and 2, M. hapla Chitwood, and M. javanica (Treub) Chitwood in greenhouse and growth chamber tests. The isogenic sets of C. annuum were `Charleston Belle' (NN) and `Keystone Resistant Giant' (nn-recurrent parent), and `Carolina Wonder' (NN) and `Yolo Wonder B' (nn-recurrent parent). Meloidogyne arenaria race 1 is pathogenic to C. annuum. `Charleston Belle' and `Carolina Wonder' exhibited high resistance to M. arenaria race 1. Their respective recurrent backcross parents, `Keystone Resistant Giant' and `Yolo Wonder B', were susceptible to M. arenaria. Meloidogyne arenaria race 2 and M. javanica are not highly pathogenic to pepper. However, `Charleston Belle' and `Carolina Wonder' both exhibited higher (P≤0.05) resistance to M. arenaria race 2 and M. javanica than `Keystone Resistant Giant' and `Yolo Wonder B'. Meloidogyne hapla is pathogenic to pepper. Both `Charleston Belle' and `Carolina Wonder' and their respective recurrent parents, `Keystone Resistant Giant' and `Yolo Wonder B', were susceptible to M. hapla. We concluded that the N gene confers resistance to M. arenaria races 1 and 2, and M. javanica in C. annuum, but the N gene does not condition resistance to M. hapla.

Grafting technology for vegetable transplants was introduced to Israel eight years ago by Hishtil Nurseries, Inc. The main goal of grafting was to find a substitute for methyl bromide, the elimination of which was pending. The use of grafted watermelon transplants soon followed. Presently, more than 40% of watermelon transplants are grafted. The chief reason for the success of grafted transplants is their tolerance to soil-borne pathogens, including Fusarium, Monosporascus, and Macrophomina. Yields of grafted transplants are often much higher, and it has been shown possible to grow watermelons with saline water (4.5). A limitation of grafted transplants is that presently, we do not have a good solution for nematodes. A drawback is that in order to get good watermelon taste and flavour, the grower needs the experience to adjust agrotechniques, especially determining the best harvest date. Grafted tomato transplants were also introduced early on. Grafted tomato transplants can have excellent resistance to fusarium crown rot, corky root, and other soil-borne pathogens. Some rootstocks have been observed to tolerate water salinity of 8 ec and still produce commercially acceptable yields. Limitations to the use of grafted tomato transplants are the lack of compatibility of some of the cultivars with the rootstocks and the breakdown of nematode resistance at high soil temperatures. Melons, eggplants, and cucumbers are grafted under some conditions.

Greenhouse experiments determined the inheritance of resistance to the peanut root-knot nematode [Meloidogyne arenaria (Neal) Chitwood race 1] in Capsicum chinense Jacq. germplasm lines PA-353 and PA-426. Evaluation of parental, F₁, F₂, and backcross populations of the crosses PA-353 × PA-350 and PA-426 × PA-350 (PA-350 is a susceptible cultigen) indicated that resistance in both C. chinense germplasm lines was conditioned by a single dominant gene. Evaluation of the F₁ × resistant parent backcross populations in the cytoplasm of their respective resistant and susceptible parents indicated that the cytoplasm of the resistant parent is not needed for full expression of resistance. Allelism tests indicated that the dominant resistance gene in both PA-353 and PA-426 is allelic to a resistance gene in C. annuum L. `Carolina Cayenne'. However, these allelism tests did not demonstrate conclusively that the M. arenaria race 1 resistance gene in C. chinense is the N gene that conditions resistance to the southern root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] in C. annuum. The ease and reliability of evaluating plants for resistance to root-knot nematodes and the availability of simply inherited sources of resistance makes breeding for peanut root-knot nematode resistance a viable objective in C. chinense breeding programs.

Peach rootstock breeding may be accelerated by utilization of molecular markers linked to the root-knot nematode resistance locus (Mi) to screen segregating populations. A genetic linkage map was constructed using RFLP markers in an F₂ population (PMP2) that is segregating for this locus. PMP2 is derived from a controlled cross of the relatively diverse peach rootstocks Harrow Blood (susceptible) and Okinawa (homozygous resistant). Bulked Segregant Analysis was applied using RAPD markers. A single small (227 base pairs) RAPD marker was found to be linked to the dominant resistant allele of Mi at a distance of 10 cM. This new marker joined the Mi locus to the RFLP linkage map and showed that two dominant RFLP markers are located between the RAPD marker and Mi. RFLPS are expensive, time-consuming and RAPD markers are unreliable, and therefore both are unsuitable for screening breeding populations. We attempted to convert the RAPD marker to a more breeder-friendly CAPS marker. The converted CAP marker was dominant. Attempts to convert the CAP marker to a co-dominant marker were not successful. The utility of the CAP marker was tested in an open pollinated F₂ population derived from the F₁ parent of PMP2 and in several rootstocks. The genetic linkage map was compared to other Prunus maps. The PMP2 linkage group containing the Mi locus can be related to the peach × almond linkage group which contains the phosphoglucomutase Pgm-1 locus.

The USDA–ARS has released a new Habanero-type pepper cultivar named TigerPaw-NR. The new cultivar is the product of a conventional recurrent backcross breeding procedure to transfer a dominant root-knot nematode resistance gene from the Scotch Bonnet accession PA-426 into the Habanero-type accession PA-350. TigerPaw-NR was derived from a single F₃BC₄ plant grown in 2002. TigerPaw-NR is homozygous for a dominant gene conditioning a high level of resistance to the southern root-knot nematode, the peanut root-knot nematode, and the tropical root-knot nematode. TigerPaw-NR has a compact plant habit and produces attractive lantern-shaped, orange-colored fruit. The results of three replicated field studies conducted at Charleston, S.C., indicate that the fruit and yield characteristics of TigerPaw-NR are comparable to those of currently available Habanero-type cultivars. A typical fruit weighs 7.8 g, is 2.7 cm wide × 4.4 cm long, and is extremely pungent (348,634 Scoville heat units). Root-knot nematodes are major pests of peppers in the United States, and all Habanero-type cultivars currently available to commercial growers and home gardeners are susceptible. The root-knot nematode resistant TigerPaw-NR is recommended for use by both commercial growers and home gardeners. Protection for TigerPaw-NR is being sought under the Plant Variety Protection Act.

Abstract

Breeding tomatoes for resistance to fruit cracking was started in Louisiana in 1960. Varieties and lines used in these tests were ‘Pinkdeal’, ‘Floralou’, ‘Glamour’, L70 and L125, as resistant to moderately resistant parents, and L92 as a very susceptible line. Crosses were made to get the possible combinations of F₁ F₂ and backcrosses of ‘Pinkdeal’, ‘Floralou’ with L92.

In crosses of ‘Pinkdeal’ and ‘Floralou’, with L92 partial dominance of genes controlling fruit crack resistance was found. The F₂ data showed a skewness of plant distribution toward the resistant parent. From the backcross progenies of ‘Pinkdeal’ × L92 × ‘Pinkdeal’, L70 was selected after 8 filial generations as a recombination having a high level of fruit crack resistance in conjunction with good horticultural characters. Tomato lines L161 and L210 resulted from a cross of L70 × L125 and these lines were pressure selected for 6 or more generations for fruit crack resistance, good horticultural characters and root knot nematode resistance. L147 resulted from a cross of ‘Glamour’ × L92; it has good horticultural characters. These lines demonstrated highly significant levels of fruit crack resistance and good productivity on field tests for several years.