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Abstract

‘Colossus 80’ southernpea [Vigna unguiculata (L.) Walp] has been released by the U.S. Department of Agriculture and Clemson University as a replacement for ‘Colossus’, a widely adapted and popular home garden and fresh-market cultivar that was released by Clemson University in 1972 (8). ‘Colossus 80’ is highly resistant to Cercospora leaf spot (CLS), a disease incited by Cercospora cruenta Sacc. (= Mycosphaerella cruenta Latham). CLS is one of the most serious foliage diseases of southernpeas in the southeastern United States and infection of susceptible cultivars results in premature defoliation and reduced yields. The original ‘Colossus’ is highly susceptible to this disease. Fery et al. (7) reported reductions of 36, 25, 10, and 3% in total seed yield, pod number, seed number per pod, and average weight per 100 seed, respectively, in a naturally infected ‘Colossus’ planting.

Development of rootstocks resistant to root-knot nematodes (Meloidogyne spp.) is a priority in grape breeding. The N allele, present in Harmony and Freedom rootstocks, confers resistance to N-avirulent strains of Meloidogyne. Extensive planting of rootstocks containing N has led to the development of N-virulent nematode strains, prompting a search for new resistance alleles. A seedling population derived from Vitis nesbittiana Comeaux was evaluated to investigate the genetic control of nematode resistance in this species. Hybridization with easily propagated rootstock selections will be required to utilize nematode resistance found in V. nesbittiana, a native of Mexico. The female parent of the population was 161-49C, a V. riparia × V. berlandieri hybrid rootstock. 161-49C does not contribute nematode resistance to its progeny. The male parent was V. nesbittiana DVIT 2236.12, an accession held in the U.S. National Plant Germplasm system. Nematode resistance of the 161-49C × V. nesbittiana DVIT 2236.12 population was assessed in greenhouse pot culture. Seedling roots were stained in an eosin-Y solution six weeks after inoculation with 1500 N-virulent M. arenaria juveniles. Resistance classes of seedlings were determined by assessing the degree of galling and number of egg masses per root system. Segregation in the seedling population was consistent with a 1:1 ratio of resistance to susceptibility, indicating that the V. nesbittiana accession is heterozygous for a dominant allele conferring resistance to N-virulent root-knot nematodes. The genetic relationship between this allele and the N allele has yet to be determined.

Abstract

‘Coastal Red’ sweet potato [Ipomoea batatas (L.) Lam.] was developed by the Univ. of Georgia at the UGA Coastal Plain Experiment Station. This cultivar combines high yield with high resistance to fusarium wilt and root-knot nematodes, resistance to soil insects, excellent storage quality, profuse plant production from bedded roots, and good baking and canning quality in a red-skinned cultivar.

Abstract

A new root-knot nematode (Meloidogyne sp.) of peach which occurs in Florida overcomes the resistance of Okinawa and Nemaguard rootstocks (Prunus spp.). Resistance to this new nematode obtained from crosses and open-pollinated seedlings of Okinawa, Chico II, and P. davidiana has a heritability (narrow sense) of 0.31 ± .04. Two selections, Fla. 14-11 and Fla. 9-4, combine the nematode resistance of Okinawa and Nemaguard with resistance to the new nematode.

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 F<sub>2</sub> 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.

The grape Vitis champinii Planchon is one source of nematode resistance in grape rootstocks. Several selections valued for their resistance to the root-knot nematode (Meloidogyne incognita), a serious pest of grape production, are used as rootstocks and in rootstock variety development. However, V. champinii-based rootstock varieties are faulted for their excess vigor and susceptibility to other root pests. Root-knot nematode populations with the ability to damage important V. champinii-based rootstocks have been identified and may become more common. Other V. champinii accessions might be sources of nematode resistance genes with different specificities or might have more suitable horticultural characteristics than V. champinii varieties in commercial use. Nine V. champinii accessions from the National Clonal Germplasm Repository, Davis, Calif., and a V. champinii rootstock variety were screened for resistance to M. incognita. Resistance was assessed by counting eggs produced per root system. Eight of ten V. champinii accessions did not support nematode reproduction. Susceptible accessions supported lower nematode reproduction than susceptible V. vinifera control varieties. Progeny testing from crosses of resistant and susceptible accessions suggests that a dominant and a recessive gene may condition root-knot nematode resistance.

Lack of a conclusive evidence of ingestion of plastid components by plant-parasitic nematodes cautions the use of plastid transformation technology for nematode resistance. Nematode-resistant effector proteins generally require ingestion by the nematode to be effective. Transgene-encoded proteins produced in plastids are not known to be exported into the cytoplasm. Disintegration of plant cell organelles after nematode feeding suggests that nematodes possibly ingest plastid components. Proof of ingestion will validate the development of plastid transformation for nematode resistance. Small subunit ribulose-1,5-bisphosphate carboxylase/oxygenase (SSRubisco) protein is prevalent in chloroplasts and thus chosen as a study molecule. The migratory endoparasitic nematodes Pratylenchus penetrans and Radopholus similis were cultured on green carrot callus containing chloroplasts. Total nematode proteins were extracted and subjected to western blot analysis using cross-reactive polyclonal antibody raised against spinach SSRubisco. Positive detection of SSRubisco occurred in protein extracts from nematodes fed on green carrot callus, but not in extracts from nematodes cultured on non-green alfalfa root callus as negative control. These results confirm the ingestion of plastid components of MW 14 kDA by migratory endoparasitic nematodes.

The southern root-knot nematode (Meloidogyne incognita) is a major pest of bell peppers (Capsicum annuum) in the United States. Since none of the leading bell pepper cultivars grown in the U.S. exhibit adequate levels of resistance, a breeding program was initiated to incorporate the N root-knot nematode resistance gene into commercial bell pepper germplasm. A backcross breeding procedure was used. The donor parent of the N gene was the open-pollinated, pimiento pepper cultivar Mississippi Nemaheart, and the recurrent parents were the open-pollinated bell pepper cultivars Keystone Resistant Giant and Yolo Wonder. A large number of homozygous resistant BC₆ populations were evaluated in field tests in 1995, and two lines (PA-440, an isoline of `Keystone Resistant Giant', and PA-453, an isoline of `Yolo Wonder') were selected for further field evaluation and seed multiplication in 1996. Results of replicated field and greenhouse tests conducted in 1996 indicate that root-knot nematode resistance has been incorporated successfully in `Keystone Resistant Giant' and `Yolo Wonder' backgrounds.