Summary Specialised plant pathogens are in many ways adapted to exploit their host plants. Infective propagules should reach the appropriate plant tissue, gain access to the tissue and negate or suppress various kinds of constitutive and inducible resistance mechanisms. The resistance type most frequently deployed in plant breeding is the racespecific resistance, where a hypersensitive response of plant tissue is elicited by an avirulence factor produced by the pathogen. The great disadvantage of this type of resistance is, that it is often ephemeral. Detailed screening of germplasm may result in the discovery of alternative defence mechanisms not associated with hypersensitivity, that may be durable. Avoidance mechanisms may reduce the chance of infection. Upright plant habit has been reported to decrease spore deposition in cereals. Crop architecture may also affect aspects such as humidity and aeration in the crop, and hence the chances for successful infection by pathogen propagules. Other examples of avoidance are leaf surface properties that interfere with leaf wettability, germ tube orientation and finding of stomata to enter the leaf. Stomata in some accessions of Hordeum chilense are excessively covered by cuticular wax that prevent rust fungal germ tubes from perceiving the stomata, resulting in failure of penetration of the pathogen into the leaf. There is evidence that in compatible host species, biotrophic pathogenic fungi induce basic compatibility by suppressing defence mechanisms. Failure of this induction results in abortion of the infection attempt. Several cases of apparently durable resistance are discussed that are based on failure of haustorium formation, and are not associated with hypersensitivity. They may represent cases where the pathogen has problems in establishing basic compatibility.

Potentially durable resistance mechanisms in plants to specialised fungal pathogens

Groundnut (Arachis hypogaea L.), a self-pollinated legume is an important crop cultivated in 24 million ha world over for extraction of edible oil and food uses. The kernels are rich in oil (48–50%) and protein (25–28%), and are source of several vitamins, minerals, antioxidants, biologically active polyphenols, flavonoids, and isoflavones. Improved varieties of groundnut with high yield potential were developed and released for cultivation world over. The improved varieties belong to different maturity durations and possess resistance to diseases, tolerance to drought, enhanced oil content, and improved quality traits for food uses. Conventional breeding procedures along with the tools for phenotyping were largely used in groundnut improvement programs. Mutations were used to induce variability and wide hybridization was attempted to tap variability from wild species. Low genetic variability has been a bottleneck for groundnut improvement. The vast potential of wild species, reservoir of new alleles remains under-utilized. Development of linkage maps of groundnut during the last decade was followed by identification of markers and quantitative trait loci for the target traits. Consequently, the last decade has witnessed the deployment of molecular breeding approaches to complement the ongoing groundnut improvement programs in USA, China, India, and Japan. The other potential advantages of molecular breeding are the feasibility to target multiple traits for improvement and provide tools to tap new alleles from wild species. The first groundnut variety developed through marker-assisted back-crossing is a root-knot nematode-resistant variety, NemaTAM in USA. The uptake of molecular breeding approaches in groundnut improvement programs by NARS partners in India and many African countries is slow or needs to be initiated in part due to inadequate infrastructure, high genotyping costs, and human capacities. Availability of draft genome sequence for diploid (AA and BB) and tetraploid, AABB genome species of Arachis in coming years is expected to bring low-cost genotyping to the groundnut community that will facilitate use of modern genetics and breeding approaches such as genome-wide association studies for trait mapping and genomic selection for crop improvement.

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Groundnut improvement: use of genetic and genomic tools

Peanut is widely used as an oilseed crop around the world and as a direct
source of human food in the United States. Several species of peanut
have been cultivated for their edible seeds, but only Arachis hypogaea
L. has been domesticated and widely distributed. Production in the
United States is completely mechanized, but in many other regions the
seeds are planted and harvested by hand. In the United States, approximately
70 percent of the peanuts are runners (small-seeded types of var.
hypogaea), 20 percent are virginias (large-seeded types of var. hypogaea),
10 percent are spanish (var. vulgaris), and less than 1 percent are valencia
(var. fastigiata) market types (Knauft and Gorbet 1989). Peruvian and
aequatoriana types are produced in only a few countries in Central and
South America

Peanut Breeding and Genetic Resources

Foreword. I. Characteristics and Aspects of Durability of Resistance. II. Durable Resistance in Well Documented Pathosystems. III. Case Studies from Developing Countries. IIIa. The DGIS/ WAU Durable Resistance Programme. IIIb. Other Selected Pathosystems. IV. Selection for Durable Resistance: the Plant Breeder's View. V. Closing Remarks. Abstracts of Posters. Author Index. Subject Index.

Durability of Disease Resistance

In this paper we review the existence of sources of resistance and the various available screening methods for resistance in grain legumes against the airborne pathogens powdery mildews, downy mildews and rusts. Available resistance against these pathogens is not abundant and is particularly in risk of erosion owing to the constant generation and introduction of new races of the pathogen. A continuous search for more resistance sources is therefore a priority in legume breeding and special emphasis should be paid to selection of resistance mechanisms that are likely to be durable and to implementation of strategies to prolong the durability of existing resistance.

Screening techniques and sources of resistance to rusts and mildews in grain legumes

The groundnut (Arachis hypogaea L.) — groundnut rust (Puccinia arachidis Speg.) pathosystem appears to have coevolved in Peru, South America, where the host is known to have been cultivated for almost 4000 years. The groundnut spreaded to the rest of the world after the Spanish and Portuguese colonization of South America. Prior to 1969 the pathogen which was largely confined to South America, but it got firmly established in all the groundnut growing countries in a short span of time (a ‘re-encounter’ phenomenon). The pathogen is highly host-specific and is known by its uredinial stage. Rust is an economically important disease on groundnut, often causing more than 50% yield losses in most groundnut growing areas. Rust resistant genotypes have been identified. The resistance is of a quantitative nature and its inheritance does not seem to be simple. Rust resistance in most genotypes is stable over a wide range of geographic locations except in a few locations, indicating possible variation in the pathogen, which needs confirmation. Rust resistance in groundnut fits neither typical race-specific nor race-non-specific patterns and appears to be an intermediate type falling in the continuum of these two extreme types.

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Origin and utilization of rust resistance in groundnut.

A total of 979 germplasm accessions was screened for resistance to rust and late leaf spot in preliminary field trials at ICRISAT Center, Patancheru, India during 1989–1990. Selected genotypes were further evaluated in separate advanced screening trials conducted with erect bunch [Spanish (var. vulgaris) and Valencia (var. fastigiata)], spreading bunch [Virginia bunch (var. hypogaea)], and runner [Virginia runner (var. hypogaea)] types during 1991–1993. Thirty‐eight genotypes (21 erect bunch, 5 spreading bunch, and 12 runner) were identified as resistant to rust, and seven (five erect bunch and two runner) to late leaf spot. Four genotypes (ICG Nos 6843, 10890,11567, and 12112) showed resistance to both rust and late leaf spot; ICG 10881 and ICG 11567 belong to the Spanish bunch group. Two genotypes [BPZ 691 and 259–2 (Red)] showed high level of resistance to late leaf spot, but were only moderately resistant to rust. The characteristic features of these sources of resistance are described, and t...

Identification of new sources of resistance to rust and late leaf spot in peanut

ICGV 87165, a high-yielding peanut (Arachis hypogaea L. subsp.
hypogaea var. hypogaea) germplasm line (Reg. no. GP-78, P1
594923) with multiple disease and insect resistance, was released
in 1994 by the Plant Materials Identification Committee of the
International Crops Research Institute for the Semi-Arid Tropics
(ICRISAT) because of its resistance to rust (caused by Puccinia
arachidis Speg.) and late leaf spot [caused by Phaeoisariopsis personata
(Berk. & Curt.) Arx], and moderate resistance to bacterial
wilt [caused by Pseudomonas solanacearum (Smith) Smith], leaf
miner (Aproarema modicella Deventar), and tobacco caterpillar
[Spodoptera litura (F.)].

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Registration of ICGV 87165 Peanut Germplasm Line with Multiple Resistance

This article reports the results of two experiments carried out to standardize a protocol for the screening of salt-tolerant groundnut genotypes. The objectives were: (i) to identify an optimum NaCl treatment; (ii) to explore the potential tolerance mechanisms; and (iii) to assess the genotypic variation for salinity tolerance in groundnut. Results showed that 100-125 mM of NaCl was suitable to screen for salinity tolerance in groundnut. The material screened in this study was very limited by large differences could be observed for response to salinity, indicating a good scope for identifying genotypes with higher level of tolerance from larger screening of diverse sets of materials. Reduction in leaf size and stem/leaf ratio was observed in response to salinity. This indicates arrest of leaf expansions, which eventually limits the area available for photosynthesis. Further research is needed to determine the mechanisms of salinity tolerance in groundnut.

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Standardization of a Protocol to Screen for Salinity Tolerance in Groundnut

Light inhibited germination of Puccinia arachidis urediniospores. The threshold value for complete inhibition of urediniospore germination was 5000 Wm−1, Wavelengths of 430, 510 and 620 nm inhibited germination but had no adverse effect on germ-tube elongation of pregerminated urediniospores. Hydrated urediniospores were more sensitive to light than non-hydrated urediniospores. Inhibition was not permanent, and spores germinated when returned to the dark.

D. McDonald

Papers

Origin and utilization of rust resistance in groundnut.

Identification of new sources of resistance to rust and late leaf spot in peanut

Registration of ICGV 87165 Peanut Germplasm Line with Multiple Resistance

Standardization of a Protocol to Screen for Salinity Tolerance in Groundnut

Photosensitivity of urediniospore germination in Puccinia arachidis