While this article describes important steps to reduce the presence of potentially seedborne pathogens in seed production fields, information contained herein constitutes suggestions only and does not guarantee a disease-free crop. Despite
Branko R. Lovic and Donald L. Hopkins
Ron R. Walcott
Plant pathogens present a serious threat to seedling establishment and the potential for plant disease epidemics under greenhouse conditions is great. Hence, pathogen exclusion by detection and elimination of infested seedlots remains a requisite tactic for seedling production and disease management. Unfortunately, the numbers of contaminated seed within a lot may be low and infested seed may be asymptomatic making their detection difficult. To address these issues seed detection assays have been developed, but many of them have shortcomings that reduce their effectiveness. Examples of frequently used seed assays include visual examination, selective media, seedling grow-out and serological assays which, while appropriate for some pathogens, often display inadequate levels of sensitivity and specificity. Recently, the polymerase chain reaction (PCR) has emerged as a tool for detecting microorganisms in many diverse environments. Thus far, it is clear that DNA-based detection systems exhibit higher levels sensitivity than conventional techniques. Unfortunately, PCR-based seed tests require the extraction of PCR-quality DNA from target organisms in backgrounds of saprophytic organisms and inhibitory seed-derived compounds. The inability to efficiently extract PCR-quality DNA from seeds has restricted the acceptance and application of PCR for seed detection. To overcome these limitations several modified PCR protocols have been developed including selective target colony enrichment followed by PCR (BIO-PCR) and immunomagnetic separation and PCR. These techniques seek to selectively concentrate or increase target organism populations to enhance detection and have been successfully applied for detecting bacteria in seed. Other techniques with great potential for rapid detection of seedborne pathogens include magnetic capture hybridization and PCR, and DNA-chip technology. Ultimately, PCR will be available for the detection of all seedborne pathogens and may supersede conventional detection methods.
A.G. Hunter, G.E. Boyhan, E.H. Simonne, and O.L. Chambliss
Seed harvested from 41 entries in the 1994 southernpea variety trial was grown in a greenhouse for evaluation of seedborne mosaic viruses. When second trifoliate leaves were fully expanded, 100 plants per plot per block (4) were evaluated for blackeye cowpea mosaic virus (B1CMV), cucumber mosaic virus (CMV), cowpea severe mosaic virus (CSMV), and southern bean mosaic virus (SBMV). The average number of plants with virus symptoms ranged from 2% (Pinkeye Pinkpod) to 44% (Bettergreen). Plants with symptoms were assayed using enzyme-linked immunosorbent assay (ELISA). At least one virus was detected with ELISA in all entries, except for `Zipper Cream' in which none were evident. All viruses were detected in seven entries. B1CMV and CMV were present in 13. CMV was present in all but `Zipper Cream', `Mississippi Cream', and `Texas Pinkeye'. Symptomatology was poorly correlated to ELISA results: six entries having all four viruses had symptoms on less than 13% of their plants.
Muhammad Bashir and Richard Hampton
By ELISA serology, we have detected and identified the following seed-borne viruses in Vigna unguiculata seedlots, processed cooperatively with the government of Denmark as potential germplasm introductions: BLACKEYE COWPEA MOSAIC, COWPEA APHID-BORNE MOSAIC, COWPEA MILD MOTTLE, COWPEA MOSAIC, COWPEA MOTTLE, COWPEA SEVERE MOSAIC, CUCUMBER MOSAIC, and SOUTHERN BEAN MOSAIC VIRUSES. Twenty-three of 155 seedlots from 10 old-world countries were found to contain one or more of these viruses. TOBACCO RINGSPOT and URD BEAN LEAF CRINKLE VIRUSES are known to be seed-borne 1n cowpea, but were not included in assays. At least six other cowpea-crop-damaging viruses, as yet inadequately characterized, are also reportedly seed-borne in cowpea in India and countries of west Africa. We are currently characterizing viruses in Vigna pre-introductions and selected germplasm accessions, with emphasis on seed-borne potyviruses. Comparisons among B1CMV and CAMV isolates, for which cowpea sources of genetic resistance have been identified, revealed a wide range of isolate pathogenicity for both viruses.
Kiersten A. Wise, Robert A. Henson, and Carl A. Bradley
evaluate the effects of fungicide seed treatments on controlling seed-borne A. rabiei in chickpea. Materials and methods Fungicide seed treatments. A chickpea seedlot (cultivar Dwelley) was obtained from a producer's field in North Dakota that was
Richard O. Hampton
Blackeye cowpea mosaic potyvirus is the most easily observable seed-borne virus in cowpeas, but is typically seed-transmitted at lower rates (i.e., 0.1 to 2%) than the less conspicuous cowpea severe mosaic comovirus or cucumber mosaic cucumovirus. All three viruses are readily vector transmissible after seed-borne inoculum reaches the field, perpetuating and spreading the viruses. Individually and particularly in mixtures, these viruses are capable of decreasing both seed quality and yield. Disease-tolerant cultivars are available, but fail to control viral diseases. Development of superior new cowpea cultivars with multiple viral-disease resistance is clearly within reach and has become essential to long-term, sustainable, profitable cowpea production. This breeding objective requires public-research supported efforts by the combined cowpea seed and processing industries. Southern bean mosaic sobemovirus is also recognized as an important cowpea pathogen, but was encountered at a much lower frequency than the above three viruses in both plant and seed samples, in 1992 and 1993.
K. Kasimor, J.R. Baggett, and R.O. Hampton
Commercial pea (Pisum sativum L.) cultivars, plant introduction (PI) lines, and Oregon State Univ. (OSU) breeding lines were tested for resistance to pathotype P2 (lentil strain) and pathotype P1 (type strain) of pea seedborne mosaic virus (PSbMV) and to bean yellow mosaic virus (BYMV) to assess the relative proportion of resistant and susceptible pea genotypes. Of the 161 commercial cultivars tested, 117 (73%) were resistant and 44 were susceptible to PSbMV-P2. Of these PSbMV-P2-resistant cultivars, 115 were tested for resistance to BYMV and all were resistant. Of the 44 PSbMV-P2-susceptible cultivars, 43 were tested for BYMV susceptibility and all were infected except two, `Quincy' and `Avon', both of which were susceptible to a BYMV isolate in another laboratory. Of 138 commercial cultivars inoculated with PSbMV-P1, all were susceptible. All PI lines and OSU breeding lines that were resistant to PSbMV-P1 were resistant also to PSbMV-P2. The high percentage of commercial cultivars resistant to PSbMV-P2 was probably attributable to the close linkage of genes sbm-2 and mo and the widespread use by breeders of BYMV-resistant `Perfection' and `Dark Skin Perfection' in developing new pea cultivars. Segregation ratios in progenies of three separate crosses between PSbMV-P2-resistant and PSbMV-P2-susceptible cultivars closely fit the expected 3 susceptible: 1 resistant ratio expected for resistance conferred by a single recessive gene.
Richard W. Robinson, Rosario Provvidenti, and Joseph W. Shail
James R. Baggett, Kathryn Kasimor, and Richard O. Hampton
Anthony P. Keinath
tuberous root in mid to late spring, and this is after the majority of the years' precipitation has fallen” (S.J. Novak, personal communication, 19 June 1997). A Seedborne Cucurbit Pathogen D. bryoniae may be present both on and in cucurbit seed. Although