R. A. Frederiksen

Induced Resistance Responses in Maize

Systemic acquired resistance (SAR) is a widely distributed plant defense system that confers broad-spectrum disease resistance and is accompanied by coordinate expression of the so-called SAR genes. This type of resistance and SAR gene expression can be mimicked with chemical inducers of resistance. Here, we report that chemical inducers of resistance are active in maize. Chemical induction increases resistance to downy mildew and activates expression of the maize PR-1 and PR-5 genes. These genes are also coordinately activated by pathogen infection and function as indicators of the defense reaction. Specifically, after pathogen infection, the PR-1 and PR-5 genes are induced more rapidly and more strongly in an incompatible than in a compatible interaction. In addition, we show that monocot lesion mimic plants also express these defense-related genes and that they have increased levels of salicylic acid after lesions develop, similar to pathogeninfected maize plants. The existence of chemically inducible disease resistance and PR-1 and PR-5 gene expression in maize indicates that maize is similar to dicots in many aspects of induced resistance. This reinforces the notion of an ancient plant-inducible defense pathway against pathogen attack that is shared between monocots and dicots.

Compendium of sorghum diseases

Compendium of sorghum diseases , Compendium of sorghum diseases , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Relations Among Sorghum Ergot Isolates from the Americas, Africa, India, and Australia

Sorghum ergot, initially restricted to Asia and Africa, was recently found in the Americas and Australia. Three species causing the disease have been reported: Claviceps sorghi in India, C. sorghicola in Japan, and C. africana in all ergot-positive countries. The objective of our study was to study the intraspecific variation in C. africana isolates in the Americas, Africa, India, and Australia. We confirmed C. africana, C. sorghi, and C. sorghicola as different species using differences in nucleotide sequences of internal transcribed spacer 1 and 5.8S rDNA regions. Sequences of this region obtained from the representative American, Indian, and Australian isolates of C. africana were identical. In addition, random amplified polymorphic DNA (RAPD) banding patterns of sorghum ergot pathogen isolates from the United States, Mexico, Puerto Rico, Bolivia, Australia, and India were evaluated with nearly 100 primers. A total of 65 primers gave identical patterns for all isolates, which confirmed that all were C. africana. The identity of RAPD pattern and rDNA sequence of Indian isolates with those of C. africana confirmed that the species is now present in India. Only 20 primers gave small pattern differences and 7 of them were used for routine testing. All of the American isolates were identical and three isolates of the same type were also found in South Africa, suggesting Africa as the origin of the invasion clone in the Americas. Australian and Indian isolates were distinguishable by a single band difference; therefore, migration from the Asian region to Australia is suspected. Another distinct group was found in Africa. Cluster analysis of the informative bands revealed that the American and African group are on the same moderately (69%) supported clade. Isolates from Australia and India belonged to another clade.

Genetics of nonsenescence and charcoal rot resistance in sorghum

SummaryNonsenescence is a delayed leaf and plant death resistance mechanism in sorghum that circumvents the detrimental effects of reduced soil moisture combined with high temperatures during post-anthesis growth. This drought-tolerance mechanism is often equated with charcoal rot resistance, a widespread root and stalk disease of great destructive potential. Therefore, the inheritance of charcoal rot resistance was investigated directly, by exposure of sorghum to Macrophomina phaseolina, the causal organism, and indirectly, by determination of the inheritance of nonsenescence. Sorghum families derived from diallel crosses between two nonsenescent, resistant inbreds (B35, SC599-11E) and two senescent, susceptible inbreds (BTx378, BTx623) were evaluated in 1989 at College Station and at Lubbock, Texas, under controlled and field conditions. We determined that nonsenescence was regulated by dominant and recessive epistatic interactions between two nonsenescence-inducing loci and a third locus with modifying effects. The same conclusion was reached for charcoal rot resistance. The presence of different genetic mechanisms within SC599-11E for nonsenescence and charcoal rot resistance verifies that these two forms of resistance are not different manifestations of a single trait, i.e., they are not to be equated with each other. We conclude that nonsenescence alone cannot account for, and should not be used as the sole breeding criterion for, resistance to charcoal rot in sorghum.

Biological species in the Gibberella fujikuroi species complex ( Fusarium section Liseola ) recovered from sorghum in Tanzania

In Fusarium section Liseola, the teleomorph is used to identify mating populations that represent different biological species when distinguishing morphological characters are absent in the anamorph. The Gibberella fujikuroi mating populations to which strains of Fusarium section Liseola belong were determined for isolates from sorghum grown at Ifakara, Ilonga and Kachiri, Tanzania. Representatives of all of the mating populations (A–F) were recovered at Ilonga, but C and E were absent at Ifakara and C was absent from Kachiri. The frequency of the different mating populations was similar at all three sites with A (21%) and F (49%) being the most frequent and C and E the least frequent, if they were recovered at all. The relative proportions of mating populations A and F in the population were significantly different from each other at Ilonga, but were not significantly different at Ifakara or Kachiri. Female fertile strains were more common within mating population A than within mating population F. The inbreeding effective population sizes for the A and F mating populations, respectively, were 69 and 91% of count based on mating type, and 88 and 53% of count based on male/hermaphrodite ratios.

Length heterogeneity in ITS 2 and the methylation status of CCGG and GCGC sites in the rRNA genes of the genus Peronosclerospora.

SummaryThe polymerase chain reaction (PCR) was used with primers complementary to conserved flanking sequences to amplify the internal transcribed spacer 2 (ITS 2) of the rDNA repeat units of five Peronoscleropora isolates, one each of P. sorghi, P. maydis, P. sacchari and tow of P. zeae. In contrast to the situation found in mostfungi that have been examined, length heterogeneity was evident in each sample. The rDNA composition of the amplified bands was confirmed by Southern hybridizations using an ITS 2 amplified from P. sorghi and cloned rDNA from Neurospora crassa as probes. Length heterogeneity was also detected in genomic DNA digests using the same probes. In addition to one dominant fragment for each isolate, there were several less frequent fragments of different sizes, and the isolate(s) for each species had a unique banding pattern for ITS 2. The absence of 5-methylcytosine residues in CCGG and GCGC sequences in the ribosomal genes of these four Peronosclerospora species was demonstrated by the production of identical banding patterns with ribosomal DNA probes following digestion of genomic DNA with MspI and HpaII, and by complete digestion with CfoI.

Contemporary Global Movement of Emerging Plant Diseases

Abstract: Plant diseases are a significant constraint to agricultural productivity. Exotic plant diseases pose a continued threat to profitable agriculture in the United States. The extent of this threat has increased dramatically in the 1980s and 1990s due to the expansion of international trade in agricultural products and frequent movement of massive volume of people and goods across national boundaries. Introduction of new diseases has not only caused farm losses, but has also diminished export revenue since phytosanitary issues are linked to international commerce. Plant pathogens and their vectors have also moved across national boundaries, sometimes naturally and at other times influenced by the recent changes in trade practices. Sorghum ergot, Karnal bunt of wheat, potato late blight, and citrus tristeza are some of the most recent examples of enhanced importance of diseases due to the introduction of plant pathogens or vectors.

A molecular marker that segregates with sorghum leaf blight resistance in one cross is maternally inherited in another.

Abstract Leaf blight-resistant sorghum accession SC326-6 was crossed to the susceptible cultivar BTx623 to analyze the genetic basis for resistance. Field scoring of inoculated F2 progeny revealed that resistance was transmitted as a dominant single-gene trait. By combining the random amplified polymorphic DNA (RAPD) technique with bulked-segregant analysis, it was possible to identify PCR amplification products that␣segregated with disease response. Primer OPD12 amplified a 323-bp band (D12R) that segregated with resistance. Creation of longer primers, or SCARs (sequence characterized amplified regions) for D12R resulted in the amplification of a single major band of the predicted size from all the resistant F2 progeny and the resistant parent SC326-6, but not from BTx623 or 24 of 29 susceptible F2 progeny. The SCAR primers also amplified a single band with DNA from IS3620C, the female parent in a cross with BTx623 that has been used to produce a recombinant inbred population for RFLP mapping. An equivalent band was amplified from all 137 recombinant inbred progeny, indicating that organelle DNA is the amplification target in this cross.

Virulence and Molecular Genotyping Studies of Sporisorium reilianum Isolates in Sorghum

Head smut, caused by the fungal pathogen Sporisorium reilianum, has been reported with increasing frequency in the grain sorghum growing areas of Texas. To facilitate analysis of changes in pathogen virulence, four inoculation techniques were examined: soil and teliospore mixture, seed coating, media placement, and syringe injection. Of the four, syringe injection was determined to be the most effective. Inoculations of sorghum host differentials BTx643, BTx7078, BTx635, SC170-6-17 (TAM2571), SA281 (Early Hegari), and Tx414 showed 23 of 32 Texas isolates were race 4. Two isolates from College Station, TX, were classified as race 1, but no race 2 or 3 isolates were found. New, virulent races 5 and 6 were identified among isolates from south Texas. Using 16 amplified fragment length polymorphism (AFLP) primer combinations, genetic diversity was assessed in DNA samples from 49 S. reilianum isolates, including 44 sorghum isolates from Texas, two from Uganda, and one from Mali; and two maize isolates from Mexico. Single-base extensions with EcoRI and MseI primers in the selective amplification increased the number of informative polymorphic bands. High genetic dissimilarity (50%) was observed between isolates originating from maize and those originating from sorghum. The resultant dendrogram, made using cluster analysis, grouped the Texas S. reilianum isolates into four small clusters with ≥82% similarity. Other than for two race 6 isolates from Weslaco, TX, no evidence for geographical or other restrictions on gene flow was evident.

Development of an infection assay for Sporisorium reilianum, the head smut pathogen on sorghum

The practical value of dry teliospores of Sporisorium reilianum as inoculum in head smut infection assays was determined from a series of studies using three different isolates of the pathogen. Overall germinability of teliospores ranged from 8.0 to 28.0% on culture media after 48 h of incubation. Teliospores germinated at slower rates in the vicinity of sorghum seedlings growing on germination paper, reaching maximum values of 11.5 to 13.0% after 120 h. These results suggest adaptation in the germination strategy of S. reilianum, probably modulated by the variable soil environment. Fifteen percent (15%) of the seedlings of a head smut susceptible maize hybrid inoculated with teliospores of S. reilianum at root protrusion openings of the mesocotyl tissues became infected, while seedlings similarly inoculated with sporidial suspensions remained uninfected. Three sorghum lines susceptible to head smut were inoculated in three separate experiments with dry teliospores of isolates from Corpus Christi and Taylor, Texas. Seeds were imbibed for 18 h, and the seedlings were planted in soil at 40% moisture content (wt/wt) and covered with a 1:50 (vol/vol) mixture of teliospores and autoclaved soil. Infection levels obtained in these experiments were 2.5 to 2.8 times higher than field infection levels on susceptible lines RT×7078 and B1. In this study, average infection levels of 65 to 79.5% and 84 to 87% for RT×7078 and B1, respectively, were obtained with Corpus Christi isolates of S. reilianum; whereas Taylor isolates infected 91.9 and 82.3% of the plants in these two lines. It is postulated that the increased infection efficiency observed with this inoculation technique results from uniform and higher levels of inoculum, timely delivered under stable soil conditions that provide the pathogen with an environment more conducive for infection.