Gary N. Odvody

Ergot: a new disease threat to sorghum in the Americas and Australia.

Sorghum, Sorghum bicolor (L.) Moench, is the world’s fifth most important cereal crop, cultivated on about 45 million hectares for food, feed, beverage, and fodder. The most significant technological change since the 1960s has been the development and use of F1 hybrid seed (14), which has lead to a dramatic improvement in the crop’s productivity. Sorghum cultivation in intensive, commercialized systems where yields average 3 to 5 t ha-1 relies almost totally on hybrid seed. In contrast, yields vary widely and average less than 1 t ha-1 in low-input production systems.

First report of sorghum ergot caused by Claviceps africana in the United States.

In March 1997, ergot was found on sorghum (Sorghum bicolor (L.) Moench) regrowth in several abandoned commercial grain sorghum fields in Cameron and Hidalgo counties in the Lower Rio Grande Valley (LRGV) of Texas. White sphacelia in florets produced honeydew containing macrospores (hyaline, oblong to oval, 10 to 25 μm × 5 to 7 μm) and microspores (hyaline, spherical, 3 μm in diameter). Macrospores germinated iteratively to form secondary conidia when placed on water agar and in situ following rain. Secondary conidia were hyaline, pyriform, with a protruding hilum, and measured 10 to 17 μm × 5 to 7 μm. High-pressure liquid chromatography analysis detected the alkaloid di-hydroergosine in sphacelia, which is unique to C. africana (1). The pathogen was also confirmed on adjacent johnsongrass (S. halepense). The spread of ergot across Texas was associated with the progressive maturation of the commercial sorghum crop as follows: LRGV (mid-May), Coastal Bend near Corpus Christi (June), Winter Garden area southwest of San Antonio (July), and the seed production region of the Texas Panhandle (mid-August). Ergot incidence ranged from a trace to 10% of the heads in (self-fertile) grain sorghum fields of the LRGV. Most heads had only a few infected florets, but a few heads had 35 to 50% of the florets infected. Only trace amounts were found in grain sorghum fields in other areas of the state. Incidence and severity of ergot were greatest in fields of male-sterile sorghums grown for forage. Ergot was generally low in primary heads of male-sterile sorghums in hybrid seed production fields but, in the absence of pollen, axillary tillers sometimes developed high levels of ergot. The major impact of sorghum ergot is expected to be in hybrid seed production fields in the High Plains of Texas. Reference: (1) D. E. Frederickson et al. Mycol. Res. 95:1101, 1991.

Effects of Bacillus thuringiensis Transgenic Corn on Corn Earworm and Fall Armyworm (Lepidoptera: Noctuidae) Densities

Abstract We examined 17 pairs of near-isogenic hybrids of Bacillus thuringiensis (Bt) (176, Mon810, and Bt11) and non-Bt corn, Zea mays L., to examine the effects of Bt on larval densities of Helicoverpa zea (Boddie) and Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) during 2 yr. During ear formation, instar densities of H. zea and S. frugiperda were recorded for each hybrid. We found that H. zea first, second, and fifth instar densities were each affected by Mon810 and Bt11 Bt corn but not by 176 corn. Surprisingly, first and second instars were found in higher numbers on ears of Mon810 and Bt11 corn than on non-Bt corn. Densities of third and fourth instars were equal on Bt and non-Bt hybrids, whereas densities of fifth instars were lower on Bt plants. S. frugiperda larval densities were only affected during 1 yr when second, and fourth to sixth instars were lower on ears of Mon810 and Bt11 hybrids compared with their non-Bt counterparts. Two likely explanations for early instar H. zea densities being higher on Bt corn than non-Bt corn are that 1) Bt toxins delay development, creating a greater abundance of early instars that eventually die, and 2) reduced survival of H. zea to later instars on Bt corn decreased the normal asymmetric cannibalism or H. zea–S. frugiperda intraguild predation of late instars on early instars. Either explanation could explain why differences between Bt and non-Bt plants were greater for H. zea than S. frugiperda, because H. zea is more strongly affected by Bt toxins and more cannibalistic.

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.

A Description of Silk Cut, a Stress-Related Loss of Kernel Integrity in Preharvest Maize

Silk cut is an important recurring problem of many commercial maize (Zea mays) hybrids exposed to late-season drought stress in southern Texas. Silk cut is the preharvest occurrence of one or more lateral splits in the kernel pericarp that expose the kernel tissues and embryo to either pre- or postharvest attack by fungi and insects. It can occur as multiple lateral splits anywhere on the seed surface but its primary and most common occurrence is as a single, lateral split at the kernel edge on either side or both sides of the embryo. Individual splits range from nearly microscopic to those that almost encircle the seed. Silk cut can be initiated at kernel moistures as high as 50% but is more commonly initiated and observed at kernel moistures of 28% and lower. In yearly observations from 1986 to 1994, silk cut was highest in incidence and severity on normal-yield-potential maize crops exposed to rapidly increasing environmental stress (decreasing soil moisture and high soil and air temperatures) during the latter stages of maturity but especially after black-layer formation. Hybrids with open ear tips and loose husks were among those most vulnerable to silk cut but incidence and severity varied widely across years, sites, and stress environments. Incidence also varied widely between adjacent plants. Total silk cut (kernels with silk cut colonized by fungi and noncolonized) on 105 and 110 ears from consecutive plants of two vulnerable hybrids averaged 23 and 31% but ranged from 0 to 94 and 0 to 97%, respectively. Average difference in silk cut incidence between ears of adjacent plants of these hybrids was 28 and 29% and individual differences ranged from 0 to 93 and <1 to 97%, respectively. Kernels on the top one-third of the ear had a higher average incidence of silk cut (44%) than those in the middle (38%) or bottom (shank end) (31%) positions.

Simple sequence repeat markers useful for sorghum downy mildew (Peronosclerospora sorghi) and related species

BackgroundA recent outbreak of sorghum downy mildew in Texas has led to the discovery of both metalaxyl resistance and a new pathotype in the causal organism, Peronosclerospora sorghi. These observations and the difficulty in resolving among phylogenetically related downy mildew pathogens dramatically point out the need for simply scored markers in order to differentiate among isolates and species, and to study the population structure within these obligate oomycetes. Here we present the initial results from the use of a biotin capture method to discover, clone and develop PCR primers that permit the use of simple sequence repeats (microsatellites) to detect differences at the DNA level.ResultsAmong the 55 primers pairs designed from clones from pathotype 3 of P. sorghi, 36 flanked microsatellite loci containing simple repeats, including 28 (55%) with dinucleotide repeats and 6 (11%) with trinucleotide repeats. A total of 22 microsatellites with CA/AC or GT/TG repeats were the most abundant (40%) and GA/AG or CT/TC types contribute 15% in our collection. When used to amplify DNA from 19 isolates from P. sorghi, as well as from 5 related species that cause downy mildew on other hosts, the number of different bands detected for each SSR primer pair using a LI-COR- DNA Analyzer ranged from two to eight. Successful cross-amplification for 12 primer pairs studied in detail using DNA from downy mildews that attack maize (P. maydis & P. philippinensis), sugar cane (P. sacchari), pearl millet (Sclerospora graminicola) and rose (Peronospora sparsa) indicate that the flanking regions are conserved in all these species. A total of 15 SSR amplicons unique to P. philippinensis (one of the potential threats to US maize production) were detected, and these have potential for development of diagnostic tests. A total of 260 alleles were obtained using 54 microsatellites primer combinations, with an average of 4.8 polymorphic markers per SSR across 34 Peronosclerospora, Peronospora and Sclerospora spp isolates studied. Cluster analysis by UPGMA as well as principal coordinate analysis (PCA) grouped the 34 isolates into three distinct groups (all 19 isolates of Peronosclerospora sorghi in cluster I, five isolates of P. maydis and three isolates of P. sacchari in cluster II and five isolates of Sclerospora graminicola in cluster III).ConclusionTo our knowledge, this is the first attempt to extensively develop SSR markers from Peronosclerospora genomic DNA. The newly developed SSR markers can be readily used to distinguish isolates within several species of the oomycetes that cause downy mildew diseases. Also, microsatellite fragments likely include retrotransposon regions of DNA and these sequences can serve as useful genetic markers for strain identification, due to their degree of variability and their widespread occurrence among sorghum, maize, sugarcane, pearl millet and rose downy mildew isolates.

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.

Survival of Claviceps africana Within Sorghum Panicles at Several Texas Locations

Survival of the sorghum ergot fungus, Claviceps africana, based on pathogenicity of recovered macroconidia used to inoculate sorghum (Sorghum bicolor), was measured in 2000 over the course of the year at five locations in Texas representing three climates. The experiment was repeated in 2001. Sphacelia associated with infected sorghum panicles were placed in nylon mesh bags and either buried at a 10-cm depth, placed on the soil surface, or suspended 61 cm above the ground. Samples were recovered after 4, 8, and 12 months and assessed for pathogenicity of surviving macroconidia by macerating tissue in water and spraying it onto panicles of flowering male-sterile sorghum in the greenhouse. Survival of ergot macroconidia in recovered panicles declined at all locations after the first 4 months that panicles were left in the field. The decline in viability during this period was greater in 2001 than in 2000. In 2000, survival after 4 months was greatest at Lubbock and Bushland, which have a continental steppe climate, than at the other three Texas locations, Weslaco and Corpus Christi, which have a subtropical subhumid climate, and College Station, which has a subtropical humid climate. However, this difference in survival was not as pronounced in 2001. Additionally, after 8 months, survival levels at all locations were similar. At the end of 12 months, infective macroconidia were found only at Lubbock in 2000, and only at Lubbock and College Station in 2001. Ergot macroconidia can survive in all major sorghum production areas of Texas; thus, conidia would not need to move long distances in order to initiate an epiphytotic.

Disease Resistance in Sorghum

Over the past decades considerable progress has been made in describing and utilizing sources of resistance to many of the important diseases of sorghum. Much of the work was interdisciplinary, international and collaborative. Procedures for developing cultivars with resistance to variable pathogens such as those causing downy mildew, anthracnose and head smut as well as those diseases of sorghum with profound environmental interactions such as stalk rot and grain mold have been incorporated into the program. New sources of resistance have been introduced into production systems by the conversion of tall, late maturing, photosensitive sorghums to dwarf, early maturing insensitive genotypes. Monitoring of resistance occurs in disease prone environments world wide, either in the form of a Uniform Nursery or a combination of Uniform Nurseries (All Disease and Insect Nursery). Current strategies include the mapping of virulence-/avirulence traits in pathogens and the development of a genetic map of the sorghum genome with resistance traits linked with RFLP or PCR markers.

Spread of ergot of sorghum (Claviceps africana) in Central Mexico.

By late August 1997, sorghum ergot (Claviceps africana Frederickson, Mantle & De Milliano) had not been detected in the Bajio area in central Mexico, the second-largest sorghum (Sorghum bicolor (L.) Moench) producing area in the country, despite earlier 1997 reports of the disease in the adjacent states of San Luis Potosi, Michoacan, and Jalisco. A mid-September survey was conducted in el Bajio, primarily in the state of Guanajuato, and adjacent areas in the states of Michoacan and Jalisco. Infected sorghum heads showing ergot symptoms of honeydew and white secondary sporulation were observed in commercial grain and hybrid seed fields in all three states. Environmental conditions, late summer rains and early low temperatures promoting abundant dew, as well as extended periods of sorghum blooming, contributed to a low and delayed incidence of ergot in Guanajuato. In Michoacan and Jalisco the higher relative humidity and rainfall (around 750 ml) probably contributed to the observed epidemic of ergot. Johnsongrass (Sorghum halepense (L.) Pers.) florets also showed ergot symptoms. Macroconidia in honeydew were hyaline, oblong to oval, slightly constricted at the center, with an average size of 15 × 7 μm, agreeing with the given description of Sphacelia sorghi McRae (2), the anamorph stage of C. africana. No sclerotia were found on any host. Ergot control in this region of Mexico is being attempted by chemical means and burning of heads, even in commercial fields with minimal incidence of ergot. This report of ergot spread complements an earlier note describing the initial detection of the disease in Tamaulipas, the largest sorghum-producing state in Mexico (1). References: (1) J. Aguirre R. et al. Plant Dis. 81:831, 1997. (2) D. E. Frederickson et al. Mycol. Res. 95:1101, 1991.