L. J. Francl

Daily inoculum levels of Gibberella zeae on wheat spikes

The inoculum level of Gibberella zeae on wheat spikes was measured during 1995 and 1996 in nine locations of Canada and the United States prone to Fusarium head blight of wheat. Spikes were exposed after exsertion and until kernel milk or soft dough stage in fields with wheat or corn residue as a source of inoculum; other spikes were exposed in a location remote from any obvious inoculum source; and in 1995 only, control plants remained in a greenhouse. After 24 h, spikes were excised and vigorously shaken in water to remove inoculum. Propagules were enumerated on selective medium and identified as G. zeae from subcultures. Significantly more inoculum was detected from fields in epidemic areas than from remote sites in an epidemic and from fields in nonepidemic areas. The median inoculum level was 20 CFU of G. zeae per spike per day in fields experiencing an epidemic, 4 CFU in locations remote from epidemic fields, 2 CFU in nonepidemic fields, and 1 CFU in locations remote from a source of inoculum in non-epidemic areas. In an epidemic region, inoculum levels near corn stubble reached up to 587 CFU of G. zeae per spike per day, and the median inoculum level of 126 CFU was significantly higher than the median of 13 CFU found near wheat residue. Inoculum was not detected or occurred sporadically during extended dry periods. While inoculum increased during rainy periods, timing of increased levels was variable. Fusarium head blight epidemics were associated with multiple inoculation episodes and coincident wet periods.

Characterization of the ToxB gene from Pyrenophora tritici-repentis.

The ToxB gene was cloned and characterized from a race 5 isolate of Pyrenophora tritici-repentis from North Dakota. ToxB contains a 261-bp open reading frame that encodes a 23 amino acid putative signal peptide and a 64 amino acid host-selective toxin, Ptr ToxB. Analysis of Ptr ToxB from heterologous expression in Pichia pastoris confirms that ToxB encodes a host-selective toxin.

Population Race Structure of Pyrenophora tritici-repentis Prevalent on Wheat and Noncereal Grasses in the Great Plains

The fungus Pyrenophora tritici-repentis, cause of tan spot of wheat, is an important foliar pathogen worldwide. Genetic variation in the fungal population prevalent in the Great Plains was studied by analysis of 270 single-spore isolates of P. tritici-repentis recovered from wheat, durum, and 10 noncereal grasses: Alti wild rye, barnyard grass, crested wheatgrass, intermediate wheatgrass, needle and thread grass, quackgrass, smooth bromegrass, sand reedgrass, slender wheatgrass, and wild barley. The isolates were grouped into five known races based on necrosis and/or chlorosis induction on standard differentials with two additional wheat genotypes ND495 and M-3. The isolates recovered from wheat were races 1, 2, and 4, while those from durum were races 1 and 5. Isolates from noncereal grasses were all race 4, except for the recovery of two isolates of race 1 from smooth bromegrass. Race 3 was not found in this study. This is the first record of barnyard grass and slender wheatgrass as alternative hosts for P. tritici-repentis. The recovery from noncereal grasses suggests that the fungus has a fairly wide host range; however, predominance of a race that is avirulent on wheat on these grasses tends to eliminate their significance in the disease epidemiology of wheat. The results indicate that P. tritici-repentis has a diverse population on wheat and noncereal grasses. For durable resistance, wheat lines should be tested against all virulent races found in the field.

Role of Host Sensitivity to Ptr ToxA in Development of Tan Spot of Wheat.

ABSTRACT Pyrenophora tritici-repentis race 2 produces Ptr ToxA, a host-selective toxin previously described as a pathogenicity factor for tan spot on wheat. The objective of this research was to evaluate the role of host sensitivity to toxin, conditioned by a single dominant gene on chromosome 5BL, in the disease development by race 2. An F(2)-derived F(6) recombinant inbred population of 108 wheat lines, produced from crosses of toxin-sensitive, disease-susceptible cv. Kulm with the toxin-insensitive, disease-resistant cv. Erik segregated 1:1 for toxin reaction. However, the population was skewed toward resistance to race 2 of the fungus. Toxin reaction accounted for 24.4% of the genetic variance for disease. Heritability estimates suggested the presence of four to five genes that influence disease reaction in the population. Toxin-insensitive mutants, previously derived Kulm, were susceptible to race 2, although disease developed more slowly on the mutants than it did on the wild-type Kulm. The data indicate that sensitivity to Ptr ToxA influences disease severity in some host genotypes without defining susceptibility.

Reaction of Ptr ToxA-Insensitive Wheat Mutants to Pyrenophora tritici-repentis Race 1

ABSTRACT The host-selective toxin Ptr ToxA is produced by races 1 and 2 of Pyrenophora tritici-repentis, causal agent of tan spot of wheat. Ptr ToxA has been causally associated with pathogenicity by the race 2 phenotype in this system. However, the role of toxin in disease caused by race 1, the most prevalent form of the fungus in the central and northern Great Plains of North America, has not been rigorously investigated. Three independent wheat lines harboring mutations for insensitivity to Ptr ToxA were derived from ethylmethane sulfonate treatment of the hard red spring wheat cv. Kulm, possessing the single dominant gene for toxin sensitivity. Each of the three mutants was insensitive to Ptr ToxA in bioassays based on necrosis development and electrolyte leakage. Each mutant was crossed to each of the other mutants and to the wild-type Kulm. Segregation data indicate that each mutant line harbors a single recessive mutation for toxin insensitivity that maps to or near the same locus, possibly the toxin-sensitivity gene. Each toxin-insensitive mutant line was susceptible to two isolates of race 1 of P. tritici-repentis. F(2) and F(3) generations derived from crosses between Kulm and each mutant segregated for toxin reaction. However, segregation for fungal reaction was not evident, and all F(3) families were tan spot susceptible regardless of toxin reaction. Host insensitivity to Ptr ToxA is not necessarily equivalent to resistance to race 1. Ptr ToxA should not be used alone as a proxy for fungal inoculations by breeding programs aimed at developing tan spot-resistant wheat.

Neural network classification of Tan spot and Stagonospora blotch infection periods in a wheat field environment.

ABSTRACT Tan spot and Stagonospora blotch of hard red spring wheat served as a model system for evaluating disease forecasts by artificial neural networks. Pathogen infection periods on susceptible wheat plants were measured in the field from 1993 to 1998, and incidence data were merged with 24-h summaries of accumulated growing degree days, temperature, relative humidity, precipitation, and leaf wetness duration. The resulting data set of 202 discrete periods was randomly assigned to 10 modeldevelopment or -validation (n = 50) data sets. Backpropagation neural networks, general regression neural networks, logistic regression, and parametric and nonparametric methods of discriminant analysis were chosen for comparison. Mean validation classification of tan spot incidence was between 71% for logistic regression and 76% for backpropagation models. No significant difference was found between methods of modeling tan spot infection periods. Mean validation prediction accuracy of Stagonospora blotch incidence was 86 and 81% for backpropagation and logistic regression, respectively. Prediction accuracies of other modeling methods were </=78% and were significantly different (P = 0.01) from backpropagation, but not logistic regression, results. The best backpropagation models of tan spot and Stagonospora blotch incidences correctly classified 82 and 84% of validation cases, respectively. High classification accuracy and consistently good performance demonstrate the applicability of neural network technology to plant disease forecasting.

First Report of Pyrenophora tritici-repentis Race 5 from North America

Tan spot, caused by Pyrenophora tritici-repentis, is an important foliar disease of wheat worldwide. The fungus produces two distinct symptoms, necrosis (nec) and chlorosis (chl), on susceptible wheat genotypes. Fungal isolates have been grouped into five races based on their ability to induce necrosis and/or chlorosis on differentials Glenlea, Katepwa, 6B365, and Salamouni (1). Moreover, the isolates were designated on their ability to induce necrosis and chlorosis as follows: nec+chl+ (necrosis and chlorosis), nec+chl- (necrosis only), nec-chl+ (chlorosis only), and nec-chl- (neither symptom). Races 3 and 5 induce extensive chlorosis (nec-chl+) on 6B365 and Katepwa, respectively. Race 5 was reported on durum from North Africa. Races 1 to 4 were described from North America (1,2). During 1998, a survey of durum fields was conducted in the primary durum-growing area of North Dakota to assess the virulence pattern of P. tritici-repentis. Fifty-two single-spore isolates were obtained from diseased leaves. The isolates were evaluated for their virulence by inoculating them individually onto 15 seedlings of each wheat differential in the greenhouse. Forty-nine of 52 isolates were grouped as race 1 (nec+chl+) and three isolates, obtained from the Langdon Experiment Research Station, were grouped as race 5 (nec-chl+). Race 5 isolates were evaluated three times and consistently induced extensive chlorosis on Katepwa. This is the first report of the occurrence of race 5 outside of North Africa. This race may threaten wheat in the United States, so cultivars and germplasm should be evaluated for resistance. More isolates are under investigation to obtain a comprehensive virulence pattern of the pathogen population in the United States. References: L. Lamari and C. C. Bernier. Can. J. Plant Pathol. 11:49, 1989; (2) L. Lamari et al. Can. J. Plant Pathol. 17:312, 1995.

Neural Networks That Distinguish Infection Periods of Wheat Tan Spot in an Outdoor Environment

ABSTRACT Tan spot of wheat, caused by Pyrenophora tritici-repentis, provided a model system for testing disease forecasts based on an artificial neural network. Infection periods for P. tritici-repentis on susceptible wheat cultivars were identified from a bioassay system that correlated tan spot incidence with crop growth stage and 24-h summaries of environmental data, including temperature, relative humidity, wind speed, wind direction, solar radiation, precipitation, and flat-plate resistance-type wetness sensors. The resulting data set consisted of 97 discrete periods, of which 32 were reserved for validation analysis. Neural networks with zero to nine processing elements were evaluated 20 times each to identify the model that most accurately predicted an infection event. The 200 models averaged 74 to 77% accuracy, depending on the number of processing elements and random initialization of coefficients. The most accurate model had five processing elements and correctly predicted 87% of the infection p...

Source strength of wheat pathogens during combine harvest

Many fungal pathogens of plants are dispersedaerially long distances and by this meansestablish disease foci and redistribute races.The goal of this research was to measure thesource strength of wheat pathogens duringharvest. Two fields in North Dakota wereplanted with spring wheat cultivar 2375 andcombined from windrows. A Rotorod model 20 airsampler attached to a helium balloon was liftedto 30 m and activated when the debris cloudfrom the combine passed the sampler location.The sampler was positioned 100 to 200 mdownwind from the field edge and 100 m upwindof the combine in field one. In this field,concentrations of Puccinia triticinauredospores were 1663 spores/m3 downwindand 19 spores/m3 upwind; also,Cochliobolus sativus conidia were 732spores/m3 downwind and 32 spores/m3upwind. In field two, downwind samples weretaken at heights of 6 m and 30 m on the fieldedge. In this field P. triticina sporeconcentration was 840 spores/m3 downwind.Also in field two, conidia of Pyrenophoratritici-repentis and teliospores ofUstilago sp. were each observed atconcentrations of 9 spores/m3. Many sporesappeared desiccated, but C. sativusspores germinated after 24 h in a moistchamber. Spore concentrations at the fieldmargin meant a minimal estimate of 10 billionspores were dispersed from the field per hourof combine operation; therefore, wheatharvesting liberates immense numbers of fungalpathogen spores, many of which then can bedispersed long distances in wind currents.