L. M. Reid

Interaction of Fusarium graminearum and F. moniliforme in Maize Ears: Disease Progress, Fungal Biomass, and Mycotoxin Accumulation

ABSTRACT To investigate the interaction between two major ear-rotting pathogens, maize ears were inoculated with either Fusarium graminearum, F. moniliforme, or an equal mixture of the two. Silk and kernel tissues were periodically harvested throughout the growing season so that a time course of the experimental variables (disease severity, ergosterol content, fungal DNA content, and mycotoxin concentration) could be recorded. Over the 3 years tested (1992 to 1994), the highest levels of disease and ergosterol were found in the F. graminearum treatment, followed by the mixture treatment (F. graminearum plus F. moniliforme) and, finally, the F. moniliforme treatment. Kernel ergosterol content and disease rating were correlated for both pathogens, but the highest correlation coefficients were obtained in the F. graminearum treatment. The DNA analysis revealed that, in the mixed inoculum, F. moniliforme had a greater growth rate than did F. graminearum. In 1994, appreciable F. moniliforme from natural inoculum was found in the F. graminearum treatment. Fumonisin B(1) levels did not differ between the F. moniliforme treatment and the mixed inoculum treatment. The effect of temperature on the growth rate of the two species explained some of the field results, with temperatures in the silks being more favorable to F. moniliforme. Data on the growth rate on silks obtained by the incorporation of radiolabeled precursor to ergosterol demonstrated that F. graminearum was able to grow well at 26 to 28 degrees C, whereas F. moniliforme grew well over a broader range, including at higher temperatures.

Effect of inoculation time and point of entry on disease severity in Fusarium graminearum, Fusarium verticillioides, or Fusarium subglutinans inoculated maize ears1

To determine if differences exist in the ability of three Fusarium species (F. graminearum, F. verticillioides, and F. subglutinans) to infect maize ears as the silks and kernels mature, one moderately resistant and two susceptible hybrids were inoculated at two points of entry (silk channel and kernels) in 1994 and 1995. Inoculations were conducted nine times for each part of the ear starting from silk emergence. For all three species, the greatest silk channel inoculated disease severities occurred when the ears were inoculated in the early stages of silk development, with a peak in susceptibility around 1–6 days after silk emergence, followed by a rapid decrease in severity. With kernel inoculations, a general decrease in disease severity occurred with time for all species. Fusarium verticillioides had the lowest disease severity of the three species. With silk channel inoculations, F. subglutinans resulted in higher disease severity than F. graminearum; however, the opposite was found with kernel inoculations, with F. graminearum producing the greatest amount of disease symptoms.Key words: Fusarium graminearum, Fusarium verticillioides, Fusarium subglutinans, maize, corn, inoculation, disease severity.

A Mathematical Simulation of Growth of Fusarium in Maize Ears After Artificial Inoculation

ABSTRACT Fusarium spp. in maize can contaminate the grain with mycotoxins if environmental conditions are favorable for fungal growth. To quantify the relationship between growth of Fusarium spp. and environmental conditions, a mathematical model was developed to simulate growth of F. graminearum and F. verticillioides on maize ears following silk inoculation in field experiments from 1992 to 1995. Each species was inoculated separately and as a mixture of the two for 3 of the 4 years on one maize hybrid. Disease progress in ears was measured by a visual rating scale that was converted to percent visual infection. Measurements were made at regular time intervals after silks were inoculated 5 days after silk emergence. Differential equations were used to relate growth rates of Fusarium spp. in maize ears to hourly air temperature and relative humidity and to daily precipitation. Integration of these equations over time produced quantitative estimates of fungal growth. Model calculations compared well with measurements (R(2) = 0.931, standard error of estimate [SEE] = 2.11%) of percent visual disease infection of maize ears over 3 years. The model was tested against a second set of data (R(2) = 0.89, SEE = 5.9%) in which silks were inoculated at nine different times after first silk emergence for each of 2 years (1994 and 1995) with the two species of fungi on the same maize hybrid. At this time, a silk function was developed to account for changes in the susceptibility of silks to disease. F. graminearum responded to wet conditions more than F. verticillioides, and for the conditions of this experiment, grew much faster than F. verticillioides when inoculated separately. When they were inoculated together, F. graminearum growth rates were much lower, indicating some interference by F. verticillioides. During 1993, weather conditions before inoculation reduced the growth of both species in silks.

Effects of inoculation position, timing, macroconidial concentration, and irrigation on resistance of maize to Fusarium graminearum infection through kernels

To standardize a kernel wound inoculation technique for evaluation of maize genotypes for resistance to gibberella ear rot, four parameters associated with its application were investigated: inoculation position, time of inoculation after silk emergence, macroconidial concentration, and irrigation. Two moderately resistant (Pride K127 and Funk G4106) and two susceptible (Pioneer 3953 and Dekalb DK-415) hybrids were inoculated through the husk by wounding 3-4 kernels in the centre of the ear with four stainless steel pins that had previously been dipped in a macroconidial suspension of Fusarium graminearum. Disease severity was determined at harvest. In four experiments conducted in 1993 and 1994, ears were inoculated using three inoculation positions (tip, centre, and butt of the ear); seven conidial suspension concentrations ranging from 0 to 2 X 10(6) conidia/mL; 12 (1993) and 9 (1994) different times of inoculation, ranging from 0 to 44 days post-silk emergence; and wounding and a silk channel injection techniques under irrigated and nonirrigated conditions. Disease severity was highest with inoculations made in the centre of the ear. Disease severity increased with increasing conidial concentrations but decreased with increasing kernel/silk age. Irrigation tended to lower disease severity and did not significantly enhance differentiation between genotypes. Best differentiation between genotypes was obtained when inoculations were made in the centre of the ear at approximately 15 days post-silk emergence and with a suspension of at least 10(5) conidia/mL suspension.

Crop rotation and nitrogen effects on maize susceptibility to gibberella (Fusarium graminearum) ear rot

An experiment was established in 1992 in eastern Ontario, Canada to determine the effects of crop rotation (continuous maize, soybean-maize and alfalfa-maize) and nitrogen (N) amendment [0, 100 and 200 kg N ha−1 of fertilizer (NH4NO3), and 50 and 100 Mg ha−1 (wet wt.) each of stockpiled and rotted dairy manure] on maize production and soil properties. From 1997 to 1999, an additional study was added to the experiment to investigate treatment effects on the susceptibility of maize hybrids to gibberella ear rot. A moderately resistant and a susceptible hybrid were planted in each plot and inoculated with a macroconidial suspension of Fusarium graminearum by both the silk channel injection and the kernel-wound techniques. At harvest, ears were rated for the severity of disease symptoms and harvested kernels were analyzed for the mycotoxin deoxynivalenol (DON). The greatest number of significant N effects were found in the continuous maize treatments and with the susceptible hybrid. Most N amendments decreased both disease severity and DON accumulation in the susceptible hybrid. The most consistent effect was a decrease in disease severity with 100 kg N ha−1 fertilizer and an increase in disease severity with the higher rate of 200 kg N ha−1. This study is the first to report on the effects of soil N amendments on gibberella ear rot susceptibility.

Effect of macroconidial suspension volume and concentration on expression of resistance to Fusarium graminearum in Maize

The effects of volume and concentration of macroconidial suspensions applied to maize silks on resistance to Fusarium graminearum infection were investigated on three maize hybrids (Pride K127, Pioneer 3953, and Dekalb DK-415). Plants were inoculated in the field by injecting a spore suspension into the silk channel of individual ears. Disease severity and deoxynivalenol (DON) concentration in grain were determined at harvest. In three experiments, plants were inoculated with 1) six conidial suspension volumes ranging from 0,5 to 10 ml in 1991 and 1992; 2) seven spore concentrations ranging from 0 to 2 × 10 6 spores per milliliter in 1990, 1991, and 1992; and 3) three spore concentrations (2 × 10 5 , 5 × 10 5 , and 2 × 10 6 spores per milliliter) inoculated at six silk ages (1, 3, 5, 7, 12, and 18 days after silk emergence) in 1991 and 1992. On average, Pride K127 was most resistant, followed by Pioneer 3953 and Dekalb DK-415. Disease severity, and in most cases DON, increased with increased conidial suspension volume and concentration, with greater effects in susceptible hybrids. Disease severity decreased with silk age over all spore concentrations. When screening for resistance to infection via the silk, 2 ml of a 5 × 10 5 spores per milliliter suspension injected into the silk channel at approximately 6 days after silk emergence should give adequate differentiation of genotypes

Comparison of techniques for inoculating maize silk, kernel, and cob tissues with Fusarium graminearum

Six inoculation techniques differing in the method of application of a conidial suspension and in the part of the ear inoculated were evaluated for their effectiveness in assessing maize (Zea mays) resistance to ear rot caused by Fusarium graminearum. Silk channel injection and ear-tip flooding inoculation techniques were carried out 7 days after silk emergence. The other four techniques (wound-spray, kernel-stab, pipe cleaner, and cob-tip) were carried out 15 days after silk emergence. A 7-class rating scale was used to assess disease severity at harvest. Significant differences (P < 0.05) in incidence and severity of ear rot symptoms were detected among the inbred lines and inoculation techniques. There were significant inbred x inoculation technique interactions, but inoculation techniques intended to measure the same resistance mechanism ranked inbred lines similarly in three of the four environments. All inoculation techniques except the ear-tip flooding technique identified CO325 as the most resistant inbred. Among the techniques used, the silk channel and the kernel-stab techniques appeared to be the most effective in measuring silk and kernel resistance, respectively.

Stability of maize resistance to the ear rots caused by Fusarium graminearum and F. verticillioides in Argentinian and Canadian environments

Sources of resistance to Fusarium spp. are needed to develop maize hybrids resistant to the accumulation of fungal mycotoxins in the grain. In a search for resistant germplasm in 1999 and 2000, a set of Argentinian maize populations was evaluated in Ottawa, Canada, for resistance to ear rots after inoculation with local isolates of Fusarium verticillioides and F. graminearum. Sixteen of these populations, varying in observed resistance levels, were re-evaluated in 2003 and 2004 in Pergamino, Argentina, using local isolates of the same fungi. Conidial suspensions of each fungal species were inoculated into the silk channel of primary ears. Disease severity was assessed after physiological maturity using a scale based on the percentage of visibly infected kernels. Genotype effect was more important than genotype-by-fungal species or genotype-by-fungal species-by-environment interaction effects. In addition, disease severity levels associated with each fungal species were positively correlated (P < 0.05) (r = 0.90, r = 0.81, r = 0.87 and r = 0.53, in Ottawa 1999 and 2000, and Pergamino 2003 and 2004, respectively). Populations ARZM 01107, ARZM 07138, ARZM 10041, ARZM 13031, ARZM 16002 and Pora INTA exhibited the highest and most stable resistance to both species. Considering that disease resistance exhibited low specificity to the environment and to the fungal species in evaluations conducted in a wide range of environments and with fungal isolates collected from different hemispheres, the most resistant populations are potential sources of genes for stable resistance to these Fusarium spp.

CO441 corn inbred line

CO441 is a short-season corn inbred line with improved resistance to silk infection by Fusarium graminearum. Resistance to infection via wounded kernels is also high in CO441. This inbred exhibits excellent combining ability and average to superior lodging resistance in hybrids. Key words: Corn, maize, Zea mays, cultivar description, ear rot, Fusarium

Fumonisin B1 accumulation and severity of fusarium ear rot and gibberella ear rot in food-grade corn hybrids in Ontario after inoculation according to two methods

Fusarium verticillioides and Fusarium graminearum are pathogens of corn causing fusarium ear rot and gibberella ear rot, respectively. The mycotoxins fumonisin B1 (FB1) and deoxynivalenol (DON) are produced by F. verticillioides and F. graminearum, respectively, and commercial millers monitor the levels of these toxins in corn to avoid problems with contamination in finished foods. The objectives of the present study were: (1) to evaluate the susceptibility of corn hybrids to F. graminearum and F. verticillioides, following silk-channel or kernel-wounding inoculations, (2) to evaluate the variability in FB1 production among food-grade corn hybrids, following silk or wound inoculation with F. verticillioides, and (3) to determine the association between the severity of fusarium ear rot and FB1 to see if visual ratings of fusarium ear rot are predictive of the tendency of corn hybrids to accumulate FB1. The experiments were conducted in the field from 1999 to 2002 with different sets of commercial hybrids each year. Mean fusarium ear rot severity and FB1 accumulation varied among years regardless of the inoculation method. Severity of fusarium ear rot and gibberella ear rot (rating on a scale of 1–7) varied respectively from 1.5 to 2.7 and 1.8 to 3.9, with silk inoculation, and from 2.0 to 2.3 and 2.8 to 4.2, with wound inoculation. For a same inoculation method, hybrids responses differed between F. graminearum and F. verticillioides inoculations, likely influenced by the different environmental conditions favoured by each fungus. There was a high level of variation in FB1 accumulation among the hybrids within each year; for example, FB1 accumulation across hybrids grown under the same environmental conditions ranged from not detected to 211.2 ppm in 2001. Severity of fusarium ear rot and FB1 accumulation after wound inoculation with F. verticillioides were correlated, suggesting that severity symptoms of fusarium ear rot after wound inoculation might be useful to select hybrids that are less likely to accumulate FB1.