Marcia McMullen

Scab of wheat and barley: a re-emerging disease of devastating impact

cab can be a devastating disease affecting all classes of wheat and other small grains. This fungal disease, also called Fusarium head blight (FHB), has the ability to completely destroy a potentially high-yielding crop within a few weeks of harvest. Lush, green fields become blighted seemingly overnight (Figs. 1 and 2). Frequent rainfalls, high humidities, and/or heavy dews that coincide with the flowering and early kernel-fill period of the crop favor infection and development of the disease. Damage from head scab is multifold: reduced yields, discolored, shriveled “tombstone” kernels (Figs. 3 to 5), contamination with mycotoxins, and reduction in seed quality. The disease also reduces test weight and lowers market grade. Difficulties in marketing, exporting, processing, and feeding scabby grain are experienced. In North America, Fusarium graminearum Schwabe (teleomorph Gibberella zeae (Schwein.) Petch; synonym = G. saubinetti) predominates among several Fusarium species that can cause scab (4,5,8,40,48,60). Other species may predominate in cooler climates or where crops other than wheat and corn are dominant (8,40,48,60). F. graminearum also is associated with stalk and ear rot of corn and may cause a root rot of cereals. The fungus persists and multiplies on infected crop residues of small grains and corn. The chaff, light-weight kernels, and other infected head debris of wheat and barley, returned to the soil surface during harvest, serve as important sites of overwintering of the fungus. Continued moist weather during the crop growing season favors development of the fungus, and spores are windblown or water-splashed onto heads of cereal crops. Wheat and barley are susceptible to head infection from the flowering (pollination) period up through the soft dough stage of kernel development. Spores of the causal fungus may land on the exposed anthers of the flower and then grow into the kernels, glumes, or other head parts. Excellent descriptions of the disease cycle and spore stages of the causal fungi have been published (4,8,21,40,48). Mycotoxins are frequently associated with the growth and invasion of cereal grains by scab fungi. The most common toxin associated with F. graminearum– infected grain is vomitoxin (deoxynivalenol). Vomitoxin is known to cause vomiting and feed refusal in nonruminant animals and poses a threat to other animals and humans if exposure levels are high (45). The presence of mycotoxins in infected grain further exacerbates the losses that scab can cause. Recent articles have reviewed the epidemiology, management, and history of scab outbreaks in the United States, Canada, Europe, Asia, and South America (5,40,45,48). As these papers indicate, numerous research and survey reports have described the worldwide occurrence and epidemic levels of scab during the past century. Yield loss reports have not always been based on replicated research trials, but extensive surveys of producers’ fields have provided assessments of head blighting severity, which were translated into yield loss estimates. In the United States, scab was found in 31 of 40 states surveyed in 1917, with losses estimated at 288,000 metric tons (10.6 million bushels), primarily in Ohio, Indiana, and Illinois (4). Scab caused an estimated loss of 2.18 million metric tons (80 million bushels) of winter and spring wheat throughout the United States in 1919 (14). Extensive field surveys

Head blighting potential of Fusarium species associated with spring wheat heads

Seven Fusarium species isolated from spring wheat heads were tested for pathogenicity. Only F. graminearum and F. culmorum caused severe blighting of wheat heads following inoculation in the greenhouse. F. tricincium, F. sporotrichioides, F. equiseti, F. acuminalum, and F. poae did not blight heads but sometimes caused damage to inoculated spikelets.

Efficacy and Stability of Integrating Fungicide and Cultivar Resistance to Manage Fusarium Head Blight and Deoxynivalenol in Wheat

Integration of host resistance and prothioconazole + tebuconazole fungicide application at anthesis to manage Fusarium head blight (FHB) and deoxynivalenol (DON) in wheat was evaluated using data from over 40 trials in 12 U.S. states. Means of FHB index (index) and DON from up to six resistance class-fungicide management combinations per trial (susceptible treated [S_TR] and untreated [S_UT]; moderately susceptible treated [MS_TR] and untreated [MS_UT]; moderately resistant treated [MR_TR] and untreated [MR_UT]) were used in multivariate meta-analyses, and mean log response ratios across trials were estimated and transformed to estimate mean percent control ( ) due to the management combinations relative to S_UT. All combinations led to a significant reduction in index and DON (P < 0.001). MR_TR was the most effective combination, with a of 76% for index and 71% for DON, followed by MS_TR (71 and 58%, respectively), MR_UT (54 and 51%, respectively), S_TR (53 and 39%, respectively), and MS_UT (43 and 30%, respectively). Calculations based on the principle of treatment independence showed that the combination of fungicide application and resistance was additive in terms of percent control for index and DON. Management combinations were ranked based on percent control relative to S_UT within each trial, and nonparametric analyses were performed to determine management combination stability across environments (trials) using the Kendall coefficient of concordance (W). There was a significant concordance of management combinations for both index and DON (P < 0.001), indicating a nonrandom ranking across environments and relatively low variability in the within-environment ranking of management combinations. MR_TR had the highest mean rank (best control relative to S_UT) and was one of the most stable management combinations across environments, with low rank stability variance (0.99 for index and 0.67 for DON). MS_UT had the lowest mean rank (poorest control) but was also one of the most stable management combinations. Based on Piephos nonparametric rank-based variance homogeneity U test, there was an interaction of management combination and environment for index (P = 0.011) but not for DON (P = 0.147), indicating that the rank ordering for index depended somewhat on environment. In conclusion, although the magnitude of percent control will likely vary among environments, integrating a single tebuconazole + prothioconazole application at anthesis with cultivar resistance will be a more effective and stable management practice for both index and DON than either approach used alone.

Corn yield loss estimates due to diseases in the United States and Ontario, Canada from 2012 to 2015.

Annual decreases in corn yield caused by diseases were estimated by surveying members of the Corn Disease Working Group in 22 corn-producing states in the United States and in Ontario, Canada, from 2012 through 2015. Estimated loss from each disease varied greatly by state and year. In general, foliar diseases such as northern corn leaf blight, gray leaf spot, and Goss’s wilt commonly caused the largest estimated yield loss in the northern United States and Ontario during nondrought years. Fusarium stalk rot and plant-parasitic nematodes caused the most estimated loss in the southernmost United States. The estimated mean economic loss due to yield loss by corn diseases in the United States and Ontario from 2012 to 2015 was

Distribution and Severity of Pasmo on Flax in North Dakota and Evaluation of Fungicides and Cultivars for Management

76.51 USD per acre. The cost of disease-mitigating strategies is another potential source of profit loss. Results from this survey will provide scientists, breeders, government, and educators with data to help inform and prioritize research, policy, and educational efforts in corn pathology and disease management. M U E L L E R E T A L . , P L A N T H E A L T H P R O G R E S S 1 7 (2 0 1 6 )

MANAGING PLANT DISEASE RISK IN DIVERSIFIED CROPPING SYSTEMS

Pasmo, caused by Septoria linicola, reduces flax (Linum usitatissimum) yield in the Canadian provinces of Manitoba and Saskatchewan, but little is known about its distribution and effect on yield in North Dakota. Field surveys for pasmo were conducted in 74 and 87 flax fields across 19 and 23 North Dakota counties in 2002 and 2003, respectively. The surveys indicated that pasmo was present in 17 and 18 counties in 2002 and 2003, respectively. County mean plant incidences ranged from 0 to 21% and 0 to 84.5% in 2002 and 2003, respectively. County mean pasmo severity ranged from 0 to 38.8% and 0 to 29.3% in 2002 and 2003, respectively. Significant (P ≤ 0.07) positive Pearson correlations were detected between total rainfall accumulated for June to August and pasmo severity in 2002 and 2003 and for rainfall and pasmo incidence in 2003. Field trials were conducted to determine the effect of fungicides and flax cultivars on pasmo severity and flax yield. Pasmo severity was significantly (P ≤ 0.05) reduced with azoxystrobin and sulfur fungicides compared with the untreated control. Flax yields were significantly (P ≤ 0.05) greater in azoxystrobin- and prothioconazole-treated plots than in the untreated control plots. Cv. Omega had significantly lower pasmo severity than the other three cultivars, but cv. Rahab 94 had the greatest yield of all the cultivars. Based on the results presented, pasmo is an important disease of flax in North Dakota, and its distribution is widespread throughout the flax-production region. Fungicides such as azoxystrobin and prothioconazole appear to be excellent potential tools for pasmo management.