Tar Spot: An Understudied Disease Threatening Corn Production in the Americas.

Tar spot of corn has been a major foliar disease in several Latin American countries since 1904. In 2015, tar spot was first documented in the United States and has led to significant yield losses of approximately 4.5 million t. Tar spot is caused by an obligate pathogen, Phyllachora maydis, and thus requires a living host to grow and reproduce. Due to its obligate nature, biological and epidemiological studies are limited and impact of disease in corn production has been understudied. Here we present the current literature and gaps in knowledge of tar spot of corn in the Americas, its etiology, distribution, impact and known management strategies as a resource for understanding the pathosystem. This will in tern guide current and future research and aid in the development of effective management strategies for this disease.

Screening for Resistance to Sugarcane Brown Rust With Controlled-conditions Inoculation.

Breeding for resistance is the primary control measure for brown rust of sugarcane. Resistance screening utilizing natural infection symptom severity ratings provides erratic results. Therefore, a method accomplishing infection and disease expression under controlled conditions was evaluated to determine whether it could provide accurate resistance ratings for seedlings and clones with known and unknown reactions. Seedlings from crosses between parents with different levels of resistance were inoculated with increasing concentrations of urediniospores. Inoculum concentration affected disease severity and the frequency of resistant progeny in crosses. Brown rust resistance is a heritable trait; however, parental reaction was not a consistent determinant of progeny distribution across resistance rating categories. These results suggest that seedling inoculation may be misleading for the evaluation of brown rust resistance. Clone resistance reactions could not be reliably determined for susceptible clones in single inoculations. Ratings for controlled-conditions inoculation and field natural infection severity were not correlated. Multiple inoculations under controlled conditions accurately identified resistant and susceptible clones, with severe infection resulting from any single inoculation indicating susceptibility. Therefore, controlled-conditions inoculation has the potential to be useful in limited studies to characterize parents in a recurrent selection program and for basic studies of resistance to brown rust.

Variability in Puccinia melanocephala Pathogenicity and Resistance in Sugarcane Cultivars.

Brown rust, caused by Puccinia melanocephala, is an important disease of sugarcane worldwide, controlled primarily with host plant resistance. Disease response shifts from resistant to susceptible have been repeatedly observed for cultivars. However, information is limited concerning pathogen variability related to host reactions. To evaluate variability in the pathogen population and characterize resistance responses in different host genotypes, seven cultivars were inoculated with four urediniospore collections from three cultivars. Greenhouse-grown plants were inoculated under controlled conditions favorable for infection and disease development. Severity assessed as leaf area occupied by lesions, lesion density, and lesion size was determined and compared. Three cultivars that shifted from resistance to high susceptibility while under cultivation exhibited differential disease severity when inoculated with spore collections from two of the respective cultivars. Two cultivars exhibited consistent moderate to high levels of quantitative resistance against all spore collections and two cultivars, including one with the Bru1 resistance gene, were highly resistant to all collections. Differential reactions were best revealed by assessing percent leaf area. Pathogenic variability related to host genotype was confirmed, and quantitative resistance was detected that could be useful to improve breeding and selection for effective, durable resistance to brown rust.