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.

First Report of Spruce Needle Drop Associated with Setomelanomma holmii on Spruce in Illinois.

Spruce needle drop (also known as sudden needle drop of spruce, both referred to as SNEED) is a condition consistently associated with the fungus Setomelanomma holmii. While the pathogenicity of this fungus is still under investigation, it has been reported to cause the discoloration and needle drop of older needles on white spruce (Picea glauca) and Colorado blue spruce (P. pungens) (1). In the spring and summer of 2011, the University of Illinois Plant Clinic received several samples of Norway spruce (P. abies) and Colorado blue spruce branches that all exhibited the same symptoms: browning of older needles followed by needle drop. Solitary, black, globose, 105 to 230 µm diameter pseudothecia were observed on the upper surface of the branches and bud scales with a dissecting microscope. Many of the fruiting structures contained setae. When examined under a compound microscope, asci were bitunicate, 60 to 100 × 10.5 to 15 µm (mean 71.9 × 12.7 µm), and contained eight pale brown, three-septate, 15.5 to 22.5 × 5.5 to 9 µm (mean 19.4 × 5 µm) ascospores. On the basis of the asci and ascospores, the fungus was tentatively identified as S. holmii. To confirm the identification, single-ascospore colonies were cultured on acid potato dextrose agar. DNA was extracted from the mycelia of a single colony and amplified with PCR using universal fungal primers (2). The amplified DNA was sequenced and assembled using the primers NS1, NS3, NS5, and NS7 in the forward direction and NS2, NS4, NS6, and NS8 in the reverse direction (2). When the consensus sequence was compared with all accessions in the National Center for Biotechnology Information GenBank nucleotide collection, it had 100% homology to the S. holmii 18S ribosomal gene (Accession No. AY161121, isolated from a sample of P. pungens collected in France). S. holmii was first observed in the United States in 1998 (1). Although it has been tentatively identified on the basis of morphological characteristics in many states in the Midwest, this is the first time it has been conclusively identified in Illinois to our knowledge. Additionally, this is the first time Norway spruce has been confirmed as a host of S. holmii based on genetic sequencing in Illinois. Sporulation has been observed from mid-May through early August. Currently, the presence of S. holmii has been confirmed in 14 counties in central and northern Illinois. References: (1) A. Rossman et al. Can. J. Bot. 80:1209, 2002. (2) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al. eds. Academic Press, San Diego, CA, 1990.