First report of coffee leaf rust caused by Hemileia vastatrix on coffee (Coffea arabica) in Hawaii.

Coffee is one of the most economically valuable specialty crops for which Hawaii is famous. It is produced commercially on >6,900 acres across six islands by more than 1,470 growers. It has a raw crop value of $55.9 million, while the value-added benefits of coffee-related industries exceed $148.5 million (USDA, 2021). In addition to high product quality, Hawaii also has the distinction of being the last major coffee growing region that is free of coffee leaf rust (CLR), a highly damaging disease caused by the obligate parasitic fungus Hemileia vastatrix Berk. & Broome. On October 21, 2020, a coffee grower in Maui County, Hawaii reported the presence of coffee leaf rust-like symptoms on coffee plants (var. 'Typica') at their farm with many trees heavily defoliated. Foliar symptoms consisted of yellowish-orange, circular lesions that often coalesced. On abaxial surfaces, these lesions appeared powdery. Urediniospores were mostly reniform, (25)27-34(36) × (17)18-28(29) µm, strongly echinulated on the upper (convex) surface and smooth on the lower (concave) surface, with hyaline to pale yellow-orange walls. Urediniospores incubated in molecular grade water for 10 min at 95 oC served as template for PCR targeting the internal transcribed spacer region of fungi using primers ITS1/ITS2, ITS3/ITS4, and ITS1/ITS4 (White et al., 1990). Amplification products underwent direct Sanger-based sequencing. Following primer sequence trimming, the sequence reads were assembled using CAP3 (Huang and Madan, 1999) and deposited in GenBank (Accession MW228837). Blastn analysis revealed > 99% nucleotide identity with isolates of H. vastatrix from Mexico (eg. KX260251) and Brazil (eg. MF627828). A voucher material from which both the morphological and molecular assays were performed was deposited in the National Fungus Collection (BPI 924818). Subsequent surveys on the islands of Hawaii, Lanai, and Oahu revealed the presence of coffee plants with symptoms of coffee leaf rust. To confirm pathogenicity, urediniospores from a symptomatic plant growing in Holualoa, Hawaii, were collected in gelatin capsules using a G-R Electric Manufacturing Portable Vacuum Pump with a mini cyclone spore adapter. The concentration of spores was adjusted to 1 x 105 spores/ml in sterile water using a Brightline Hemocytometer. The spore suspension was brushed onto the abaxial side of leaves from two C. arabica var. 'Typica' plants using a camel hair paintbrush. Two control plants were mock-inoculated with sterile water. Plants were placed in a dark humid chamber set at 22 oC for 48 hours, after which it was adjusted to 12 hours light under cool white fluorescent lighting. After 20 days small, chlorotic spots were visible on the adaxial leaf surface and a few spots contained orange urediniospores on the abaxial surface. Lesions expanded by day 34, followed by necrosis of the center area of several leafspots and leaf abscission was observed at day 70. The experiment was conducted twice. H. vastatrix was confirmed by both morphological and molecular examination of urediniospores recovered from the observed lesions. All mock-inoculated control plants remained asymptomatic. To our knowledge, this is the first report of CLR on coffee in Hawaii. This discovery is of great concern since CLR threatens not only the yield and quality of Hawaii-grown coffee, but also the economic viability of this historic and culturally important industry. Statewide monitoring for CLR continues and further work is needed to mitigate the impact of this discovery through the development of short- and long-term management strategies.