Janice Y. Uchida

Molecular characterization of closteroviruses infecting Cordyline fruticosa L. in Hawaii

In Hawaii, common green ti plants (Cordyline fruticosa L.) have been shown to harbor Cordyline virus 1 (CoV-1) which, along with Little cherry virus 1 (LChV-1), and Grapevine leafroll-associated virus 7 (GLRaV-7), form a distinct clade within the family Closteroviridae. Preliminary work has indicated that, aside from CoV-1, three additional closteroviruses may infect common green ti plants in Hawaii. In this study, pyrosequencing was used to characterize the genomes of closteroviruses infecting a single common green ti plant. The sequence data confirmed the presence of CoV-1 as well as three additional closteroviruses. Although all four viruses had the same general genome organization, the sequence divergence between the RNA-dependent RNA polymerase, heat shock protein 70 homolog, and coat protein ranged from 22 to 61%, indicating these represent four distinct closterovirus species. The names CoV-2, CoV-3, and CoV-4 are proposed for the three new viruses. Phylogenetic analyses placed CoV-2, CoV-3, and CoV-4 in the same clade as CoV-1, LChV-1, and GLRaV-7.

Tracking down worldwide Puccinia psidii dispersal

Puccinia psidii causes rust disease on many host species in the Myrtaceae [1]. First reported in 1884 on guava in Southern Brazil [2], the rust has since been detected on several myrtaceous in South America, Central America, Caribbean, Mexico, USA: in Florida, California, and Hawaii. More recently, P. psidii was reported in Japan infecting M. polymorpha[3]. Of special note is that a rust was found infecting Myrtaceae species in Australia, where the fungus was reported as Uredo rangelii, based on the tonsure found on the urediniospores surface. However, DNA sequence data did not differentiate that rust from P. psidii[4], and the same tonsure patch, was also observed on rust urediniospores collected from several host species in Brazil [unpublished data].We have hypothesed that P. psidii was introduced into Hawaii through California by trade of rust infected myrtaceous plants, and that P. psidii populations from South America are distinct from the rust populations that became estabilished in California and Hawaii.

A Phytophthora palmivora extracellular cystatin-like protease inhibitor targets papain to contribute to virulence on papaya

Papaya fruits, stems, and leaves are rich in papain, a cysteine protease that has been shown to mediate plant defense against pathogens and insects. Yet the oomycete Phytophthora palmivora is a destructive pathogen that infects all parts of papaya plants, suggesting that it has evolved cysteine protease inhibitors to inhibit papain to enable successful infection. Out of five putative extracellular cystatin-like cysteine protease inhibitors (PpalEPICs) from P. palmivora transcriptomic sequence data, PpalEPIC8 appeared to be unique to P. palmivora and was highly induced during infection of papaya. Purified recombinant PpalEPIC8 strongly inhibited papain enzyme activity, suggesting that it is a functional cysteine protease inhibitor. Homozygous PpalEPIC8 mutants were generated using CRISPR/Cas9-mediated gene editing via Agrobacterium-mediated transformation (AMT). Increased papain sensitivity of in-vitro growth and reduced pathogenicity during infection of papaya fruits were observed for the mutants compared with the wild-type strain, suggesting that PpalEPIC8, indeed, plays a role in P. palmivora virulence by inhibiting papain. This study provided genetic evidence demonstrating that plant-pathogenic oomycetes secrete cystatins as important weapons to invade plants. It also established an effective gene-editing system for P. palmivora by the combined use of CRISPR/Cas9 and AMT, which is expected to be applicable to other oomycetes.

Fusarium species as pathogen on orchids

The recent surge in demand for exotic ornamental crops such as orchids has led to a rise in international production, and a sharp increase in the number of plant and plant products moving between countries. Along with the plants, diseases are also being transported and introduced into new areas. Fusarium is one of the major diseases causing pathogens infecting orchids that is spreading through international trade. Studies have identified several species of Fusarium associated with orchids, some are pathogenic and cause symptoms such as leaf and flower spots, leaf or sheath blights, pseudostem or root rots, and wilts. Infection and damage caused by Fusarium reduces the quality of plants and flowers, and can cause severe economic losses. This review documents the current status of the Fusarium-orchid interaction, and illustrates challenges and future perspectives based on the available literature. This review is the first of Fusarium and orchid interactions, and integrates diverse results that both furthers the understanding and knowledge of this disease complex, and will enable the development of effective disease management practices.

Differentiation and Distribution of Cordyline Viruses 1-4 in Hawaiian ti Plants (Cordyline fruticosa L.)

Common green ti plants (Cordyline fruticosa L.) in Hawaii can be infected by four recently characterized closteroviruses that are tentative members of the proposed genus Velarivirus. In this study, a reverse-transcription polymerase chain reaction (RT-PCR) assay developed to detect and distinguish Cordyline virus 1 (CoV-1), CoV-2, CoV-3, and CoV-4 was used to determine: (i) the distribution of these viruses in Hawaii; and (ii) if they are involved in the etiology of ti ringspot disease. One hundred and thirty-seven common green ti plants with and without ti ringspot symptoms were sampled from 43 sites on five of the Hawaiian Islands and underwent the RT-PCR assay. Eleven ornamental ti varieties were also sampled and assayed. Based on this survey, it appears none of the CoVs are involved in the etiology of ti ringspot. The observation of a non-uniform geographic distribution of the CoVs in common green ti, combined with the presence of CoVs in seed-derived ornamental varieties, suggests active vector transmission. Eight herbarium specimens collected between 1903 and 2003 from plants on the island of Oahu also underwent the RT-PCR assay. Amplifiable RNA was isolated from accessions collected in 1985 or later, however only the 2003 accession was found to harbor CoVs.

Plant Disease Prevention and Management in Sustainable Agricultural Systems

This chapter emphasizes the importance of devoting more efforts to create sustainable farming system for plant disease prevention and management in the tropics. We highlight sustainable pest management approaches that: (1) enhance high biological diversity through polyculture instead of the conventional preference of monoculture cropping systems; (2) increase ecosystem community stability by promoting natural enemies of multiple pests and pathogens; (3) stimulate inherent plant defenses; (4) improve plant health by maintaining nutrient cycling and energy flow; and (5) target vulnerable stages of a pest or pathogen through the understanding of their ecology. The use of therapeutic approaches, whether biological, chemical, or physical, is in violation of the five fundamental approaches for sustainable pest management listed above. The foundation for disease management in sustainable agricultural systems should be based on an understanding of the total agroecosystem composite of the biology of pathogens, host plant defenses, plant mixtures, soil, natural enemies, and timing of cultural practices to avoid creating a conducive environment for disease development. Successful examples of sustainable pest management strategies using these approaches are listed for key plant pathogens. While the literature review on sustainable pest management approaches for different groups of plant pathogens has been reviewed separately, future work in developing sustainable pest management should design approaches that can target multiple pests and plant pathogens concurrently through environmentally friendly and renewable strategies.