Ceratobasidium sp. is associated with cassava witches' broom disease, a re-emerging threat to cassava cultivation in Southeast Asia.

Cassava witches' broom disease (CWBD) is a devastating disease of cassava in Southeast Asia (SEA), of unknown etiology. Affected plants show reduced internodal length, proliferation of leaves and weakening of stems. This results in poor germination of infected stem cuttings (i.e., planting material) and significant reductions in fresh root yields and starch content, causing economic losses for farmers and processors. Using a metagenomic approach, we identified a fungus belonging to the Ceratobasidium genus, sharing more than 98.3-99.7% nucleotide identity at the Internal Transcribed Spacer (ITS), with Ceratobasidium theobromae a pathogen causing similar symptoms in cacao. Microscopy analysis confirmed the identity of the fungus and specific designed PCR tests readily showed (1) Ceratobasidium sp. of cassava is strongly associated with CWBD symptoms, (2) the fungus is present in diseased samples collected since the first recorded CWBD outbreaks in SEA and (3) the fungus is transmissible by grafting. No phytoplasma sequences were detected in diseased plants. Current disease management efforts include adjustment of quarantine protocols and guarantee the production and distribution of Ceratobasidium-free planting material. Implications of related Ceratobasidium fungi, infecting cassava, and cacao in SEA and in other potential risk areas are discussed.

Cassava Witches' Broom Disease in Southeast Asia: A Review of Its Distribution and Associated Symptoms.

Cassava witches' broom disease (CWBD) is one of the main diseases of cassava in Southeast Asia (SEA). Affected cassava plants show reduced internodal length and proliferation of leaves (phyllody) in the middle and top part of the plant, which results in reduced root yields of 50% or more. It is thought to be caused by phytoplasma; however, despite its widespread distribution in SEA still little is known about CWBD pathology. The overarching goal of this study was to review and corroborate published information on CWBD biology and epidemiology considering recent field observations. We report the following: (1) CWBD symptoms are conserved and persistent in SEA and are distinct from what has been reported as witches' broom in Argentina and Brazil. (2) In comparison with cassava mosaic disease, another major disease of cassava in SEA, symptoms of CWBD develop later. (3) Phytoplasma detected in CWBD-affected plants belong to different ribosomal groups and there is no association study available indicating phytoplasma as the causing agent of CWBD. These findings are essential clues for designing surveillance and management strategies and for future studies to better understand the biology, tissue localization and spatial spread of CWBD in SEA and other potential risk areas.

Induction of aphid resistance in tobacco by the cucumber mosaic virus CMV∆2b mutant is jasmonate-dependent.

Cucumber mosaic virus (CMV) is vectored by aphids, including Myzus persicae. Tobacco (Nicotiana tabacum 'Xanthi') plants infected with a mutant of the Fny strain of CMV (Fny-CMVΔ2b, which cannot express the CMV 2b protein) exhibit strong resistance against M. persicae, which is manifested by decreased survival and reproduction of aphids confined on the plants. Previously, we found that the Fny-CMV 1a replication protein elicits aphid resistance in plants infected with Fny-CMVΔ2b, whereas in plants infected with wild-type Fny-CMV this is counteracted by the CMV 2b protein, a counterdefence protein that, among other things, inhibits jasmonic acid (JA)-dependent immune signalling. We noted that in nontransformed cv. Petit Havana SR1 tobacco plants aphid resistance was not induced by Fny-CMVΔ2b, suggesting that not all tobacco varieties possess the factor(s) with which the 1a protein interacts. To determine if 1a protein-induced aphid resistance is JA-dependent in Xanthi tobacco, transgenic plants were made that expressed an RNA silencing construct to diminish expression of the JA co-receptor CORONATINE-INSENSITIVE 1. Fny-CMVΔ2b did not induce resistance to M. persicae in these transgenic plants. Thus, aphid resistance induction by the 1a protein requires JA-dependent defensive signalling, which is countered by the CMV 2b protein.

The Effects of Cucumber Mosaic Virus and Its 2a and 2b Proteins on Interactions of Tomato Plants with the Aphid Vectors Myzus persicae and Macrosiphum euphorbiae.

Cucumber mosaic virus (CMV), a major tomato pathogen, is aphid-vectored in the non-persistent manner. We investigated if CMV-induced volatile organic compounds (VOCs) or other virus-induced cues alter aphid-tomato interactions. Y-tube olfactometry showed that VOCs emitted by plants infected with CMV (strain Fny) attracted generalist (Myzus persicae) and Solanaceae specialist (Macrosiphum euphorbiae) aphids. Myzus persicae preferred settling on infected plants (3 days post-inoculation: dpi) at 1h post-release, but at 9 and 21 dpi, aphids preferentially settled on mock-inoculated plants. Macrosiphum euphorbiae showed no strong preference for mock-inoculated versus infected plants at 3 dpi but settled preferentially on mock-inoculated plants at 9 and 21 dpi. In darkness aphids showed no settling or migration bias towards either mock-inoculated or infected plants. However, tomato VOC blends differed in light and darkness, suggesting aphids respond to a complex mix of olfactory, visual, and other cues influenced by infection. The LS-CMV strain induced no changes in aphid-plant interactions. Experiments using inter-strain recombinant and pseudorecombinant viruses showed that the Fny-CMV 2a and 2b proteins modified tomato interactions with Macrosiphum euphorbiae and Myzus persicae, respectively. The defence signal salicylic acid prevents excessive CMV-induced damage to tomato plants but is not involved in CMV-induced changes in aphid-plant interactions.

Modelling and manipulation of aphid-mediated spread of non-persistently transmitted viruses.

Aphids vector many plant viruses in a non-persistent manner i.e., virus particles bind loosely to the insect mouthparts (stylet). This means that acquisition of virus particles from infected plants, and inoculation of uninfected plants by viruliferous aphids, are rapid processes that require only brief probes of the plant's epidermal cells. Virus infection alters plant biochemistry, which causes changes in emission of volatile organic compounds and altered accumulation of nutrients and defence compounds in host tissues. These virus-induced biochemical changes can influence the migration, settling and feeding behaviours of aphids. Working mainly with cucumber mosaic virus and several potyviruses, a number of research groups have noted that in some plants, virus infection engenders resistance to aphid settling (sometimes accompanied by emission of deceptively attractive volatiles, that can lead to exploratory penetration by aphids without settling). However, in certain other hosts, virus infection renders plants more susceptible to aphid colonisation. It has been suggested that induction of resistance to aphid settling encourages transmission of non-persistently transmitted viruses, while induction of susceptibility to settling retards transmission. However, recent mathematical modelling indicates that both virus-induced effects contribute to epidemic development at different scales. We have also investigated at the molecular level the processes leading to induction, by cucumber mosaic virus, of feeding deterrence versus susceptibility to aphid infestation. Both processes involve complex interactions between specific viral proteins and host factors, resulting in manipulation or suppression of the plant's immune networks.