A Virus Infecting Hibiscus rosa-sinensis Represents an Evolutionary Link Between Cileviruses and Higreviruses

Hibiscus (Hibiscus spp.) are popular ornamental and landscape plants in Hawaii which are susceptible to foliar diseases caused by viruses belonging to the genera Cilevirus and Higrevirus (family Kitaviridae). In this study, a virus infecting H. rosa-sinensis plants displaying foliar symptoms consistent with infection by a kitavirus, including yellow chlorotic blotches with a green perimeter, was characterized. The genome consisted of two RNAs 8.4 and 4.4 kb in length, and was organized most similarly to cileviruses, but with important distinctions. These included the location of the p29 homolog as the 3′-terminal open reading frame (ORF) of RNA2 instead of its typical locus at the 3′-end of RNA1; the absence of a p15 homolog on RNA2 and the adjacent intergenic region which also harbors small putative ORFs of unknown function; and the presence of an ORF encoding a 10 kDa protein at the 3′-terminal end of RNA1 that was also found to be present in the hibiscus green spot virus 2 genome. Spherical particles approximately 55–65 nm in diameter were observed in infected leaf tissue, and viral RNA was detected by reverse-transcription PCR in individual mites collected from symptomatic plants tentatively identified as Brevipalpus yothersi. Although phylogenetic analyses placed this virus between the higrevirus and cilevirus clades, we propose the tentative taxonomic placement of this virus, designated hibiscus yellow blotch virus (HYBV), within the genus Cilevirus.

Growth-inhibiting Fungicides Affect Detection of Phytophthora ramorum from Infected Foliage and Roots

Growth-inhibiting fungicides are used routinely to control common and regulated Oomycete pathogens. This study investigated whether such fungicides could affect detection of Phytophthora ramorum from plant tissue, both foliage and roots. Whole plants of Rhododendron × ‘Cunningham's White’ were inoculated with P. ramorum and treated 3 days later with fosetyl-Al, mefenoxam, or propamocarb. The foliage was sampled over time to see if fungicides prevented successful culturing of the pathogen from infected leaf tissue or interfered with detection using real-time PCR or ELISA. Mefenoxam significantly reduced the ability to culture the pathogen from leaves for the first 6 weeks while recovery from leaves treated with other fungicides did not differ from water-treated controls; detection using PCR or ELISA was not affected by fungicide application. The roots of Viburnum cuttings were inoculated with P. ramorum and then treated 4 days later with fosetyl-Al, mefenoxam, or propamocarb. The amount of inoculum in flow through water samples taken weekly for 5 weeks was quantified and percent root colonization determined at the end of the experiment. Propamocarb had no effect on inoculum production or root infection, while viable inoculum production was significantly decreased in fosetyl-Al- or mefenoxam-treated plants over 5 weeks, and root colonization was significantly decreased. Accepted for publication 23 January 2014. Published 18 March 2014.

Response of grapevine leaves to Plasmopara viticola infection by means of measurement of reflectance and fluorescence signals

Response of grapevine leaf tissue naturally infected byPlasmopara viticolain field was measured by means of chlorophyll fluorescence and reflectance signals. Three susceptible grapevine varieties (Cabernet Sauvignon, Pinot Blanc and Pinot Gris) were used in this study. Since the infection impairs photosynthetic activity, distribution of FV/FMparameter (maximum quantum yield of Photosystem II) over the leaf was effective to discriminate healthy and naturally infected leaf tissue. FV/FMwas reduced ~ 25% in all infected leaf parts. Infected leaf spots expressed significantly altered chlorophyll fluorescence induction kinetics expressing much slower electron transport rate both on donor and acceptor site of PSII. Vegetation reflectance indices followed the variations in pigment content after the fungal infection. R750/R700 (R2= 0.877) and CRI (carotenoid reflectance index; R2=0.735) were the most potent to follow changes in chlorophylls and carotenoids contents, respectively. Infected leaf tissue exhibited decrease in chlorophyll a (~50 %) as well as carotenoids (~70%). We conclude that combination of chlorophyll fluorescence and reflectance measurements can be used as an effective non-invasive tool for an early detection ofPlasmopara viticolain field as well as for estimation of the level of infection.

Colletotrichum gloeosporioides infection induces differential expression of glutathione S-transferase genes in Malva pusilla

Among a collection of 840 expressed sequence tags of Malva pusilla leaves infected with Colletotrichum gloeosporioides f. sp. malvae (Cgm), a total of four different glutathione S-transferase (GST) (EC 2.5.1.18) genes were identified, each showing a different pattern of expression following infection. MpGSTU1 and MpGSTU2 were members of the class tau GSTs, MpGSTF1 was a member of the class phi GSTs, and MpGSTZ1 belonged to the class zeta GSTs. Infection by Cgm occurs by a hemibiotrophic process with an initial biotrophic phase preceding the necrotrophic phase and the appearance of symptoms. Expression of MpGSTZ1 progressively increased during infection, corresponding directly with the growth of the pathogen. Expression of MpGSTU2 was similar to that of MpGSTZ1, except for a greater increase during the late necrotrophic phase. MpGSTU1 expression remained relatively constant throughout the infection, whereas MpGSTF1 expression was induced primarily during the conversion from the biotrophic to necrotrophic phases of infection. Incubation of healthy mallow leaves in the dark resulted in decreased expression of MpGSTF1 and MpGSTU1, but not MpGSTZ1 and MpGSTU2. The differential expression patterns indicate that these mallow GST genes play a variety of roles in healthy and fungal-infected leaf tissue. The nucleotide sequences reported in this paper have been submitted to GenBank under the accession numbers AY206003, AY206001, AY206002, AY206000 and AY205999.

Survival and Persistence of Alternaria dauci in Carrot Cropping Systems

Alternaria dauci was recovered in California from carrot crop residue and from volunteer carrot plants in fallow carrot fields. The fungus was not recovered from common weeds surrounding fallow fields. To evaluate further the survival of A. dauci on carrot crop residue, infected carrot leaf tissue was placed in fields or in soil in greenhouse pots, and recovered over time. In California, A. dauci was recovered from infected leaf tissue in both fallow and irrigated fields for as long as 1 year. In Florida, A. dauci was recovered from infected leaf tissue in fallow fields for up to 30 weeks. In greenhouse experiments, A. dauci was recovered from infected leaf tissue for as long as 1 year in dry soil, but only up to 30 weeks in soil that was watered weekly. To determine the infectivity of A. dauci borne on carrot crop residue, infected carrot crops were incorporated into organic and mineral field soils, and soil samples were collected over time. Carrot seed were planted in collected soil, and seedling infection by A. dauci was recorded. Seedling infection was detected up to 13 and 14 weeks after crop incorporation in organic and mineral soil, respectively. Seedling infection was detected for up to 5 weeks in soil that remained dry compared with 3 weeks in flooded soil.

A physiological study of Panicum maximum infected by Phyllachora paspalicola

Detailed morphological studies of the leaf pathogen Phyllachora paspalicola (tarspot) on its host Panicum maximum Jacq. have been previously described. This research reports physiological changes in leaf tissue infected by P. paspalicola. Aspects of photosynthetic and nitrogen metabolism were studied in healthy and tarspot-infected leaf tissue. Concentrations of glucose, fructose, and sucrose in infected and noninfected leaves were also measured. Gross and net photosynthetic rates declined significantly in tarspot-infected leaves at high disease intensities (>25%). Chlorophyll content declined in leaves exhibiting >25% infection. Transmission electron microscopy revealed disruption of lamellae and formation of large lipid bodies in infected mesophyll and bundle sheath cells. Significant changes in some C4 enzymes, such as NADP malate dehydrogenase, aspartate aminotransferase, ribulose-1,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylase, were noted in tarspot-infected leaves. Small differences in the activities of NADP–malate enzyme and alanine aminotransferase were also observed. Activity of the respiratory enzyme succinate dehydrogenase was also stimulated in intensely infected leaves. An increase in nitrate levels was measured as ascopore sporogenesis occurred. At sporulation and during postsporulation, total nitrogen declined in leaves exhibiting intense infection (>25%). A small increase in glucose and fructose content occurred in leaves exhibiting mature fruiting bodies, whereas a significant (p = 0.05) increase in sucrose content was observed in infected leaves.