George H. Duncan

Effect of humidity on infection of rose petals by dry-inoculated conidia of Botrytis cinerea

Cut flowers of rose cv. Scarlet Pimpernel were dusted with dry conidia of Botrytis cinerea from 10- to 14-d-old cultures and incubated at 15 °C in a range of humidities controlled to within 2% relative humidity (r.h.). Petals developed lesions within 24 h at, or above 94% r.h. of the air entering the controlled environment chamber containing rose flowers in the absence of water droplets, and necrosis was severe after 48 h. Petals were cleared, stained in aniline blue and viewed by fluorescence microscopy to show that conidia germinated to form only short germ tubes. They lacked appressoria or any other complex infection apparatus and colonized petals extensively within 48 h. Low temperature scanning electron microscopy confirmed that the conidia germinated in the absence of surface water and penetrated the epidermis without the formation of appressoria or evident external surface changes in the cuticle around germ tubes at the point of penetration of host cells. In freeze-fractures of infected petals, the apices of fungal hyphae caused no apparent deformation of host cell walls during penetration of, or emergence from, the mesophyll cells, indicating that cell wall-degrading enzymes are secreted and retained at the hyphal tip.

Satellite-like Properties of Small Circular RNA Molecules in Particles of Lucerne Transient Streak Virus

Summary The isometric particles of lucerne transient streak virus (LTSV) contain linear ssRNA of mol. wt. approx. 1.4 × 106 (RNA-1) and ssRNA of mol. wt. approx. 1.2 × 105 in both a linear and circular form (RNA-2). Unfractionated LTSV RNA induced necrotic local lesions in leaves of Chenopodium amaranticolor whereas RNA-1, partially separated from RNA-2 by gel electrophoresis, induced many chlorotic local lesions but few necrotic ones. Cultures obtained from either lesion type continued to induce only chlorotic (C isolate) or necrotic lesions (N isolate) on subsequent passage. Apart from the lesion type in Chenopodium species, the isolates were indistinguishable except that particles of isolate N contained both RNA-1 and RNA-2 whereas those of isolate C contained RNA-1 but no RNA-2; RNA-2 was also not detected in leaves inoculated with isolate C. RNA-2 alone did not infect C. amaranticolor but when it was added to inocula of RNA from isolate C, a proportion of the lesions induced were necrotic and this proportion increased with increasing concentration of RNA-2. The infectivity of RNA-1 was destroyed by treatment with snake venom phosphodiesterase or proteinase K but the ability of RNA-2 to alter the lesion type induced by isolate C was unaffected by these treatments, suggesting that the molecules of RNA-2 are biologically functional and do not need a genome-linked protein for this activity. These results suggest that RNA-2 found in particles of LTSV is not part of the virus genome but may be a satellite RNA that affects symptom expression.

Cecidophyopsis mites (Acari: Eriophyidae) on Ribes spp. (Grossulariaceae)

Abstract The authors present a review and morphological study of the eriophyid mites in the genus Cecidophyopsis occurring on gooseberries and currants of the genus Ribes (Grossulariaceae). Three species are redescribed and illustrated: Cecidophyopsis ribis (Westwood 1869) causing galling on black currants, Ribes nigrum; C. selachodon Eyndhoven 1967 causing galling on red currants, R. rubrum; and C. grossulariae (Collinge 1907), a foliar and bud mite on gooseberries, R. grossularia, and red and black currants. Two new species are described: C. aurea n. sp., causing bud galls on golden currants, R. aureum; and C. alpina n. sp. causing bud galls on alpine currants, R. alpinum. Biological notes, character tables and a key are presented for separating the five species.

Diffraction Studies of Tulip Virus X Particles

Summary X-ray diffraction from oriented tulip virus X (TVX) particles and optical diffraction from electron micrographs show that the virus particles have a helical structure with a pitch of about 3.25 nm and a true repeat in five turns of the helix. Both X-ray and optical diffraction indicate that the number of subunits in the repeat period is 5q + 4, where q is an integer. The X-ray diffraction patterns suggest that 9 ⩾ q ⩾ 6, and the optical diffraction patterns suggest that q is probably 7 or 8. The X-ray diffraction patterns indicate that there is a marked feature in the virus particle at a radial position of about 3.3 nm, which is probably the RNA. The equatorial diffraction further suggests that the virus has an axial hole of about 1.5 nm radius.

ANTIGENIC ANALYSIS OF NEMATODE-TRANSMISSIBLE AND NON-TRANSMISSIBLE ISOLATES OF TOBACCO RATTLE TOBRAVIRUS USING MONOCLONAL ANTIBODIES

Murine monoclonal antibodies (MAbs) were produced against a nematode non-transmissible isolate of tobacco rattle tobravirus (TRV-PLB). Two of the MAbs (SCR 80 and 81) did not react with the serologically closely related isolate TRV-PPK20 (both isolates belong to the PRN serotype), which is readily transmissible by nematodes. When further isolates of the PRN serotype were tested, all the non-transmissible ones reacted with these two MAbs, but so also did some of those that are readily transmissible. SCR 80 and 81 were able to trap TRV-PLB particles onto electron microscope grids and reacted with metatopes located near the C terminus of the particle protein. The epitope recognized by SCR 81 was discontinuous. The MAbs that reacted with both TRV-PLB and TRV-PPK20 recognized either continuous cryptotopes (SCR 78) or discontinuous neotopes (SCR 79 and 82).