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Potato virus B, and some other viruses with reactions in potato varieties different from any previously described, are strains of virus X. All produce intracellular inclusions which vary with different hosts and virus strains. Except with virus B, the inclusions are larger and more frequent in potato than in tobacco or tomato. All give systemic infection when inoculated to tobacco, tomato and potato varieties in which they are carried or cause mosaic symptoms; some give systemic infection when inoculated to varieties in which they cause top‐necrosis, whereas others give only local lesions. Potato virus C is a strain of virus Y: in tobacco and a few potato varieties both produce similar symptoms, but in those varieties in which Y causes leaf‐drop streak, C causes top‐necrosis. C causes systemic infection when inoculated to tobacco and to potato varieties in which it causes mosaic symptoms, but not when inoculated to potato varieties in which it causes top‐necrosis. Virus C was not transmitted by M. persicae. Viruses C and Y produce a few small intracellular inclusions in potato and tobacco. Virus A is not related to Y or X: no inclusions were found in plants infected with A alone.

In plants infected with leaf‐roll virus a type of phloem obliteration and necrosis occurs which is distinct from any abnormality produced by other pathogens or arising from physiological causes. The necrosis occurs in the primary phloem only of the bicollateral bundles. The affected tissue reacts with phloro‐glucinol in HCI. It was present in all of 179 plants of 33 varieties showing secondary leaf‐roll which were examined and was not found in any of 83 healthy plants of 20 varieties. The amount of necrosis varies in different plants. If the disease is severe, necrosis may extend to almost all parts of the plant except the stolon, tubers and roots. If the infection is mild it may be confined to a very few strands in two or three nodes near the base of the main stem. Phloem necrosis can always be found before leaf rolling is apparent. In primary leaf‐roll, slight necrosis can be found in the stem near the bases of the lowest rolled leaves and sometimes in the petioles. A technique is suggested for the use of this symptom in diagnosis.

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Extensive root disorganization is associatedwithall recognizable stages of the suddendeath disease of cloves, the final symptoms being those of a rapid wilt. In the early stages of the disease, however, the water status of an affected plant is more favourable than that of a healthy one, in spite of the root disorganization. In the diseased plant, the transpiring power and assimilation rate of the leaves are greatly lowered, probably because of partial closing of the stomata. The consequent reduction in transpiration appears to account for the smaller water deficit found in the earlier stages of the disease.These results are considered to furnish additional evidence in favour of the hypothesis that the disease is caused by a pathogen.