INTRODUCTION In this Fourth Supplement (to NZ. Dep. sci. industr. Res. Bull, 10 1) are listed 95 diseases on 114 host plants. It includes new records, additional host records, and diseases recorded in publications since the preparation of the Third Supplement (Brien and Dingley 1957). New records include Alternaria dianthicola on Dianthus caryophyllus and Fusarium bulbigenum var. batatas on Ipomoea batatas, while additional host records contain Phytophthora cinnamomi on a number of host plants and Puccinia iridis on nine species of Iris. Diseases of some importance which have been recorded since the Third Supplement are Ustilago violaceae, Septoria passiflorae, shell-bark virus, citrus exocortis virus and citrus tristeza virus.

The probable races of bean rust present in New Zealand are 10, 17, and 28, as identified in United States of America, with possibly another unidentified race which attacks the differential line "U.S. 780".In crosses of the resistant variety Westralia with susceptible varieties, heterozygote plants show a delayed, but definite infection with rust in field and glasshouse trials. Homozygous susceptible plants show equal susceptibility with susceptible parents. Susceptibility can thus be termed recessive or dominant according to the time of recording the disease symptoms in segregating populations.The usefulness of this heterozygote reaction for selection in breeding material is discussed.