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Guava wilt, caused by Nalanthamala psidii, has become an important disease of guava (Psidium guajava) in Taiwan since the 1970s. This study was conducted to develop a semiselective medium for detecting N. psidii in soil and in tissues of diseased guava trees. Among 9 carbon and 21 nitrogen compounds tested in a modified Czapek-Dox medium, the most effective carbon and nitrogen sources for mycelial growth of N. psidii were sucrose and glycine, respectively. Among eight fungicides tested, iprodione at 5 μg ml–1 and azoxystrobin at 1 μg ml–1 were the most effective fungicides for detection of N. psidii in artificially infested soil or in naturally infected guava debris. Based on the requirement for carbon and nitrogen sources and response to fungicides, a semiselective medium designated as modified sucrose-glycine semiselective medium (mSGSSM) was developed for isolation of N. psidii, using the modified Czapek-Dox medium containing 3% sucrose, 0.3% glycine, iprodione at 5 μg ml–1, azoxystrobin at 1 μg ml–1, streptomycin at 200 μg ml–1, and neomycin at 200 μg ml–1. Colonies of N. psidii on mSGSSM at 30°C for 5 to 10 days were white to orange with sparse aerial hyphae. N. psidii was detected more accurately and efficiently on mSGSSM than on other media, including potato dextrose agar, modified Nash-Snyder medium, and modified Czapek-Dox medium. This semiselective medium is effective in detection of N. psidii from various parts of diseased guava trees and in soil; therefore, it would be a useful medium for etiological, ecological, and epidemiological studies of guava wilt.

Guava (Psidium guajava) wilt, caused by Nalanthamala psidii, has been a destructive disease in Taiwan, Thailand, Malaysia and South Africa since it was first reported in guava in 1926. However, the primary infection site of the pathogen remains controversial. This study was conducted to elucidate the importance of root infection by N. psidii in guava orchards in Taiwan. During 2010–2012, a total of 23 guava trees were sampled from three orchards and the isolation frequency of N. psidii from wounded twigs and roots was analysed. The overall isolation frequency of N. psidii was significantly higher (P < 0·01) from roots than trimmed and pruned twigs. Nalanthamala psidii was detected in 53·5% of the root samples from a guava tree showing mild symptoms of wilt and in 0·9% of the root samples from another guava tree without visible symptoms of wilt; however, no N. psidii was detected in the tissues of the trimming and pruning wounds in either tree. Cross sections of diseased guava branches showed that N. psidii colonized the lower part more heavily than the upper part of the branch. When guava trees were inoculated by applying N. psidii‐infested soil to injured roots, wilt symptoms developed within 6–13 months. Furthermore, N. psidii infection of guava seedlings via root contact with wilted guava trunks was recorded for the first time in Taiwan, demonstrating the importance of root infection in guava wilt disease.

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