Development of a Semiselective Medium for Detection of Nalanthamala psidii, Causal Agent of Wilt of Guava

Abstract

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.

Importance of root infection in guava wilt caused by Nalanthamala psidii

“…psidii could survive in soil and cause wilt disease on guava trees via wounded roots, N . psidii ‐infested soil was prepared using the method described in previous studies (Huang et al ., ; Hong et al ., ). Agar plugs (5 mm diameter) containing mycelial mats of N .…”

“…psidii . The wound sites were surface sterilized with 75% ethanol for 10–15 s, excised, placed on modified sucrose glycine semiselective medium (mSGSSM) (Hong et al ., ) in Petri dishes (9 cm diameter) with six to eight pieces per dish, incubated at 30°C for 7 days and then examined for N . psidii .…”

“…After scoring for disease severity, tissues from trimming and pruning wounds on twigs and branches of diseased plants were collected for the isolation of N. psidii. The wound sites were surface sterilized with 75% ethanol for 10-15 s, excised, placed on modified sucrose glycine semiselective medium (mSGSSM) (Hong et al, 2013) in Petri dishes (9 cm diameter) with six to eight pieces per dish, incubated at 30°C for 7 days and then examined for N. psidii.…”

“…However, there are still no studies describing the ecology and infection of N. psidii in the field. In a previous study, N. psidii was more readily isolated from roots than from wounded twigs of wilted plants (Hong et al, 2013). These conflicting reports (Kurosawa, 1926;Lim & Manicom, 2003;Hong et al, 2013) suggest the need for further studies to verify the importance of N. psidii infection in belowground tissues versus aboveground tissues in guava orchards, in order to develop effective methods for the management of this disease.…”

“…In a previous study, N. psidii was more readily isolated from roots than from wounded twigs of wilted plants (Hong et al, 2013). These conflicting reports (Kurosawa, 1926;Lim & Manicom, 2003;Hong et al, 2013) suggest the need for further studies to verify the importance of N. psidii infection in belowground tissues versus aboveground tissues in guava orchards, in order to develop effective methods for the management of this disease. The objectives of this study were to investigate the infection ecology of N. psidii in naturally infested orchards and to determine whether N. psidii in artificially infested soil could serve as an inoculum for guava wilt disease.…”