Estimates of plant disease severity are crucial to various practical and research-related needs in agriculture. Ordinal scales are used for categorizing severity into ordered classes. Certain characteristics of quantitative ordinal scale design may affect the accuracy of the specimen estimates and, consequently, affect the accuracy of the resulting mean disease severity for the sample. The aim of this study was to compare mean estimates based on various quantitative ordinal scale designs to the nearest percent estimates, and to investigate the effect of the number of classes in an ordinal scale on the accuracy of that mean. A simulation method was employed. The criterion for comparison was the mean squared error of the mean disease severity for each of the different scale designs used. The results indicate that scales with seven or more classes are preferable when actual mean disease severities of 50% or less are involved. Moreover, use of an amended 10% quantitative ordinal scale with additional classes at low severities resulted in a more accurate mean severity compared to most other scale designs at most mean disease severities. To further verify the simulation results, estimates of mean severity of pear scab on samples of leaves from orchards in Taiwan demonstrated similar results. These observations contribute to the development of plant disease assessment scales to improve the accuracy of estimates of mean disease severities.

Fusarium oxysporum is a large complex cosmopolitan species composed of plant pathogens, human opportunistic pathogens, and nonpathogenic isolates. Many plant pathogenic strains are known based on host plant specificity and the large number of plant species attacked. F. oxysporum is an opportunistic pathogen in humans with a compromised immune system. The objectives of this study were: (1) to develop a specific marker to detect human opportunistic F. oxysporum (HOFo) isolates; (2) to determine whether or not HOFo isolates can colonize and cause disease symptoms in plants; and (3) to assess Taiwan isolates sensitivity to two agro-fungicides. The primer pair, Primer 5/ST33-R, specifically amplifying Taiwan and international reference HOFo isolates was developed and used to detect and assess the distribution of a Taiwan isolate in inoculated tomato plants and tomato and cucumber fruit. Taiwan HOFo isolate MCC2074 was shown to colonize tomato roots, hypocotyls, and cotyledons, but did not show any visible symptoms. Four days after surface inoculation of tomato and cucumber fruit with the same isolate, MCC2074 was detected in the pericarp and locular cavities of both tomato and cucumber fruit and in columella of tomato fruit. Three Taiwan HOFo isolates were found to be moderately sensitive to azoxystrobin and highly sensitive to difenconazole.

Mango (Mangifera indica L.) is an economically important tropical fruit in southern Taiwan. In February 2019, new leaf blotches distinct from anthracnose lesions were noticed on mango leaves in Meinong, Kaohsiung (N22°54'43.7" E120°32'59.3"). Symptoms were circular to irregular lesions with easily torn centers and were cream to light brown with dark brown margin on both leaf surfaces. Similar symptoms were observed on mango leaves in Yujing, Tainan (N23°07'31.3" E120°27'18.2") in July of the same year. We surveyed the disease incidence on 60 mango trees consisting of three cultivars, 'Irwin', 'Yu-win No.6' and a native cultivar in a commercial farm by randomly examining five shoots of each tree. The disease incidences of 'Irwin', the native cultivar and 'Yu-win No.6' were 25%, 37% and 73%, respectively. Diseased tissues from the two locations were surface sterilized and incubated on potato dextrose agar (PDA) for pathogen isolation. Seven isolates (Mgk3, TMg2-2.2, TMg3-1.2, TMg3-2.1, TMg4-1, TMg6-3, and TMg8-1.1) from different locations and cultivars were selected for further study. Pycnidia were produced on 7-day-old PDA cultures. Conidiogenous cells were hyaline and short cylindrical phialides. Conidia were hyaline, aseptate, thick-walled, 20-24 × 11-16 μm, and subcircular to ellipsoid with 1-2 large oil droplets and a markedly flat protruding hilum at the base. These morphological features presenting in the seven isolates were identified as a member of Pseudoplagiostoma (Cheewangkoon et al. 2010). Pathogenicity assays were conducted by the point inoculation method on 10 intact young leaves growing on 'Irwin' mango plants in a greenhouse at 20-25℃. Each leaf was inoculated at six points on the abaxial surface with point inoculation. Each point was inoculated with a 10 μl conidial suspension (106 conidia/ml) of isolate TMg 8-1.1. Sterilized water was used as control. Shoots with inoculated leaves were covered with translucent plastic bags for 2 days. At 7 days post-inoculation (dpi), 70% of conidia inoculated points (n = 60) displayed symptoms resembling the field symptoms, but sterilized water inoculated points (n = 60) did not. The six other isolates were inoculated on detached leaves by the point inoculation method. All inoculated leaves were maintained in humid containers at 25℃ with 12 h light/dark regime, and displayed similar lesions at 7 dpi. Fungi re-isolated from the symptomatic leaves showed the same morphological characteristics observed in the Pseudoplagiostoma originally isolated from diseased tissues. Internal transcribed spacer (ITS), TUB2 and LSU gene sequences of the seven isolates (ITS accession nos.: MN818659 to MN818665; TUB2 accession nos.: MW415921 to MW415927; LSU accession nos.: MN876849 to MN876855) were amplified with primer sets ITS4/V9G, T1/Bt-2b and LSU1Fd/LR5, respectively, and used for molecular identification (Cheewangkoon et al. 2010). The seven isolate were 95.8%, 99-100% and 99.8% identical to Pseudoplagiostoma mangiferae Dayarathne, Phookamsak & K. D. Hyde KUMCC 18-0179 (the ex-type strain) for the ITS gene (MK084824), TUB2 gene (MK084823) and LSU gene (MK084825), respectively. Phylogenetic analysis based on concatenated sequences of ITS and LSU genes was performed by the Maximum Likelihood method. All seven isolates were clustered in a well-supported clade with P. mangiferae KUMCC 18-0179 with 100% bootstrap value. Based on the pathogenicity and morphological characteristics, the pathogen was identified as P. mangiferae which was reported as a new species associated with mango leaf blight in Yunnan, China (Bezerra et al. 2019; Cheewangkoon et al. 2010; Crous et al. 2012; Crous et al. 2018; Phookamsak et al. 2019; Suwannarach et al. 2016). The newly emerging leaf blotch may become a prevalent disease of mango in future.