Development of a single uredinium inoculation method for Puccinia kuehnii, the causal agent of sugarcane orange rust

Abstract

The use of resistant varieties is the ideal method to control rusts. Nevertheless, knowing the pathogen’s diversity is fundamental to the success of this measure. Diversity can be analyzed phenotypically and/or genotypically. For phenotypic diversity, the reaction of genotypes is assessed by means of inoculations of the pathogen generally obtained from several uredinia. One handicap of this technique is its impossibility to detect diversity among these uredinia, assuming that they are all homogenous. Therefore, the aim of the present study was to develop a single uredinium technique for Puccinia kuehnii to be used in studies of rust diversity in sugarcane. The comparison between the two inoculation methods was done by employing urediniospores from SP89-1115 on the varieties SP89-1115 (susceptible) and RB975201 (resistant). The adopted design was completely randomized with five and seven replicates, respectively, examining incubation, latency, disease score, and injured area at 14 and 21 days. The two inoculation techniques were significantly equal for the susceptible variety, considering all evaluated parameters. For the resistant genotype, a significant difference was identified in the injured area and such difference did not interfere in the classification of the reaction of the material since values were below 1%. The single uredinium technique developed in this study showed to be reliable since the genotypic profile of the inoculated fungus was similar to that of pathogens from the produced lesions.

Genetic diversity of Puccinia kuehnii, the causal agent of orange rust of sugarcane, from Brazil

“…The following sugarcane cultivars were used in the first screening experiment: RB835054 (72 young plants), RB867515 (308 plants), RB935744 (308 plants), RB966928 (72 plants) and RB975201 (308 plants). They were all classified as resistant to orange rust (Araújo et al, 2013, Porto & Urashima, 2018. In the subsequent experiment, cultivars RB935744 and SP81-3250 were employed, one as resistant and the latter as susceptible.…”

“…They were classified as highly susceptible and thus employed as orange rust spreader in the sugarcane breeding centre in Araras city, São Paulo state. Fourto six-leaf young plants were inoculated using the single-pustule method specifically developed for orange rust of sugarcane by Porto and Urashima (2018). Briefly, single pustules of type 6 lesion, according to the description of Tai et al (1981), were selected and detached from the diseased leaves.…”

“…Briefly, single pustules of type 6 lesion, according to the description of Tai et al (1981), were selected and detached from the diseased leaves. This tissue piece containing urediniospores was rubbed over an area of 5 cm 2 of the abaxial part of a new sugarcane leaf, previously humidified with autoclaved distilled Milli-Q water and 0.01% of Tween 20; the urediniospore concentration in each inoculation was 1.10 5 spores per ml (Porto & Urashima, 2018). Negative controls were prepared on the distal area of the same leaves, towards the tip of the leaf by repeating the spread motion using a piece of healthy leaf over a 5 cm 2 area.…”