First report of orange rust caused by Puccinia kuehnii on sugarcane in South Africa.

In February 2022, rust-like lesions were observed on commercial sugarcane cultivar N76 in the North Coast area of KwaZulu-Natal, South Africa (29°23'45.95"S; 31°10'3.91"E). Moderate to severe leaf symptoms were present throughout the 0.2 ha field that was planted in August 2021. The symptoms differed from those of the two sugarcane rusts commonly observed in South Africa, namely, brown rust (Puccinia melanocephala H. & P. Sydow) and tawny rust (Macruropyxis fulva L.A. Martin, S.A. McFarl. and L.A. Castl.) (Martin et al., 2017). Similar symptoms were subsequently observed on several additional cultivars in other regions of the industry. Up to five leaves were collected from each contaminated field and examined under a dissecting microscope. Uredinial lesions were orange-brown, cinnamon-brown to red-brown in colour and were 0.5 to 5 × 0.2 to 0.5 mm. Uredinia within the lesions were 300 to 820 × 28 to 40 µm, mainly hypophyllous although epiphyllous uredinia were occasionally observed on severely infected leaves. Uredinia were ellipsoid to fusiform in shape. Urediniospores were transferred to microscope slides and examined at 400x magnification. The urediniospores were morphologically similar to Puccinia kuehnii E. J. Butler, causal agent of sugarcane orange rust (Glynn et al., 2010). They were orange to cinnamon-brown in colour, mostly obovoid to pyriform in shape and moderately echinulate. They were variable in size (25 to 55 × 22 to 27 µm), most with obvious apical thickening of up to 10 µm. Inconspicuous paraphyses were occasionally present but no telia or teliospores were observed. Identity of the pathogen was confirmed using primers Pk2-F and Pk1-R, targeting the internal transcribed spacer (ITS1-5.8S-ITS2) region (Glynn et al., 2010). The 384-bp sequence (GenBank Accession no: ON329808) matched 100% with P. kuehnii submissions on Genbank and was distinct from those of P. melanocephala and M. fulva. Rust symptoms observed on cultivar Co301 in South Africa from 1941 were previously reported to be caused by P. kuehnii (Gorter, 1977; Crous et al., 2000). However, the pathogen was later correctly identified as P. melanocephala (Bailey, 1979). This is therefore the first report of P. kuehnii infecting sugarcane in South Africa, after being detected periodically on a spore trap located in Komatipoort, Mpumalanga (25°33'2.83"S 31°57'22.19"E) since 2016. An Emergency Response Plan, developed in anticipation of an orange rust incursion in South Africa, guided the initial response of the industry. This included the application of a fungicide (a.i. azoxystrobin / cyproconazole) pre-emptively registered for orange rust on sugarcane in South Africa in infected fields, notification of key contacts in southern Africa, and industry-wide surveys to determine prevalence and provide an indication of varietal susceptibility. Long term management will include screening for varietal resistance and the registration of additional fungicides against the disease. Research on the conditions favouring infection will allow the inclusion of orange rust in the local sugarcane rust forecasting model, while trials to assess the effect on yield will provide an indication of the impact of the disease on production.

A process-based model to simulate sugarcane orange rust severity from weather data in Southern Brazil.

Forecasting the severity of plant diseases is an emerging need for farmers and companies to optimize management actions and to predict crop yields. Process-based models are viable tools for this purpose, thanks to their capability to reproduce pathogen epidemiological processes as a function of the variability of agro-environmental conditions. We formalized the key phases of the life cycle of Puccinia kuenhii (W. Krüger) EJ Butler, the causal agent of orange rust on sugarcane, into a new simulation model, called ARISE (Orange Rust Intensity Index). ARISE is composed of generic models of epidemiological processes modulated by partial components of host resistance and was parameterized according to P. kuenhii hydro-thermal requirements. After calibration and evaluation with field data, ARISE was executed on sugarcane areas in Brazil, India and Australia to assess the pathogen suitability in different environments. ARISE performed well in calibration and evaluation, where it accurately matched observations of orange rust severity. It also reproduced a large spatial and temporal variability in simulated areas, confirming that the pathogen suitability is strictly dependent on warm temperatures and high relative air humidity. Further improvements will entail coupling ARISE with a sugarcane growth model to assess yield losses, while further testing the model with field data, using input weather data at a finer resolution to develop a decision support system for sugarcane growers.

Reaction of sugarcane genotypes to brown and to orange rust by leaf whorl inoculation / Reação de genótipos de cana-de-açúcar às ferrugens marrom e alaranjada por inoculação no cartucho foliar

ABSTRACT: In this research eleven sugarcane genotypes were classified in relation to their resistance to brown rust, and eleven to their resistance to orange rust. Artificial inoculation was carried out in the leaf whorl of 165-day-old plants in the city of Paranavaí, Paraná State, Brazil, in 2017. The evaluation was performed 30 days after inoculation, using a rating scale. Among the genotypes tested for brown rust, four were classified as susceptible, six as moderately susceptible and one presented moderate resistance. For orange rust, three genotypes were classified as susceptible, seven as moderately susceptible and one as moderately resistant. The evaluation and classification of the reaction of sugarcane genotypes to the rusts is an important tool that assist in preliminary trials and selection of promising genotypes for more advanced stages of breeding programs and provides information to producers on the choice of cultivars to be planted.

RESUMO: Neste trabalho onze genótipos de cana-de-açúcar foram classificados em relação à sua resistência à ferrugem marrom, e onze quanto à resistência à ferrugem alaranjada. Foi realizada a inoculação no "cartucho foliar" de colmos com 165 dias, no município de Paranavaí, PR, em 2017. A avaliação foi feita aos 30 dias após a inoculação, utilizando uma escala de notas. Dos genótipos avaliados para ferrugem marrom, quatro foram classificados como suscetíveis, seis como moderadamente suscetíveis e um apresentou resistência moderada. Para a ferrugem alaranjada, três genótipos foram classificados como suscetíveis, sete como moderadamente suscetíveis e um como moderadamente resistente. A avaliação e classificação da reação de genótipos de cana-de-açúcar às ferrugens é uma ferramenta importante que auxilia em ensaios preliminares, na escolha de genótipos promissores para fases mais avançadas dos programas de melhoramento além de fornecer informações aos produtores na seleção das cultivares que serão plantadas.

Physiological changes in sugarcane in function of air and ground application of fungicide for orange rust control / Alterações fisiológicas na cana-de-acúcar em função de aplicações aéreas e terrestre de fungicida no controle da ferrugem alaranjada

The application of fungicides in different operating conditions is a usual practice for maintaining the productive potential in sugarcane varieties considered susceptible to orange rust, however, the physiological effects provided by the different application methods are unknown. The objective of this study was to evaluate the photosynthetic responses (gas exchange and chlorophyll content) in the SP81-3250 sugarcane variety, in function of different operational conditions of the aerial and ground application of fungicide in the orange rust control. Two application of fungicides of the chemical groups Strobilurins and Triazoles were carried out in the experimental units in different treatments. In the aerial applications, two application rates (30 and 40 L ha-1) and three nozzle orientations (0º, 90º and 135º) and in the ground application was used 200 L ha-1 and flat fan spray nozzles with air induction (AI11004-VS). Gas exchange evaluations were performed with an IRGA and amount of chlorophyll a and b with a chlorophyll meter. Data were analyzed using Student's t-test for independent samples, at 0.05 significance. The aerial application provided better photosynthetic responses and chlorophyll a and b contents in leaf limb compared to ground application. Significant differences were detected in gas exchange and chlorophyll content between application rates and angulation of the spray nozzles in the boom. Fungicide applications provided increments of more than 19 t ha-1 compared to the control, depending on the spraying technique employed. Aerial application with 30 L ha-1 and 0° of deflection is a viable option to provide safer applications as a function of the larger droplet size.

A aplicação de fungicidas em diferentes condições operacionais é uma prática usual para manter o potencial genético em variedades de cana-de-açúcar consideradas susceptíveis à ferrugem laranja, porém os efeitos fisiológicos proporcionados pelos diferentes métodos de aplicação são desconhecidos. O objetivo deste estudo foi avaliar as respostas fotossintéticas (troca gasosa e teor de clorofila) na variedade de cana-de-açúcar SP81-3250, em função de diferentes condições operacionais de aplicação aérea e terrestre de fungicida no controle da ferrugem alaranjada. Duas aplicações de fungicidas dos grupos químicos Estrobilurinas e Triazóis foram realizadas nas unidades experimentais em diferentes tratamentos. Nas aplicações aéreas, foram utilizadas duas taxas de aplicação (30 e 40 L ha-1) e três orientações do ângulo dos bicos (0º, 90º e 135º) e na aplicação terrestre 200 L ha-1 e pontas de pulverização de jato plano com indução de ar (AI11004 -VS). As avaliações de trocas gasosas foram realizadas com analisador de gás IRGA e a quantidade de clorofila a e b com um medidor de clorofila. Os dados foram analisados utilizando o teste t de Student para amostras independentes, com um valor de 0,05 de significancia. A aplicação aérea proporcionou melhores respostas fotossintéticas e os teores de clorofila a e b no membro foliar em comparação com a aplicação terrestre. Foram detectadas diferenças significativas na troca gasosa e no teor de clorofila entre as taxas de aplicação e a angulação dos bicos de pulverização na barra. As aplicações de fungicidas proporcionaram incrementos de mais de 19 t ha-1 em relação ao tratamento controle, dependendo da técnica de pulverização empregada. A aplicação aérea com 30 L ha-1 e 0° de deflexão é uma opção viável para proporcionar aplicações mais seguras em função do maior tamanho das gotas.

Detection of Puccinia kuehnii Causing Sugarcane Orange Rust with a Loop-Mediated Isothermal Amplification-Based Assay.

Puccinia kuehnii is a fungal pathogen that causes orange rust in sugarcane, which is now prevalent in many countries. At the early stage of disease, it is almost indistinguishable from brown rust, which is caused by Puccinia melanocephala. Although several PCR assays are available to detect these diseases, the loop-mediated isothermal amplification (LAMP)-based assay has been reported to be more economical and easier to perform. Under isothermal conditions, DNA is amplified with high specificity and rapidity. Moreover, visual judgment of color change without further post-amplification processing makes the method convenient. The present study was undertaken to detect P. kuehnii genomic DNA using four primers corresponding to a unique DNA sequence of P. kuehnii. The LAMP assay was found to be optimal when 8 mM MgSO4 was used and the reaction was incubated at 63 °C for 90 min. Positive samples showed a color change from orange to green upon SYBR Green I dye addition. Specificity of the LAMP test was checked with DNA of P. melanocephala, which showed no reaction. Sensitivity of the LAMP method was observed to be the same as real-time PCR at 0.1 ng, thus providing a rapid and more affordable option for early disease detection.

Reação de cultivares e época de avaliação da ferrugem alaranjada da cana-de-açúcar / Reaction of commercials cultivars and assessment date of orange rust of sugarcane

Os objetivos deste estudo foram avaliar a reação de 28 cultivares de cana-de-açúcar à ferrugem alaranjada, em campo, estabelecendo uma época de avaliação adequada para os programas de melhoramento, e determinar os períodos de incubação e latência, incidência e severidade na cultivar padrão suscetível, em casa-de-vegetação. No campo, a severidade foi avaliada em cana-planta (dez meses) e em cana-soca (três, cinco, sete e nove meses). A partir destes dados, calculou-se a área abaixo da curva de progresso da doença (AACPD). Para estabelecer a época de avaliação adequada, calculou-se a correlação de Pearson entre os resíduos das datas com os da AACPD. Em casa-de-vegetação, inoculou-se a cultivar RB72454 e avaliou-se incidência e severidade. Das cultivares avaliadas, 14 foram suscetíveis à ferrugem alaranjada, sendo discriminadas em quatro grupos de suscetibilidade. A avaliação em maio (cana-soca, aos nove meses), foi a que mais se correlacionou com a AACPD, sendo a época adequada para avaliar a reação de cultivares à doença, no norte do Estado do Paraná. O período de incubação e latência na cultivar padrão suscetível foi de sete e dez dias, respectivamente. A cultivar RB72454 é a única altamente suscetível à ferrugem alaranjada e as demais avaliadas apresentam menores graus de suscetibilidade.

The aims of this study were to assess the reaction of 28 sugarcane cultivars to orange rust, establishing the most appropriate date for the breeding programs assess the reaction of cultivars, and, in greenhouse, determine the incubation and latency periods, the incidence and the severity in the susceptible standard cultivar. In the field, the severity was assessed in plant cane (ten months) and ratoon crops (three, five, seven and nine months). The area under the disease progress curve (AUDPC) was calculated from these data. To establish the appropriate date of assessment, the Pearson correlation was calculated between the residues of assessment dates and the AUDPC. In greenhouse, the cultivar RB72454 was inoculated and the incidence and severity were assessed. Among 28 cultivars, 14 showed some degree of susceptibility of orange rust and they may be discriminated into four groups of susceptibility. The assessment in May was the most correlated with AUDPC, being the appropriate date for assessing susceptibility of cultivars to orange rust, in the northern region of Paraná State. The incubation and latency periods in the susceptible standard cultivar were seven and ten days, respectively. The cultivar RB72454 is the only highly susceptible to orange rust and other cultivars assessed have lower degrees of susceptibility.

Cobertura de gotas de pulverização obtida com diferentes pontas e taxas de aplicação na parte aérea da cana-de-açúcar / Spray droplet coverage achieved with different nozzles and application rates in the aerial part of sugar cane

A aplicação de produtos fitosssanitários no setor sucroalcoleiro, até recentemente, estava focada principalmente no controle de plantas daninhas. Com o aparecimento da ferrugem alaranjada, como doença de importância agrícola na cultura da cana-de-açúcar, passou a ser imperioso conhecer como a aplicação chega na parte aérea da planta. Dessa forma, o presente estudo foi desenvolvido com o objetivo de avaliar a qualidade da deposição da calda em pulverização terrestre, conduzido em esquema fatorial 2x2x2 (duas pontas de pulverização, duas taxas de aplicação e duas posições dos cartões de papéis hidrosensíveis no dossel da planta), seis repetições, com dez papéis cada. Foram utilizadas as pontas de pulverizacão XR110.02 (jato plano de uso ampliado) e TTJ60 - 110.02 (jato plano duplo, com ângulo de 60 graus entre si) utilizando-se as taxas de aplicação de 120 e 160L ha-1. Os cartões de papel hidrossensível utilizados para avaliar a deposicão das pulverizações foram distribuidos sobre as folhas de cana-de-açúcar, em duas posições distintas em relação ao dossel da planta (parte superior e inferior, aproximadamente, 0,80 e 0,50 metros do solo, dispostos na horizontal e inclinado em relação à superfície do solo, respectivamente). A cultivar utilizada foi 'RB 96-6928', com altura de 0,80m aos 120 dias do plantio. Um scaner de mesa foi utilizado para digitalizar as imagens dos papéis e avaliou-se as coberturas através do programa Conta-Gotas®. Os dados de deposição/cobertura foram submetidos à análise de variância e as médias comparadas pelo teste Tukey ao nível de 5% de probabilidade de erro. No dossel superior da cana-de-açúcar observou-se maior cobertura, quando comparado ao dossel inferior, sendo que não se observaram diferenças entre as taxas de aplicação de 120 e 160L ha-1,. A ponta TTJ60-110.02 promoveu cobertura superior à ponta XR 110.02 quando se utilizou 160L ha-1 (40,34 e 36,50%, respectivamente).

The terrestrial application of agricultural defensives in the sugar-alchool producer sector until recently was focused on weed control. After the appearance of orange rust, a disease of agricultural importance in sugar cane production, it has become imperative to know how the application arrives at the plant canopy. In such a way, the present study was developed with the objective of evaluate the quality of the deposition in terrestrial spraying, lead in factorial project 2x2x2 (two nozzles of spraying, two rates of application and two positions of the water-sensitive papers in the canopy of plant), six repetitions, with ten papers each. It has been used the nozzles of spraying XR110.02 (plain jet of extended range) and TTJ60 - 110,02 (double plain jet, with angle of 60 degrees between itself) using the rates of application of 120 and 160L ha-1. The water-sensitive papers cards used to evaluate the deposition of the sprayings had been distributed on the leaves of sugar cane in two distinct heights (upper and lower, approximately 0,80 and 0,50 meters above ground level, arranged in horizontal and inclined with respect to the road surface, respectively). A table scanner was used to generate the digital images of the papers and evaluated the coverings through the Conta-Gotas® program. The deposition data/covering had been submitted to the variance analysis and the averages compared with the Tukey test at 5% of significance. It was observed in the superior canopy of the sugarcane a bigger covering when compared with the inferior, there was no difference between the application rates, 120 and 160L ha-1. The TTJ600110.02 nozzle promoted bigger covering the XR110.02 nozzle when using 160L ha-1 (40,34 and 36,50%, respectively).