Evaluation of Cane Genotypes Under Sprinkler Irrigation at Early Selection Stage for Tolerance to Sugarcane Streak Mosaic Virus (SCSMV) at Ferké Sugar Estates in Ivory Coast

Sugarcane streak mosaic virus (SCSMV) became the major endemic disease of economic importance in Ivorian sugar estates almost two years ago, which spreads very fast across plantations and varieties. The study aimed to determine resistant sugarcane genotypes against SCSMV in Ferké sugar estates. It involved five experiments conducted at first selection stage under sprinkler irrigation, following a randomized complete block design (RCBD) with 20 to 30 different genotypes, two check varieties included, all in 4 replicates. Experiments were planted in October or December 2018, and expected to be harvested in November/December 2019 and 2020 as plant cane and first ratoon, respectively. Disease incidence and severity across all experiments were determined at 3 to 4 months, i.e. at early formative growth stage where symptoms due to SCSMV could be easily observed and recognized in the field. In each of the five selection trials conducted on both Ferké 1 & 2 sugar estates, highly significant differences in disease incidence and severity were observed between genotypes as well as crop cycles (plant cane and first ratoon). Except for one trial (B3-24 in Ferké 1), Genotype x crop cycle interactions were significant or highly significant, which showed that the majority of genotypes tested behaved differently from plant cane to first ratoon towards the disease. Particularly, the percentage of resistant genotypes decreased from 50 to 3.4% whereas that of highly susceptible ones increased from 4.2 to 92.4%. This shows the level of challenge to be tackled in the control of SCSM disease through sugarcane breeding and selection. At the end of the current selection stage under way, i.e. after harvest of first ratoon, only the best yielding genotypes among the resistant ones will undergo the advanced selection stage.


Introduction
Sugarcane is one of the important cash crops grown in Ivory Coast, covering approximately 35,500 ha of land surface with an average yield of 70-75 t of cane/ha irrigated and rainfed crops combined. Sugarcane is mostly cultivated under irrigated conditions contributing to around 90% of Ivorian sugar production, i.e. about 200,000 t/year provided from four different factories.
Sugarcane streak mosaic (SCSM) is a newly emerging disease in the country, which has been infecting that crop since 2017. The virus was known about fifteen years ago in Asian and Latino American sugarcane producing countries where it was first reported in 2004 and 2005 respectively in Brazil (Gonçalves et al, 2004;Gonçalves et al, 2007a;Gonçalves et al, 2007b;Gonçalves et al, 2011;Gonçalves et al, 2012) and Java, Indonesia (Chatenet et al, 2005;Kristini et al, 2006). It was reported that at least two strains of SCSMV were infecting sugarcane in Asia (Putra et al, 2015a). SCSMV is a Poacevirus from the Potyviridae family, which is easily transmitted through plant extracts and vertically through sugarcane cuttings. It infects a limited species of Poaceae family such as sugarcane, sorghum, maize, Dacttyloctenium aegyptium, Pennisetum glaucum, Digitaria delilis as reported by several investigators (Hema et al, 2001;Xu et al, 2010;Putra et al, 2014). However, no insect vector of the pathogen was reported so far (Putra et al, 2015a, Addy et al, 2017. Several species of aphids have been tested including Aphis cracivora, Rhopalosiphum maidis and Ceratovacuna lanigera and it was reported that they could not transmit the virus (Hema et al, 2001;Putra et al, 2014). It was reported that SCSMV could be detected using RT-PCR on sugarcane aphid colony (Melanaphis indosacchari), and still the mechanism of virus transmission needs to be studied so as to develop an integrated pest management strategy (Brown, 1997).
Across Ivorian sugar estates, except for a limited number of varieties like M2593/92, M1400/86, SP70-1143, R584 and R91-2021, all main cultivars were found moderately or highly susceptible to the disease. It is the case of varieties like FR80-69, R579, SP70-1006, SP71-1406, SP71-8210, SP81-3250, R573, R570, Co997 and R575. Cane and sugar yield losses due to SCSMV occurred in Zuénoula sugar estate in central Ivory Coast where the disease broke out in 2017 on variety R575, were estimated to 20 to 30% over the last two cropping seasons (2017-18 and 2018-19). The disease expansion was so fast in Zuénoula plantations that 20% of land under cultivation with moderately or highly susceptible varieties were replanted every year instead of 10% as usual (Béhou & Péné, 2019). The disease broke out in Ferké sugar estates of northern Ivory Coast in June 2018 on variety FR80-69, a highly susceptible one, and within 12 months, it has spread to all major varieties cultivated like R579, SP71-1406, SP70-1006 and SP71-8210, with a severity score of 2 or 3, 4 being the highest symptom level. Total cropped land concerned was estimated to 2000 ha, i.e. about 25%. The threat on sugar production was so crucial that a replantation strategy was planned to be implemented over the next three or four years.
The study aimed to determine under irrigation resistant sugarcane genotypes of Reunion and Ivorian origin against SCSMV.

Site Characteristics
The study was carried out on four sugarcane plantations (B3-13, B3-24, V4-43, V8-01) and an experimental station (P3-61), at Ferké 1 and Ferké 2 sugar estates, in northern Ivory Coast (9°20' -9°60' N, 5°22' -5°40' O, 325 m a.s.l.). The prevailing climate is tropical dry with two seasons: one, from November to April, is dry and the other, from May to October, is wet. The dry season is marked by the northern trade wind, which blows over mid-November to late January. The rainfall pattern is unimodal with a focus on August and September which total amount of rainfall reaches almost half of the average annual rainfall (1200 mm) with an average daily temperature of 27 °C. Average maximum and minimum daily air temperatures are 32.5 and 21 °C, respectively. To meet sugarcane crop water requirements, the total amount of irrigation water required reaches 700 mm/year (Konan et al, 2017a,b). Both Ferké sugar mill plantations cover around 15 500 ha with 10 000 ha under irrigation and 3 500 ha of rainfed village plantations, lie mainly on shallow or moderately deep soils built up on granites. Main soil units encountered are ferralsols and temporally waterlogged soils in valley bottoms of Bandama and Lokpoho river basins with a sandy-clay texture.

Experimental Design
All experiments were carried out over 12 months as plant cane following a randomized complete block design (RCBD) with 20 to 30 cane genotypes and two commercial varieties as control (M2593/92 and R579), in 3 replicates. Each sugar estate was equipped with a weather station where parameters required to determine crop ET 0 like solar radiation, average daily air temperature, relative air moisture, and wind speed were measured. Rainfall data were recorded from different rain gauges L1-105, but also P3-61, V4-15 and V8-32 located close to Ferké 1 and Ferké 2 experiments, respectively. Ferké 1 experiments were planted on October 25 and December 11, 2018 were expected to be harvested 11 or 12 months later, i.e. in November or December 2019 and 2020, respectively, as plant cane and first ratoon. Those of Ferké 2 were planted on October 12, December 20 and 29, 2018 and expected to be harvested in November or December 2019 and 2020, respectively. Each plot was composed of 5 dual rows of 5 m long with 0.50 m and 1.90 m of row spacings. Field management in terms of fertilizer and herbicide applications were done according to usual practices in commercial plantations. NKP fertilizer (16-8.5-23%) was applied mechanically at the routine rates of 500 kg/ha in rainfed plant cane. Preemergence chemical weeding based on pendimethalin combined with clorimuron-ethyl (3.5 l/ha) was achieved mechanically two days after planting.

Genotype Infections Investigated
Three to four months after planting, all genotypes being tested were observed for symptom detection of SCMV (sugarcane streak mosaic Virus) under natural conditions. Ratings recorded were based on symptoms observed on sugarcane leaves. Four different levels of SCSM disease symptoms were as follows: (1) mild streak, (2) moderately streak, (3) high streak and (4)  Disease incidence (Inc) is defined as the percentage of infected tillers, no matter the severity, over the total number of tillers observed within two central micro-plots made of one dual row of 5 m long with respect to each genotype and replication.

Statistical Analyses
The quantitative data recorded in this study were subjected to the analysis of variance, using statistical procedures described by Gomez and Gomez (1984) and reported by Shitahum et al (2018) with the assistance of R software package version 3.5.2. Differences between means of treatments were determined from HSD's test.

Climatic Conditions Over Plant Cane and First Ratoon Crop
As expected, both experimental sites presented a similar rainfall patterns with a per-humid season taking place from June to October (Fig 1)

Cane Genotypes as Affected by SCSM Disease in FerkÉ 1 Experiments
In both Ferké 1 trails (B3-13 and B3-24 field plots), highly significant differences (P<0.1) in disease incidence and severity were observed between genotypes as well as crop cycles (Table 1). Genotype x crop cycle interactions were significant or highly significant in B3-13 trial, while not significant in B3-24 trial. This shows that in B3-13 trial, cane genotypes behaved differently from plant cane to first ratoon towards the disease. Particularly, many asymptomatic genotypes assumed to be tolerant or resistant in plant cane became susceptible in first ratoon. In contrast, for the B3-24 trial, genotypes tested behaved similarly over both crop cycles in terms of their ranking, although the disease symptoms worsened sometimes in first ratoon.
High values of coefficient of variation determined in both experiments (70-87%) could be explained by natural infection of disease, although that one became endemic over the last two years in Ferké sugar estates. Similar values were reported in survey on major endemic diseases conducted in the same agro-ecology 2019).

Cane Genotypes as Affected by SCSM Disease at Three Months in Ferké 2 Experiments
In the three trails of Ferké 2 location (P3-61, V4-43 and V8-01 field plots), highly significant differences (P<0.1) in disease incidence and severity were observed between genotypes as well as crop cycles (Table 2). Genotype x crop cycle interactions were significant in the P3-61 trial and highly significant in V4-43 and V8-01 trails. This corroborates the fact the majority of genotypes tested behaved differently from plant cane to first ratoon towards the disease. Particularly, many asymptomatic or moderately resistant genotypes in plant cane became susceptible or highly susceptible in first ratoon. Similarly to both Ferké 1 trails, high values of coefficient of variation determined in both experiments (78.5 -101%) could be explained by natural infection of disease.

Plant Cane
Higher percentage of disease resistant genotypes were observed in V4-43 and P3-61 experiments conducted in Ferké 2 with 72 and 52%, respectively (Fig. 2). The lowest percentage of resistant genotypes was observed in B3-13 experiment conducted in Ferké 1, with 37%. Highly susceptible genotypes were observed in B3-13 and V8-01 experiments carried out in Ferké 1 and Ferké 2, respectively, with 16 and 9% of genotypes tested.
Symptom observations in this study were made at three or four months, therefore at early grand growth stage of sugarcane where infection could be recognized visually without any virus detection equipment like ELISA chain (Enzyme Linked Immuno Sorbent Assay) or RT-PCR (Reverse Transcription -Polymerase Chain Reaction). However, observations made later at that growth stage (5-9 months) and the formative growth stage (10-12 months) showed much higher values of disease severity ranging from 2 to 3 (-) on susceptible genotypes compared with 0.1 to 0.5 (-) determined at early grand growth stage. That is why our suggestion would be to observe at 5-7 months, where sugarcane fields are still easily accessible for growth measurements and disease control.  In the first ratoon crop, higher percentage of highly susceptible genotypes to SCSM disease (86-95.7%) were observed in all experiments conducted in both sugar estates (Fig. 3). Percentage of resistant genotypes observed in Ferké 2 ranged from 4.3 to 6.9% while none of them was observed in both Ferké 1 trials. That of susceptible genotypes ranged from 5.3 to 6.9% observed in both Ferké 1 trials as well as V4-43 trail of Ferké 2. In total, about 92% of genotypes tested were highly susceptible whereas 4.2 and 3.4% were susceptible and resistant, respectively (Fig 4, table 4).  In the sense that it is not eliminated by hot water treatment (HWT), in contrast to most economically important diseases in sugarcane frequent in sub-Saharan Africa and elsewhere like leaf scald (Xanthomonas albinineans), smut (Sporisorium scitamineum), yellow leaf disease (SCYLD), orange rust (Puccinia kuehnii), RSD (Leifsonia xyli subsp. Xyli), and brown rust (Puccinia melanocephala). Other challenges lie in the fact that no insect vector of SCSMV is formally identified yet, its transmission mechanism from vector to crop being still unknown and it spreads very fast across sugarcane fields and varieties, with significant yield reduction as reported in Brazil, Indonesia and Ivory Coast (Gonçalves et al, 2007b;Magarey et al, 2018;Béhou & Péné, 2019. Prophylactic measures based on agricultural practices regarding sanitation of planting and harvesting tools or machinery as well as search for resistant cane varieties must be given a top priority. In this regard, sugarcane selection starting from true seed constitutes a great opportunity to broaden the genetic diversity of plant material being tested in line of SCSMV thread. This program is being under way since 2014-15 in Ivory Coast, and much earlier in Senegal and some Central African countries (Cameroon, Tchad and Congo), in collaboration with R&D institutes like eRcane (Reunion Island) and MSIRI (Mauritius).

Conclusions
In each of the five selection trails conducted on both Ferké 1 & 2 sugar estates, highly significant differences in disease incidence and severity were observed between genotypes as well as crop cycles (plant cane and first ratoon). Except for one trial (B3-24 in Ferké 1), Genotype x crop cycle interactions were significant or highly significant, which showed that the majority of genotypes tested behaved differently from plant cane to first ratoon towards the disease. Particularly, the percentage of resistant genotypes decreased from 50 to 3.4% whereas that of highly susceptible ones increased from 4.2 to 92.4%. This shows the level of challenge to be tackled in the control of SCSM disease through sugarcane breeding and selection. At the end of the current selection stage under way, i.e. after harvest of first ratoon, only the best yielding genotypes among the resistant ones will undergo the advanced selection stage.