Incidence, Severity, and Prevalence of Sorghum Diseases in the Major Production Regions in Niger

Sorghum ranks second to pearl millet as the most important cereal in Niger and is used primarily for food, feed, and other uses. During the 2022 growing season, 96 fields from the five major sorghum production regions of Dosso, Maradi, Tahoua, Tillabéri


Introduction
In Sub-Saharan Africa, sorghum [Sorghum bicolor (L.) Moench] supplies the daily caloric needs for millions of inhabitants (Frederickson & Odvody, 2000;Mundia et al., 2019;Pereira & Hawkes, 2022). Its uses include baked foodstuff, alcoholic beverages, fiber, starch, paper, syrup, biofuel, and in the health industry to combat or reduce the impact of various human maladies (Gonzalez et al., 2011;Araujo et al., 2017;Upadhyaya et al., 2017;Khalid et al., 2022). Sorghum is drought tolerant and often grown under marginal lands with low farm inputs, especially in several resource poor Countries (Gonzalez et al., 2011;Rooney et al., 2007, Chala et al., 2010Ignacimuthu & Premkumar., 2014). In Niger, 3,519,085 ha were harvested, producing 1,207,237 metric tons during the 2021 season (FAOSTAT). Compared to other countries, including some in Africa, sorghum yield in Niger fluctuates from year to year but is still low, ranging from 193 kg/ha to a little over 406 kg/ha (Anonymous, 2021); Mundia et al., 2019;Hamidou et al., 2018;Anonymous, 2019). Globally, Africa produces the most sorghum and West Central African Countries are responsible for 90% of the production (Weltzien & Rattunde, 2022). Nevertheless, sorghum yield/ha in Niger and other African Countries in the drier tropics is still low and this is due to several factors such as lower farm inputs, the unpredictable weather patterns, type of sorghum cultivars/landraces used, demand from urban areas where people prefer the consumption of the other major cereals, wheat, maize, and rice, lack of research, pests and diseases (Pereira & Hawkes, 2022;Mundia et al., 2019;Kadi Kadi et al., 2005). In Niger, sorghum production ranks second to pearl millet in importance, and is used primarily as a staple food and for animal feed, especially their stalks (Hamidou et al., 2018;Manssour et al., 2014;Ousmane et al., 2017). The world"s population is projected to reach 9.1 billion by 2050 and is estimated that 3 billion tons increase of cereal for food and non-food purposes will be needed (Mundia et al., 2019;FAO, 2009). The projected increases in cereal production coupled with other factors such as climate change will likely increase fungal, bacterial, viral and other diseases. Some of these diseases can cause both yield and quality losses, and in addition, many of these pathogens have the capacity to produce mycotoxins that can negatively impact human and animal health (Sashidha et al., 1992;Leslie et al., 2005;Isakeit et al., 2008;Prom et al., 2011a;Funnell-Harris et al., 2013;Cuevas et al., 2016). As a result, an integrated effort, including disease management will be critical in ensuring food security and environmental resilience. To determine the relative importance of these diseases, four major sorghum producing regions in Niger were surveyed in Niger during the 2019 growing season (Prom et al., 2020). The 2019 survey did not include the second ranked sorghum production region of Zinder and severity of the various diseases, which is a critical component in estimating economic loss. This study was conducted to investigate the occurrence, distribution, and severity of foliar and panicle diseases affecting sorghum in farmers" fields from the top five major production regions of Niger and to validate the information gathered during the 2019 growing season.

Material and Methods
The five major sorghum production regions of Dosso, Maradi, Tahoua, Tillabé ri, and Zinder in Niger were surveyed for foliar and panicle diseases during the 2022 growing season (Figure 1). Climatic condition for the four regions of Dosso, Maradi, Tahoua, and Tillabé ri is Sahelian; however, northern Tahoua has a Sahelo-Saharian, while the southern of Gaya in Dosso region has a Sahelo-Sudanian type of climate. Zinder region has a dry subtropical climate while the northern part belongs to the Sahara Desert (Table 1) (Hamidou et al., 2018;Anonymous, 2019;Anonymous, 2021). The annual rainfall ranges from 435 mm in Tahoua with most of the rains falling in July and August to 700 mm (Dosso) and Gaya experiencing total rains primarily in June to September that exceed 800 mm. The soil type for the region of Zinder is Lithosols while the other regions have mainly Ferruginous tropical soil, with localities of Bengou and to a lesser degree Tara also having a hydromorphous soil type (Table 1) (Hamidou et al., 2018;Anonymous, 2019;Anonymous, 2021). During the rainy season, the maximum and minimum temperatures for the surveyed regions also are noted in table 1 (Hamidou et al., 2018;Anonymous, 2019;Anonymous, 2021). Table 2 shows the average yield, rank in term of annual sorghum production, and farming systems among the surveyed regions of Niger (Moussa, 2013;Anonymous, 2015;Anonymous, 2022). A total of 96 farmers" fields along paved and unpaved roads, including National and Secondary (RN1, RN2, RN3) were arbitrarily surveyed and plants at late flowering to hard dough stages of development were assessed for diseases to cover the whole field, a W-shaped pattern was used. Along the roads, stops were made at intervals of 30 km. At each stop, 2 -5 fields (40 plants per field) were surveyed for disease incidence, severity, and prevalence. The formulae for incidence and prevalence noted below have been previously described by Prom et al. (2020). x 100 Disease Severity scale: The surveyed fields were planted with landraces and advanced lines with different leaf sizes; as a result, a modified Horsfall-Barratt rating scale of 0 -11 (Campbell & Madden, 1990) with Chala et al. (2007) range where 1 = 1-10, 2 = 11-20, 3 = 21-30, 4 = 31-40, 5 = 41-50, 6 = 51-60, 7 = 61-70, 8 =71-80, 9 = 81-90, 10 = 91-100, and 11 = 100 was employed. Their mid-points, where 1 = 5.5, 2 = 15.5, 3 = 25.5, 4 = 35.5, 5 = 45.5, 6 = 55.5, 7 = 65.5, 8= 75.5, 9 = 85.5, 10 = 95.5, and 11 = 100 were used to calculate the mean severity.

Results
During the 2022 growing season, 96 farmers" fields from five major sorghum producing regions of Dosso, Maradi, Tahoua, Tillabè ri, and Zinder were surveyed for foliar and panicle diseases. Most of the fields evaluated were planted with different landraces with a few exceptions that were planted with improved varieties released by the National Institute of Agricultural Research of Niger. The fields surveyed ranged in size from 0.2 to 1 ha and were mostly reasonably maintained by regular hand hoeing. The cropping systems used by the vast majority of sorghum producers in the surveyed regions were pure sorghum stands and intercropping with early and late varieties, millet-sorghum-cowpea, millet-cowpea-peanut, millet-sorghum-cowpea-hibiscus (Table 2).

Disease Incidence, Severity, and Prevalence
In each surveyed field, 40 plants were assessed using a W-shaped pattern to cover the whole field. Across the 96 fields surveyed, 19 diseases anthracnose, gray leaf spot, grain mold, zonate leaf spot, bacterial leaf stripe, long smut, oval leaf spot, leaf blight, bacterial leaf blight, target leaf spot, bacterial leaf streak, sooty stripe, covered smut, rough leaf spot, downy mildew, head smut, maize dwarf mosaic virus, ladder leaf spot, and crazy top were documented (Table 2). Figure 2 shows 8 of the different diseases observed: 2a shows anthracnose infected leaf, 2b leaf blight infected leaf, 2c oval leaf spot, 2d zonate leaf spot, 2e rough leaf spot, 2f sooty stripe, 2g target leaf spot, and 2h scalariform lesions of ladder leaf spot. The smuts and crazy top observed during the 2022 survey are presented in figure 3, with 3a long smut, 3b head smut, 3c covered kernel, and 3d crazy top. All the fields surveyed had plants that were infected with leaf blight (Table 3). Out of the 96 production fields surveyed, 85 fields had anthracnose, 74 fields were infected with oval leaf spot, and rough leaf spot, respectively. The prevalence of long smut was (46%), zonate leaf spot (43%), and gray leaf spot (38%), and target leaf spot (36%). Out of the 96 fields surveyed, downy mildew and maize dwarf mosaic virus were observed in one field. In the regions of Dosso and Zinder, anthracnose was found in all the surveyed fields while oval leaf spot was detected in all surveyed fields in Maradi and Zinder (Table 4). Among the regions, the highest prevalence of zonate leaf spot (91%), target leaf spot (70%), gray leaf spot (65%), sooty stripe (43%), and crazy top (35%) was recorded in the region of Dosso. Out of the 20 fields assessed in region Tillabè ri, long smut was found in 18 fields whereas rough leaf spot (92%) was most prevalent in Zinder region (Table 4).
The highest mean incidence of leaf blight (95%) was recorded in the regions of Tahoua and Zinder (Table 5).
Higher mean incidence of Target leaf spot (54%), rough leaf spot (45%), and long smut (40%) was noted in the region of Tahoua while oval leaf spot (80%) anthracnose (56%), and gray leaf spot mean incidences were highest in Maradi region (Table 5). Compared to other regions, the highest mean severities of leaf blight (30%), long smut (29%), grain mold (18%), and anthracnose (13%) were recorded in Maradi region (Table 5). Ladder leaf spot (17%) was more severe in the region of Zinder while plants surveyed in the region of Tahoua had more severe target leaf spot infection. The severity of head smut was 100% where the entire panicles were replaced with teliospres ( Figure 3B) in all the regions where the disease was observed. The highest severity of covered kernel smut (24%) was noted in the regions of Maradi and Tahoua. Table 6 shows selected locations in Dosso region that can be considered as "hot spot" for leaf blight resistance screening. Other locations such as Dan gado in Maradi, Tounfafi and Dogueraoua in Tahoua region, Tondi Banda and Karma in Tillabè ri, and Gargoumsa in the region of Zinder also can be considered as "hot spot" for leaf blight resistance evaluation sites. In all surveyed regions, there are sites where anthracnose resistance evaluations can be conducted (Figure 4). Except for the region of Tillabè ri, sites were identified with incidence of 85% and above of zonate leaf spot, oval leaf spot and rough leaf spot in Dosso, Maradi, Tahoua, and Zinder regions where sorghum germplasm can be evaluated for resistance ( Figure 5). The lower prevalence and incidence of grain mold was expected because the majority of the plants were assessed at late flowering to early hard dough stages.

Discussion
The multi-uses of sorghum and its adaptability in marginal agro-ecological zones makes it indispensable for people and animals living in drier tropics (Frederiksen & Odvody, 2000;Pereira & Hawkes, 2022;Kangama, 2017;Prom et al., 2020). Global population is projected to increase to 9.1 billion by 2050 which will require increases in cereal production, including sorghum to around 3 billion tons for food, feed and other uses (FAO, 2009). Future increases in cereal production will likely increase diseases that will impact yield and quality for both food, feed, and non-food use (Prom, 2017;Prom et al., 2020).
In this study, 96 farmers" fields in the five major sorghum producing regions of Dosso, Maradi, Tahoua, Tillabèri and Zinder in Niger were surveyed for incidence, prevalence, and severity of various diseases during the 2022 growing season. Across, the regions, leaf blight, incited by Exserohilum turcicum was the most prevalent (100%) disease followed by anthracnose (81%), oval leaf spot (72%) and rough leaf spot (68%). Beshir et al. (2015) detected leaf blight in all 45 sorghum fields surveyed in Central Sudan. In Niger, 121 fields were surveyed in 2019 to determine the distribution of sorghum diseases and the results revealed that anthracnose (99%) was the most prevalent disease followed by leaf blight (89%), long smut (76%), and rough leaf spot (67%) (Prom et al., 2020). Out of 384 fields surveyed for the presence of sorghum diseases in lower eastern Kenya, 272 fields were infected with anthracnose and 270 fields exhibited leaf blight infection (Koima et al., 2022). In Western Kenya, anthracnose, leaf blight, gray leaf spot, and zonate leaf spot were the most dominant diseases noted in sorghum fields (Ngugi et al., 2002). The prevalence of anthracnose (93.7%) and leaf blight (84.8%) were reported by Teferi & Wubshet, (2015) after inspecting 76 fields in South Tigray, Ethiopia, while Tsedaley et al. (2016) surveyed 117 fields in Southwestern and Western Ethiopia and noted the presence of anthracnose in all the fields. Pande et al. (1993) noted that anthracnose, oval leaf spot, sooty stripe, and gray leaf spot were the most prevalent diseases across fields in four major growing climatic zones in Nigeria. Surveys conducted in 37 fields in Tanzania and 134 fields in Uganda revealed that leaf blight and rust were more prevalent in Tanzania while anthracnose and zonate were more frequently observed in Uganda (Njoroge et al., 2018).
The current survey in Niger validated the information that was gathered during the 2019 survey. There were many similarities between the surveys conducted in 2019 and 2022 in Niger, with few exceptions. In 2019, a total of 21 diseases were observed and the most prevalent disease was anthracnose (99%) followed by leaf blight (89%) (Prom et al., 2020) while in the current 2022 survey, 19 diseases were detected with leaf blight (100%) found in all fields followed by anthracnose with 81% prevalence. In the 2019 survey, maize mosaic virus, loose smut, and bacterial leaf spot were observed in low frequency (Prom et al., 2020) but absent during the 2022 survey. Ladder leaf spot was noted in the 2022 survey but not observed during the 2019 survey. In both surveys conducted in Niger in 2019 and 2022, anthracnose, leaf blight, long smut, oval leaf spot, zonate leaf spot, rough leaf spot, head smut, and target leaf spot were the most prevalent diseases found in sorghum production fields. There was a major difference between the 2019 and 2022 surveys. In the 2022 survey, severity of the sorghum diseases observed was recorded but not in 2019. Disease severity which has a direct impact on yield is one of the critical components in estimating the relative economic losses due to plant diseases. In this survey, the highest severities of leaf blight, anthracnose, and long smut were noted in the region of Maradi. Head smut severity was 100% in the region where the disease was detected (Table 5). In covered kernel smut and long smut infections (Figure 3a and 3c), individual ovules and spikelets are replaced by smut sori while other ovules and spikelets within the same panicle develop normal seeds (Frederiksen, 2000;Kollo, 2000). In contrast to head smut infection, symptoms may vary from sterile panicles to entire panicles been replaced with teliospores (Prom et al., 2011b) so that grain losses can be directly predicted from the percent infected plants.

Conclusion
In conclusion, this study confirmed the distribution of sorghum diseases and sites "hot spot" for leaf blight, anthracnose, oval leaf spot, rough leaf spot and zonate leaf spot for resistance evaluations in the major production regions in Niger. This work also is significant because the information generated on the incidence, severity, farm sizes are vital components that can be utilized by scientists, producers, funding agencies, and government officials to estimate relative economic impact of these various sorghum diseases in Niger. USDA is an equal opportunity provider and employer.

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