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Plant Health Research

Reaction of Sorghum Lines to Zonate Leaf Spot and Rough Leaf Spot

Louis K. Prom, USDA-ARS, Southern Plains Agriculture Research Center, College Station, TX 77845; Thomas Isakeit, Department of Plant Pathology and Microbiology, Texas A&M University, College Station 77843; Hugo Cuevas, USDA-ARS, Tropical Agriculture Research Station, Mayaguez, PR 00680; William L. Rooney, Department of Soil and Crop Sciences, Texas A&M University, College Station 77843; Ramasamy Perumal, Kansas State University, Agricultural Research Center, Hays 67601; and Clint Magill, Department of Plant Pathology and Microbiology, Texas A&M University, College Station 77843

Accepted for publication 8 December 2015. Published 14 December 2015.

ABSTRACT

Prom, L. K., Isakeit, T., Cuevas, H., Rooney, W. L., Perumal, R., and Magill, C. 2015. Reaction of sorghum lines to zonate leaf spot and rough leaf spot. Plant Health Progress doi:10.1094/PHP-RS-15-0040.

Abundant, frequent rains along with humid and cloudy conditions during the early part of the 2015 growing season provided conducive conditions for an unusually severe outbreak of zonate leaf spot and rough leaf spot in a block of sorghum lines at the Texas A&M AgriLife Research Farm, Burleson County, near College Station, Texas. This afforded the opportunity to evaluate these lines for reaction to these

diseases, which normally occur at trace levels in this area. Out of 181 sorghum lines, including a subset of the sorghum association panel, a total of 13 lines, including Dorado, Sureno, PI576434, PI656005, PI656034, PI656075, PI656024, and PI598070, were resistant to both diseases. These lines could be used in breeding programs to introgress the genes for resistance to both diseases into other adapted lines.

INTRODUCTION

Generally, the climate of a particular growing area affects which diseases are predominant and their levels of severity from year to year. For example, grain mold of sorghum, caused by various fungi, is severe in areas with wet weather conditions after anthesis to grain maturity, but less severe or absent in areas with drier weather conditions during grain maturity (Bandyopadhyay et al. 2000). For the next century, substantial global climate change has been forecast, with most cropped areas predicted to become warmer and drier (Christensen et al. 2007). Under these changed conditions, it seems likely that the frequency or severity of epidemics caused by fungal pathogens will be altered (Burdon et al. 2006), but to date there is very little empirical evidence to suggest in which direction. The unpredictable year-to-year weather change due to global warming warrants continuous evaluation of the germplasm to identify potential sources of major and minor diseases. Sorghum [Sorghum bicolor (L.) Moench] is grown in diverse environments in which the crop is constantly being challenged by several plant pathogens, including those that incite zonate leaf spot and rough leaf spot (Frederiksen and Odvody 2000; Odvody and Madden 1984; Zummo and Broadhead 1984).

Gloeocercospora sorghi Bain & Edgerton ex Deighton causes zonate leaf spot on sorghum, corn, millet, and other grasses (Frederiksen and Odvody 2000; Odvody and Madden 1984). The pathogen overwinters as sclerotia on infected plant tissues and is endemic in South Texas (Odvody and Madden 1984). The soilborne sclerotia germinate and the rain-splashed conidia are

dispersing to initiate the infection process (Odvody and Madden 1984). The symptoms (Fig. 1) of zonate lesions are roughly circular to semicircular if close to the leaf margins and, depending on the variety of the host, with alternating bands of dark purple, red, tan, or straw-color, giving it the characteristic concentric or zonate appearance (Franklin 2000; Odvody and Madden 1984; Palakshappa and Hiremath 2003; Purohit et al. 2013). Seedling infection can result in defoliation and death of the plant (Franklin 2000). Infected plants can exhibit damage to photosynthetic leaf area as high as 85% under humid and cloudy weather conditions,

Corresponding author: Louis K. Prom. Email: louis.prom@ars.usda.gov.

doi:10.1094 / PHP-RS-15-0040 © 2015 The American Phytopathological Society

FIGURE 1 Zonate lesions are roughly circular to semicircular if close to the leaf margins and, depending on the variety of the host, with alternating bands of dark purple, red, tan, or straw-color, giving it the characteristic concentric or zonate appearance.

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and yield losses have ranged from 32 to 60% (Agnihotri and Pandey 1977; Grewal 1988).

Rough leaf spot is found in many sorghum growing regions and is incited by Ascochyta sorghina Sacc. (Zummo 2000; Zummo and Broadhead 1984). The disease appears to be confined to Sorghum spp. and is most prevalent in humid areas (Zummo 2000). The pathogen perpetuates in infected crop debris and spreads by airborne pycnidiospores under wet conditions (Zummo 2000). The disease is easily identified on infected leaves by the sandpapery roughness (Fig. 2) due to the hard, black, raised pycnidia produced by the pathogen (Zummo 2000). In drier sorghum producing regions, losses can be minimal (Zummo and Broadhead 1984).

This paper reports the response of sorghum germplasm, including a subset of the sorghum association panel (Casa et al. 2008), to zonate leaf spot and rough leaf spot infections. This data collection was a unique opportunity caused by unusually wet, humid weather conditions that occurred in a growing area where these diseases occur in trace amounts.

FIELD EVALUATION A total of 181 sorghum lines, including a subset of 171 lines

from the sorghum association panel, were evaluated for resistance against G. sorghi and A. sorghina at the Texas A&M AgriLife Research Farm, Burleson County, near College Station, TX, during the 2015 growing season. The sorghum association panel was compiled by Casa et al. (2008) for association mapping and consists of 377 accessions selected from all major cultivated races, important U.S. breeding lines, and their progenitors. Seed samples for the evaluation were provided by the USDA-ARS Plant Genetic Resources Conservation Unit, Griffin, GA. Accessions were planted in a randomized complete block design and replicated. Seed was planted in 6-m rows at 0.31-m spacing between rows. Field preparation included plowing the previous fall and incorporation of NPK according to local recommendation. To control weeds and seedling insects, a pre-emergent insecticide ‘Counter 20 CR’ (BASF Group, Southfield, MI) and the herbicide atrazine (Syngenta Crop Protection Inc. Greenboro, NC) were applied before planting.

DISEASE RATINGS AND STATISTICAL ANALYSIS Disease occurring under natural conditions was assessed at or

before the soft dough stage of development using a 1 to 5 scale for zonate leaf spot by Odvody and Madden (1984), where: 1 = no disease (resistant); 2 = 1 or more diseased sheaths per plant at 25 to 50% (moderately resistant); 3 = 1 or 2 diseased sheaths per plant at 51 to 100% (moderately susceptible); 4 = 3 or more diseased sheaths per plant at 25 to 50% (susceptible); and 5 = all diseased sheaths at 100. For the rough leaf spot assessment, a 0 to 4 rating scale by Zummo and Broadhead (1984) was used, where: 0 = no infection; 1 = fewer than 50% of the plants in each plot infected, with infected areas very small; 2 = more than 50% of plants in each plot infected but the percentage of diseased leaf area considered too small to affect yield or quality; 3 = all plants in each plot infected, with less than 25% of leaf area of each plant damaged, but still considered severe enough to cause some reduction in yield or quality; and 4 = all plants in each plot infected, with more than 25% of leaf area of plant destroyed by the disease. The data were analyzed using the command PROC GLIMMIX (SAS version 9.2, SAS Institute Inc., Cary, NC). Differences in means among sorghum lines were determined at the 5% probability level based on the Tukey-Kramer test.

SOURCES OF RESISTANCE Zonate leaf spot and rough leaf spot are considered to cause

minor yield losses in drier sorghum growing regions. However, these diseases under humid and wet conditions can cause significant losses in both yield and quality, especially on forage and sweet sorghum (Franklin 2000; Grewal 1988; Zummo 2000; Zummo and Broadhead 1984). G. sorghi and A. sorghina can persist in infected debris on the soil, so even with trace levels of disease in one season, with continuous sorghum cropping in the same field, inoculum is always available and disease severity can significantly increase in subsequent years with a change to more conducive weather (Franklin 2000; Odvody and Madden 1984; Zummo 2000; Zummo and Broadhead 1984). During the 2015 growing season, the total precipitation and number of days with rain for April (12.2 cm, 19 days), May (24.71 cm, 21 days), and June (13.2 cm, 13 days) (Table 1) was higher than those for the same three months in the previous five years. April and May 2015 rainfall totals of 12.2 and 24.7 cm were recorded in College Station, TX, respectively, and exceeded the number of rainy days when compared to the last five years (Table 1). According to the office of the Texas state climatologist, the month of May 2015 was the wettest on record for Texas, having received an average of 22.4 cm statewide. In the College Station area, high rainfall with more numerous rainy days during the entire growing season favored the infection of zonate leaf spot in 2015.

In this study, differences in response to these two diseases among the 181 sorghum germplasm evaluated were observed. Twenty-four lines, including Dorado and Sureno, were free of zonate leaf spot infection, indicating that these lines were resistant to the disease (Table 2). Across the lines, 29% were moderately resistant and 38% were moderately susceptible to susceptible. PI656029, PI642992, and PI576425 recorded the highest incidence (100%) of zonate leaf spot, whereas PI656103 and PI656117 (2.5%) had the lowest incidence. Other lines exhibited zonate leaf spot incidences ranging from 5 to 97%.

The average relative humidity in the growing season 2015 was higher than the six year average (Table 1), which was conducive to rough leaf spot disease but with significantly less disease

FIGURE 2 Rough leaf spot is characterized by the sandpapery roughness due to the hard, black, raised pynidia produced by the pathogen.

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severity when compared with zonate leaf spot infection. A total of 96 out of 181 sorghum lines had no rough leaf spot (Table 2). Thirty-nine percent of the lines tested had symptoms of rough leaf spot at levels that were not significant enough to affect the yield

or quality. In contrast, 8% of the lines evaluated were severely infected (rating of 3) to the extent of diminishing both grain yield and quality.

TABLE 1 Weather parameters at the AgriLife Research Farm, Burleson County, Texas for six years (2010 to 2015).

Year April May June

Tmax Tmin Precip DP RH Tmax Tmin Precip DP RH Tmax Tmin Precip DP RH

2010 26.3 14.7 2.8 15.0 74 32.2 20.5 5.1 6.0 71 33.3 24.4 22.9 13.0 80 2011 31.1 17.4 0.1 4.0 66 31.7 19.1 8.6 7.0 70 36.8 23.8 7.3 4.0 69 2012 28.5 17.0 1.5 7.0 72 31.3 20.3 4.6 10.0 75 34.9 23.2 5.4 8.0 74 2013 25.1 13.1 4.2 15.0 67 29.6 18.0 17.1 10.0 68 35.0 23.0 3.2 4.0 63 2014 25.7 14.4 3.1 11.0 64 28.8 17.3 22.9 9.0 67 32.7 23.3 4.1 16.0 70 2015 26.3 16.5 12.2 19.0 73 28.7 19.7 24.7 21.0 76 32.3 22.8 13.2 13.0 72 Average 27.2 15.5 4.0 11.8 69 30.4 20.0 13.3 10.5 71 34.2 23.4 9.4 9.6 71

Abbreviations: Tmax = maximum temperature (°C); Tmin = minimum temperature (°C); Precip = total precipitation in cm; DP = Number of days with precipitation; and RH = average relative humidity (%).

TABLE 2 Reaction of sorghum germplasm to zonate leaf spot and rough leaf spot.

Lines

Zonate leaf spotx Rough leaf spotx Lines

Zonate leaf spot Rough Leaf spot Score Incidence Score Incidence

SC 719-11E 4.3ay 97.0 0.0 PI659693 3.5abc 92.5 0.0 PI576390 4.0ab 90.0 1.0 PI576401 3.0abc 85.0 1.0 PI656030 4.0ab 95.0 0.0 PI655991 3.0abc 72.5 0.0 PI659691 4.0ab 90.0 0.0 PI656028 3.0abc 95.0 1.0 PI656078 3.5abc 77.5 3.0 PI576399 3.0abc 80.0 0.0 PI655971 3.5abc 65.0 1.0 PI576393 3.0abc 85.0 0.0 PI655977 3.5abc 77.5 1.0 PI656007 3.0abc 70.0 2.0 PI653617 3.5abc 92.5 1.0 PI655992 3.0abc 85.0 0.0 PI656121 3.5abc 95.0 0.0 PI576385 3.0abc 87.5 2.0 PI656119 3.5abc 67.5 0.0 PI656116 3.0abc 72.5 0.0 PI576387 3.5abc 65.0 0.0 PI576418 3.0abc 70.0 0.0 PI656087 3.5abc 85.0 0.0 PI655981 3.0abc 80.0 2.0 PI642791 3.5abc 85.0 0.0 PI655972 3.0abc 75.0 2.0 PI576425 3.5abc 100.0 3.0 PI641874 3.0abc 65.0 0.0 PI656098 3.5abc 85.0 1.5 PI597976 3.0abc 80.0 1.0 PI656003 3.5abc 90.0 2.0 PI655985 3.0abc 60.0 3.0 PI656112 3.5abc 90.0 3.0 PI655976 3.0abc 85.0 2.0 PI656029 3.5abc 100.0 1.0 PI655994 3.0abc 85.0 1.0 PI656108 3.5abc 87.0 0.0 PI655986 3.0abc 70.0 3.0 PI656109 3.5abc 90.0 3.0 PI656100 3.0abc 42.5 0.0 PI656019 3.5abc 95.0 1.0 PI576391 3.0abc 70.0 1.0 PI597973 3.0abc 97.5 2.0 PI656002 3.0abc 80.0 1.0 PI659970 3.0abc 79.3 0.0 PI656021 3.0abc 85.0 3.0 PI656086 3.0abc 75.0 0.0 PI597982 3.0abc 75.0 1.0 PI656082 3.0abc 62.5 1.0 PI642992 3.0abc 100.0 3.0 PI656073 3.0abc 90.0 0.0 PI659696 3.0abc 67.5 0.0 PI655990 3.0abc 70.0 2.0 PI656033 2.5abc 70.0 0.0 PI656096 3.0abc 90.0 0.0 PI656079 2.5abc 70.0 2.0 PI656083 3.0abc 70.0 0.0 PI651496 2.5abc 80.0 0.0 PI656035 3.0abc 77.5 0.0 PI656097 2.5abc 75.0 1.0 PI656000 3.0abc 95.0 0.0 PI656118 2.5abc 55.0 0.0 PI651492 3.0abc 80.0 0.0 PI656015 2.5abc 80.0 0.0 PI576422 3.0abc 87.5 1.0 PI656106 2.5abc 55.0 0.0 PI656001 3.0abc 87.5 1.0 PI576394 2.5abc 87.5 2.0 PI576396 3.0abc 77.5 0.0 PI597980 2.5abc 70.0 0.0 PI597966 3.0abc 97.5 0.0 PI656093 2.5abc 70.0 1.0 PI642793 3.0abc 85.0 0.0 PI576426 2.5abc 75.0 3.0 PI597972 3.0abc 95.0 0.0 PI655974 2.5abc 70.0 0.0 PI655973 3.0abc 85.0 1.0 PI656092 2.5abc 80.0 2.0

(continued)

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TABLE 2 (continued) Reaction of sorghum germplasm to zonate leaf spot and rough leaf spot.

Lines

Zonate leaf spotx Rough leaf spotx Lines

Zonate leaf spot Rough Leaf spot Score Incidence Score Incidence

PI656113 3.0abc 62.5 2.0 PI653616 2.5abc 77.5 2.0 PI597971 3.0abc 92.5 0.0 PI576437 2.5abc 52.5 1.0 PI655984 3.0abc 75.0 0.0 PI656101 2.5abc 60.0 0.0 PI655993 3.0abc 97.5 0.0 PI656023 2.5abc 75.0 2.0 PI656074 2.5abc 90.0 0.0 PI656070 2.0abc 30.0 0.0 PI659695 2.5abc 75.0 2.0 PI655980 2.0abc 45.0 1.5 PI597964 2.5abc 77.5 0.0 PI656107 2.0abc 72.5 2.0 PI656072 2.5abc 85.0 2.0 PI656111 2.0abc 57.5 1.0 PI607931 2.5abc 90.0 3.0 PI656017 2.0abc 5.0 0.0 PI656089 2.5abc 85.0 1.0 PI659970 2.0abc 15.7 0.0 PI641849 2.5abc 90.0 1.0 PI656080 2.0abc 52.5 0.0 PI656105 2.5abc 50.0 0.0 PI641836 2.0abc 80.0 1.0 PI641824 2.5abc 90.0 3.0 PI656090 2.0abc 35.0 2.0 PI655983 2.5abc 70.0 0.0 PI656102 2.0abc 42.5 0.0 PI656020 2.5abc 95.0 1.0 PI656120 2.0abc 47.5 0.0 PI655975 2.5abc 90.0 1.0 PI656110 2.0abc 60.0 3.0 PI655987 2.5abc 87.5 0.0 PI656018 2.0abc 75.0 0.0 PI655996 2.5abc 60.0 0.0 PI576386 2.0abc 50.0 0.0 PI656104 2.5abc 50.0 0.0 PI655988 2.0abc 77.5 0.0 PI655998 2.5abc 72.5 0.0 PI656013 2.0abc 95.0 3.0 PI655978 2.5abc 55.0 0.0 PI655997 2.0abc 50.0 2.0 RTx2536 2.3abc 37.0 0.0 PI656115 2.0abc 67.5 0.0 ATx623 2.3abc 33.6 0.0 PI613536 2.0abc 75.0 0.0 PI656014 2.0abc 50.0 3.0 PI655982 2.0abc 17.5 0.0 SC748-5 2.0abc 44.8 0.0 PI656016 2.0abc 72.5 0.0 BTx623 2.0abc 36.4 0.0 PI576381 2.0abc 52.5 2.0 PI656081 2.0abc 45.0 0.0 PI656006 2.0abc 45.0 2.0 PI656094 2.0abc 75.0 0.0 PI656071 2.0abc 15.0 1.0 PI656091 2.0abc 57.5 0.0 PI656022 1.5bc 10.0 0.0 PI656117 1.5bc 2.5 1.0 PI629040 1.5bc 37.5 0.0 PI576428 1.5bc 30.0 0.0 PI656099 1.5bc 35.0 0.0 PI656009 1.5bc 35.0 1.0 PI656095 1.5bc 32.5 2.0 PI597961 1.5bc 20.0 0.0 Dorado 1.0c 0.0 0.0 PI656114 1.5bc 5.0 1.0 Sureno 1.0c 0.0 0.0 PI656069 1.5bc 12.5 1.0 PI656034 1.0c 0.0 0.0 PI655979 1.5bc 5.0 0.0 PI576434 1.0c 0.0 0.0 PI656010 1.5bc 15.0 0.0 PI655999 1.0c 0.0 0.0 PI629034 1.5bc 5.0 1.0 PI629059 1.0c 0.0 2.0 PI656088 1.5bc 20.0 0.0 PI598070 1.0c 0.0 0.0 PI656078 1.5bc 45.0 2.0 PI656008 1.0c 0.0 1.0 PI609456 1.5bc 20.0 1.0 PI656031 1.0c 0.0 2.0 PI656012 1.5bc 45.0 1.0 PI601816 1.0c 0.0 0.0 PI656011 1.5bc 40.0 0.0 PI656025 1.0c 0.0 0.0 PI597965 1.5bc 37.5 0.0 PI656032 1.0c 0.0 2.0 PI585291 1.5bc 25.0 0.0 PI656004 1.0c 0.0 1.0 PI598069 1.5bc 12.5 0.0 PI655995 1.0c 0.0 3.0 PI656103 1.5bc 2.5 1.0 PI656076 1.0c 0.0 0.0 PI656085 1.0c 0.0 0.0 PI656024 1.0c 0.0 0.0 PI642998 1.0c 0.0 1.5 PI659694 1.0c 0.0 1 PI576435 1.0c 0.0 2 PI656005 1.0c 0.0 0.0 PI656026 1.0c 0.0 1 PI656075 1.0c 0.0 0.0 PI656027 1.0c 0.0 1 x A rating scale of 1 to 5 as described by Odvody and Madden (1984) was used for the zonate leaf spot assessment, where: 1 = no disease (resistant);

2 = 1 or more diseased sheaths per plant at 25 to 50% (moderately resistant); 3 = 1 or 2 diseased sheaths per plant at 51 to 100% (moderately susceptible); 4 = 3 or more diseased sheaths per plant at 25 to 50% (susceptible); and 5 = all diseased sheaths at 100. For the rough leaf spot assessment, a 0 to 4 rating scale by Zummo and Broadhead (1984) was used, where: 0 = no infection; 1 = fewer than 50% of the plants in each plot infected, with infected areas very small; 2 = more than 50% of plants in each plot infected but the percentage of diseased leaf area considered too small to affect yield or quality; 3 = all plants in each plot infected, with less than 25% of leaf area of each plant damaged, but still considered severe enough to cause some reduction in yield or quality; and 4 = all plants in each plot infected, with more than 25% of leaf area of plant destroyed by the disease.

y Means within column with the same letter(s) are not significantly different at the 5% probability level based on the Tukey-Kramer test.

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In this study, 13 lines Dorado, Sureno, PI576434, PI656005, PI656034, PI656075, PI656024, PI655999, PI656076, PI656085, PI601816, PI656025, and PI598070 were resistant to both diseases. Sarwar et al. (1988) evaluated 78 sorghum accessions in Asia and noted that 7 accessions were resistant to rough leaf spot and 26 to zonate leaf spot. Zummo and Broadhead (1984) evaluated large numbers of exotic, unreleased, and parental sweet sorghum lines in Mississippi for resistance against A. sorghina and identified two lines, MN 4055 and MN 3344, possessing high levels of resistance to the disease. Cultivar CSV 1 was resistant to rough leaf spot but susceptible to zonate leaf spot and cultivar CSV 5 was resistant to zonate leaf spot and moderately resistant to rough leaf spot, while cultivar CSV 4 was resistant to both diseases (Sharma and Jain 1978).

CONCLUSIONS Zonate leaf spot and rough leaf spot are considered minor

diseases of sorghum as they do not significantly affect yield. However, the unusually high rainfall in Texas during the 2015 cropping season greatly increased disease severity of these two pathogens, suggesting that they do have a yield-limiting potential in exceptionally wet years, and therefore this potential warrants the identification of new sources of resistance. This study identified a total of 13 lines, including Dorado, Sureno, PI576434, PI656005, PI656034, PI656075, PI656024, and PI598070, as potential resistant sources to both diseases. These lines could be used in breeding programs to introgress the genes for resistance to both diseases into other adapted lines.

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