Incidence of Zonate Leaf Spot on Sorghum Accessions Under Disease-Conducive Growing Conditions Burleson County, Texas

The frequent rains coupled with cooler than normal maximum temperatures in the early part of the 2021 growing season created conditions for severe outbreaks of zonate leaf spot infection on sorghum plants in the experimental plots at the Texas A&M AgriLife Farm, Burleson County, Texas. As a result, the incidence of zonate leaf spot was recorded for 68 accessions planted in one of the field trials. In this study, 13 sorghum accessions, including TAM428, BTx635, PI330255, PI534157, PI570841, PI609251, PI570726, and PI267588 were free of zonate leaf spot infection, indicating that they were highly resistant to the disease. The lines identified in this study may possess genes for resistance which can be useful in breeding programs for introgression of the resistant genes to elite or parental lines.


Introduction
Environmental factors such as precipitation, temperature, and relative humidity are significant contributors to pathogen infectivity and susceptibility of the host as elucidated by the disease tetrahedron [1][2][3][4][5]. Moricca and Ragazzi [2], noted that climatic factors in the Mediterranean oak forests can change the endophyte Discula quercina into a weak pathogen or an opportunistic invader of aging plant tissues or healthy trees. Severe powdery mildew infection on winter wheat in the United Kingdom was shown to be influenced by temperature, humidity, and rain in April to June, while yellow rust severity was most influenced by temperature in February to June [5]. In India, wheat stripe (yellow) rust caused by Puccinia striiformis f. sp. tritici was most damaging under cool and moist weather conditions [4]. Rodríquez-Moreno, et al. [3], noted that in Mexico, dew temperature of <13.7⁰C and mean temperature of <19.06⁰C were better determinant of leaf rust severity on winter wheat. Grain mold of sorghum [Sorghum bicolor (L.) Moench] is most severe in areas with wet weather conditions late in the growing season, but less severe or absent in areas with drier weather conditions during grain maturity [6]. Prom, et al. [7], reported higher susceptibility of sorghum accessions evaluated for anthracnose resistance during the wet season than in the dry season. The current unpredictability of weather patterns due to global warming may change or alter our understanding of different pathosystems and will warrant continuous evaluation of germplasm to identify potential sources of resistance to major and minor diseases. Sorghum, is a drought tolerant crop grown in diverse environments where it is exposed to several plant pathogens, including those that incite zonate leaf spot [8][9][10].
Zonate leaf spot on sorghum, corn, millet, and other grasses is incited by Gloeocercospora sorghi Bain & Edgerton ex Deighton [8,10]. The pathogen perennates as sclerotia on infected plant tissues and in the soil and is endemic in South Texas [8,10]. The infection process is initiated when sclerotia germinate and conidia are rainsplashed unto the leaves [10]. Depending on the host, symptoms ( Fig. 1) are characterized by roughly circular to semicircular if close to the leaf margins with alternating bands of dark purple, red, tan, or straw color giving it a concentric or zonate appearance [8,10]. Early infection on seedlings could lead to defoliation and plant death [8]. On forage sorghum, G. sorghi can cause significant yield and quality losses [11].
During early part of the 2021 growing season, frequency of rainfall events and relatively cooler temperatures occurred. These environmental conditions were favorable for zonate leaf spot disease development in Burleson County, Texas. Thus, this communication reports the response of sorghum accessions, mainly from Ethiopia to zonate leaf spot.

Field Evaluation
Due to the favorable conditions after plant emergence, a total of 68 sorghum germplasm, including 50 accessions from Ethiopia were evaluated for resistance against G. sorghi at the Texas A&M AgriLife Research Farm, Burleson County, near College Station, TX during the 2021 growing season. Seeds from Ethiopia, France, Mali, and Sudan for the evaluation were provided by the USDA-ARS, Plant Genetic Resources Conservation Unit, Griffin, Georgia. Seeds were planted in a randomized complete block design and replicated three times. Seed was planted in 6 m rows at 0.31 m spacing between rows. Field preparation included fall plowing and the application of NPK according to local recommendation. Hand weeding of the plots was conducted during the season to control weeds.

Disease Rating
Disease incidence occurring under natural conditions was assessed at or before soft dough stage of development for each replicate. Percent incidence was based on the number of plants in a row divided by the number of zonate leaf spot infected plants in the same row then multiplied by 100.

Statistical Analysis
The data were analyzed using the command PROC GLM (SAS version 9.4, SAS Institute Inc., Cary, NC). The LSMEANS statement with LINES option was used to provide Tukey-Kramer multiple comparisons between accessions at the 5% probability level.

Results
During the 2021 growing season, the mean maximum temperature for April and May were lower than the previous five years, except for April 2018 and May 2016 (Table 1). Also, more rainy days (17) and higher relative humidity (85%) for the month of May and higher total precipitation (17.1 cm) for the month of June were recorded than the previous five years. These environmental conditions were favorable for the development of zonate leaf spot in sorghum plants in the experimental plots at Burleson County, Texas.
The main effect of accessions was highly significant (P < 0.0001), indicating that the accessions responded differently to zonate leaf spot infection. In this study, 13 sorghum accessions, including TAM428, BTx635, PI330255, PI534157, PI570841, PI609251, PI570726, and PI267588 were free of zonate leaf spot infection, indicating that they were highly resistant to the disease ( Table 2). PI668717 exhibited the highest incidence of zonate leaf spot (96.7%), and this level was significantly different from the levels recorded from 41% of the accessions evaluated. A total of 18 accessions had less than 10% incidence and their infection levels ranged from 6.7% (PI669795 and PI534115) to zero.

Discussion
In drier sorghum producing regions, zonate leaf spot is considered a minor disease, causing negligible yield losses [12]. However, under humid and wet conditions, the disease can cause significant yield and quality losses, especially on forage and sweet sorghum [8,12,13]. In Asia, 78 sorghum accessions evaluated for resistance to zonate leaf spot noted that 26 accessions were resistant to the disease [14]. Prom, et al. [13], evaluated 181 sorghum lines for resistance to zonate leaf spot, 24 lines, including Dorado, Sureno, PI656034, PI576434, PI655999, and PI656075) recorded zero infection, indicating that these lines possess genes for zonate leaf spot resistance.
In most years, G. sorghi which can persist in infected debris on the soil is usually observed in low levels in grain and forage sorghum plants around Burleson County, Texas; however, with continuous sorghum cropping in the same field, inoculum is always available. And under favorable conditions early in the growing season as noted in 2015 [13], disease severity can be significant. In 2021 growing season, early and frequent rains with cooler than normal temperatures resulted in the appearance of zonate leaf spot in epidemic proportions. Out of the 68 lines evaluated for the disease in the field, 13 accessions recorded zero infection, suggesting that these accessions may possess gene for resistance to zonate leaf spot.

Conclusion
Zonate leaf spot is considered a minor disease; however, a shift in weather patterns to wetter conditions in this region, especially early in the growing season due to climate may change its status as a sorghum disease of economic importance. Therefore, the identification of new sources of resistance to the disease is warranted. The 13 accessions identified in this study may possess genes for resistance which can be useful in breeding programs for introgression of the resistant genes to elite or parental lines. Abbreviations: Tmax = maximum temperature (⁰C); Tmin = minimum temperature; Precip = total precipitation in cm; DP = Number of days with precipitation; and RH = average relative humidity (%).