Distribution and association of factors influencing emerging maize lethal necrosis disease epidemics in Southern Ethiopia

Figures & data

Figure 1. Average monthly rainfall (mm) and temperature (°C) of the surveyed (AM = Arbaminch, MA = Mihirab Arbaya, DG = Demba Gofa, KN = Konso, BR = Burjdi, DR = Derashie and BN = Bonkie) districts, Southern Ethiopia, during the 2016 and 2017 main cropping seasons.

Figure 2. Map showing districts surveyed for maize lethal necrosis disease in Southern Ethiopia, during the 2016 and 2018 main cropping seasons.

Table 1. Categorization of variables and independent variable x disease contingency table for logistic regression analysis of maize lethal necrosis disease severity (%) in seven districts (n = 240) of Southern Ethiopia, during the 2016 and 2017 main cropping seasons

Variable	Variable class	Number of field	Disease severity (%)		Variable	Variable class	Number of field	Disease severity (%)
			≤50	>50				≤50	>50
District	Arbaminch	45	0	45	N fertilizer (Kg ha^−1)	0	49	1	48
	Mihirab Abaya	45	0	45		1 to 50	136	13	123
	Bonkie	30	3	27		51 to 100	55	7	48
	Demba Gofa	40	0	40	Planting date	June	10	0	10
	Derashie	30	6	24		July	74	20	54
	Konso	30	5	25		August	126	1	125
	Burjdi	20	7	13		September	30	0	30
Year	2016	106	7	99	Crop growth stage	Vegetative	29	0	29
	2017	134	14	120		Tasseling	56	3	53
Altitude (m.a.s.l.)^1	< 1000	15	0	15		Silking	50	3	47
	1001 to 1500	179	3	176		Blister	73	8	65
	1501 to 2000	37	15	22		Milk	22	7	15
	> 2000	9	3	6		Dough	10	0	10
Maize genotype ^2	Bako hybrid	124	13	111	Insect infestation ^4	Low level	76	18	58
	Pioneer hybrid	31	0	31		Medium level	119	3	116
	Melkasa comp.	35	6	29		High level	45	0	45
	Local cultivar	50	2	48	Insect species	Aphids	42	0	42
Seed source	Bureau of Agric.	158	19	139		Stalk borer + Aphids	58	7	51
	Seed company	31	0	31		Stalk borer + Thrips	32	0	32
	Farm saved	51	2	49		Fall armyworm	108	14	94
Cropping system ^3	Sole cropping	117	0	117	Insecticide spray frequency	None	76	0	76
	Intercropping	123	21	112		Only once	114	7	107
NPS fertilizer (Kg ha^−1)	0	31	1	30		Twice and more	50	14	36
	1 to 50	119	3	116	Weed infestation ^5	Good	45	21	24
	51 to 100	90	17	73		Intermediate	144	0	144
						Poor	42	0	42

¹Areas with <1000, 1001 to 1500, 1501 to 2000 and >2000 m.a.s.l are classified as tepid lowland, tepid humid lowland, midland and highland, respectively, in Ethiopia

²Maize genotype refers to genotypes developed by agricultural research centers (Melkasa and Bako), and pioneer hybrid seed developing the private company

³Intercropping refers to farms with maize plus common bean and/or pigeon pea and/or sorghum and/or teff and/or banana and/or mango and/or papaya and/or trees

⁴Insect pest infestation was recorded as the total number of insect species presented in a quadrat with low (≤50), medium (51 to 100) and high (>100)

⁵Weed infestation was recorded as good, intermediate and low indicating weed-free, few weeds and no weeding, respectively.

Figure 3. Linear regression relating maize lethal necrosis disease severity (%) and weed population density (m⁻²) in maize fields in Southern Ethiopia, during the 2016 and 2017 main cropping seasons.

Table 2. Severity (mean ± SE) of maize lethal necrosis disease for different independent variables in the 2016 and 2017 main cropping seasons, Southern Ethiopia

Variable	Variable class	Number of field	Disease severity (%)	Variable	Variable class	Number of field	Disease severity (%)
District	Arbaminch	45	77.53 ± 1.36	N fertilizer (Kg ha^−1)	0	49	81.22 ± 1.74
	Mihirab Abaya	45	75.43 ± 1.48		1 to 50	136	72.26 ± 1.09
	Bonkie	30	69.73 ± 2.11		51 to 100	55	65.28 ± 1.29
	Demba Gofa	40	78.68 ± 1.21	Planting date	June	10	68.73 ± 0.77
	Derashie	30	68.14 ± 2.63		July	74	64.26 ± 1.72
	Konso	30	71.41 ± 2.95		August	126	74.58 ± 0.87
	Burjdi	20	54.43 ± 2.40		September	30	85.27 ± 1.39
Year	2016	106	75.85 ± 1.24	Crop growth stage	Vegetative	29	67.56 ± 1.11
	2017	134	69.84 ± 1.09		Tasseling	56	74.33 ± 1.37
Altitude (m.a.s.l.) ^1	< 1000	15	72.27 ± 3.05		Silking	50	79.92 ± 2.07
	1001 to 1500	179	76.27 ± 0.74		Blister	73	70.46 ± 1.53
	1501 to 2000	37	57.92 ± 2.38		Milk	22	65.95 ± 3.51
	> 2000	9	57.63 ± 3.79		Dough	10	68.53 ± 0.60
Maize genotype ^2	Bako hybrid	124	70.65 ± 1.14	Insect infestation ^4	Low level	76	60.53 ± 1.20
	Pioneer hybrid	31	69.38 ± 1.14		Medium level	119	74.29 ± 0.80
	Melkasa comp.	35	68.89 ± 2.34		High level	45	87.94 ± 0.76
	Local cultivar	50	81.52 ± 1.78	Insect species	Aphids	42	83.60 ± 1.17
Seed source	Bureau of Agric.	158	70.17 ± 1.02		Stalk borer + Aphids	58	69.22 ± 1.61
	Seed company	31	69.16 ± 1.01		Stalk borer + Thrips	32	84.14 ± 1.20
	Farm saved	51	81.70 ± 1.75		Fall armyworm	108	66.48 ± 1.10
Cropping system ^3	Sole cropping	117	79.81 ± 0.92	Insecticide spray frequency	None	76	83.28 ± 0.98
	Intercropping	123	65.52 ± 1.05		Only once	114	71.07 ± 0.92
NPS fertilizer (Kg ha^−1)	0	31	83.87 ± 2.24		Twice and more	50	59.33 ± 1.60
	1 to 50	119	74.58 ± 0.92	Weed infestation ^5	Good	45	54.36 ± 1.56
	51 to 100	90	65.80 ± 1.33		Intermediate	144	73.16 ± 0.64
					Poor	42	86.59 ± 0.91

¹Areas with <1000, 1001 to 1500, 1501 to 2000 and >2000 m.a.s.l are classified as tepid lowland, tepid humid lowland, midland and highland, respectively, in Ethiopia

²Maize genotype refers to genotypes developed by agricultural research centers (Melkasa and Bako) and pioneer hybrid seed developing the private company

³Intercropping refers to farms with maize plus common bean and/or pigeon pea and/or sorghum and/or Teff and/or banana and/or mango and/or papaya and/or trees

⁴Insect pest infestation was recorded as the total number of insect species presented in a quadrat with low (≤50), medium (51 to 100) and high (>100)

⁵Weed infestation was recorded as good, intermediate and poor indicating weed-free, few weeds and no weeding, respectively.

Table 3. Logistic regression model for maize lethal necrosis disease severity (%) and likelihood ratio test on independent variables in Southern Ethiopia, during the 2016 and 2017 main cropping seasons

Independent variable	df	Maize lethal necrosis disease severity, LRT ^1
		Type 1 analysis (VEF)		Type 3 analysis (VEL)
		DR	Pr > χ^2	DR	Pr > χ^2
District	6	497.25	<0.0001	134.17	<0.0001
Year	1	91.09	<0.0001	0.53	0.4680
Altitude (m.a.s.l.)	3	300.51	<0.0001	4.75	0.1913
Genotype	3	484.42	<0.0001	2.38	0.4977
Seed source	2	31.42	<0.0001	0.49	0.7814
NPS fertilizer (Kg ha^−1)	2	200.57	<0.0001	15.53	0.0004
N fertilizer (Kg ha^−1)	2	62.66	<0.0001	1.89	0.3887
Planting date	3	72.73	<0.0001	6.76	0.0800
Crop growth stage	5	36.38	<0.0001	11.44	0.0434
Cropping system	1	64.58	<0.0001	10.21	0.0004
Insect infestation	2	61.33	<0.0001	23.94	<0.0001
Insect species	3	3.05	0.3844	1.88	0.5975
Insecticide spray frequency	2	27.31	<0.0001	14.78	0.0006
Weed infestation	2	19.69	<0.0001	19.69	<0.0001

¹LRT = likelihood ratio test; VEF = variable entered first in the model; VEL = variable entered last in the model; DR = deviance reduction; Pr = probability of an χ² value exceeding the deviance reduction; χ² = chi-square; df = degrees of freedom.

Table 4. Analysis of deviance, natural logarithms of odds ratio and standard error of maize lethal necrosis disease severity (%) and likelihood ratio test on independent variables in reduced regression model in Southern Ethiopia, during the 2016 and 2017 main cropping seasons

Added variable	Residual deviance ^1	df	MLN disease, LRT ^2		Variable class	Estimate Loge (odds ratio) ^3	SE	Odds ratio
			DR	Pr > χ^2
Intercept	2074.29					0.3716	0.3087	1.450
District	1577.04	6	60.32	<0.0001	Arbaminch	0.9851	0.1268	2.678
			53.35	<0.0001	Mihirab Arbaya	0.8967	0.1228	2.452
			72.98	<0.0001	Bonkie	0.7343	0.0859	2.084
			113.73	<0.0001	Demba Gofa	1.3409	0.1257	3.822
			58.76	<0.0001	Derashie	0.7419	0.0968	2.100
			83.49	<0.0001	Konso	0.7736	0.0847	2.168
					Burjdi	0*		.
NPS fertilizer (kg ha^−1)	468.99	2	15.56	<0.0001	0	0.3793	0.0961	1.461
			4.12	0.0424	1-50	0.1012	0.0499	1.106
					51–100	0*		.
Crop growth stage	297.22	5	0.04	0.8369	Vegetative	0.0437	0.2126	1.045
			0.01	0.9407	Tasseling	0.0159	0.2142	1.016
			0.28	0.5939	Silking	0.1107	0.2076	1.117
			0.03	0.8694	Blister	−0.0324	0.1969	0.968
			1.34	0.2462	Milk	−0.2115	0.1824	0.809
					Dough	0*		.
Cropping system	232.64	1	10.19	0.0014	Sole cropping	0.1568	0.0491	1.170
					Intercropping	0*		.
Insect infestation	171.31	2	22.04	<0.0001	Low level	−0.4385	0.0934	0.645
			8.76	0.0031	Medium level	−0.2300	0.0777	0.795
					High level	0*		.
Insecticide spray freq.	140.95	2	14.77	0.0001	None	0.3140	0.0817	1.369
			7.08	0.0078	Only once	0.1514	0.0569	1.163
					Twice and more	0*		.
Weed infestation	121.27	2	16.37	<0.0001	Good	−0.3866	0.0955	0.679
			4.48	0.0343	Intermediate	−0.1525	0.0721	0.859
					Poor	0*		.

¹Unexplained variations after fitting the model

²LRT = likelihood ratio test; DR = deviance reduction; Pr = probability of an χ²value exceeding the deviance reduction

³* reference group; df = degrees of freedom; χ²= chi-square.