Reducing apple scab and frogeye or black rot infections with salicylic acid or its analogue on field-established apple trees

Figures & data

Fig. 1 Cultivar effects on incidence of apple scab and frogeye or black rot on foliage and fruit of field-established untreated ‘Honeycrisp’ and ‘Cortland’ trees during 2014, 2015 and 2016. Mean incidence of scab and frogeye on foliage was assessed in planta three times in June at weekly intervals from 50 random leaves on each tree. The incidence of scab and black rot on fruit was determined at harvest on a whole tree basis. Means are average of three or four (2016) replicates for ‘Honeycrisp’ and two or three (2016) replicates for ‘Cortland’ and error bars represent standard error of mean.

Table 1. Main cultivar effects on incidence and severity of apple scab and incidence of frogeye on foliage of field-established treated and untreated ‘Honeycrisp’ and ‘Cortland’ trees ^a.

	Mean disease incidence (%) or severity on foliage ^b
	Scab incidence		Scab severity ^c		Frogeye incidence
Cultivar/Treatment	2014	2016	2014	2016	2014	2016
‘Cortland’-inject	21.99	25.18	0.91	0.61	14.83	8.89
‘Honeycrisp’-inject	2.63	1.59	0.05	0.02	34.33	36.59
‘Honeycrisp’-spray	3.29	1.49	0.05	0.03	31.17	33.88
Grand mean	7.72	7.45	0.27	0.17	28.27	28.65
SEM ^d	0.86	1.12	0.08	0.02	4.86	5.79
‘Honeycrisp’-inject vs. ‘Cortland’-inject	P < 0.001	P < 0.001	P < 0.001	P < 0.001	P 0.038	P 0.014

^a Salicylic acid (2-benzoic acid 99%; 200 mg) or Actigard® (Acibenzolar-S-methyl 50WG; 100 mg) powder were injected into tree trunks using an EZ-Ject Lance system. Aqueous ethanol (0.095%) solutions of salicylic acid (1 mM) and Actigard® (0.1 g L⁻¹) were applied using a motorized backpack sprayer (Solo) to leaf wetness.

^b Mean incidence and severity of scab and incidence of frogeye on foliage was assessed in planta from 50 random leaves on each tree 1, 2, and 3 weeks after the second treatment application.

^c Scab severity was assessed using a 0‒10 scale based on the percentage of infected leaf area using nearest percent estimates (Bock et al. 2010).

^d Standard error of mean.

Table 2. Main cultivar effects on incidences of apple scab and black rot on fruit of field-established treated and untreated ‘Honeycrisp’ and ‘Cortland’ trees ^a.

	Percentage of infected fruit at harvest ^b
	Scab			Black rot
Cultivar/Treatment	2014	2015	2016	2014	2015	2016
‘Cortland’-inject	87.89	88.58	88.81	3.38	3.50	1.15
‘Honeycrisp’-inject	25.44	23.37	4.04	18.86	4.82	1.44
‘Honeycrisp’-spray	28.59	25.75	5.51	14.68	11.01	2.00
Grand mean	42.23	40.57	25.79	13.43	6.81	1.58
SEM ^c	5.27	3.58	2.25	3.07	3.63	0.79
‘Honeycrisp’-inject vs ‘Cortland’-inject	P < 0.001	P < 0.001	P < 0.001	P 0.021	ns	ns

^a Salicylic acid (2-benzoic acid 99%; 200 mg) or Actigard® (Acibenzolar-S-methyl 50WG;100 mg) powder were injected into tree trunks using an EZ-Ject Lance system. Aqueous ethanol (0.095%) solutions of salicylic acid (1 mM) and Actigard® (0.1 g L⁻¹) were applied using a motorized backpack sprayer (Solo) to leaf wetness.

^b Fruit showing lesions of scab or black rot at harvest on a whole tree basis.

^c Standard error of mean.

Table 3. Main effects of salicylic acid (SA) and Actigard® treatments applied to field-established ‘Honeycrisp’ and ‘Cortland’ trees on apple scab and frogeye incidence on foliage ^a.

	Mean disease incidence on foliage ^b
	Scab			Frogeye
Treatment	2014	2015 ^c	2016	2014	2015 ^c	2016
Control	9.98	40.00	10.11	26.25	8.00	23.91
SA	5.66	39.67	5.96	28.87	6.00	30.92
Actigard®	7.35	38.67	6.28	30.62	5.31	31.11
Grand mean	7.64	40.47	7.45	28.27	7.93	28.65
SEM ^d	0.96	2.01	0.78	2.54	1.72	1.56
	P 0.006	ns	P < 0.001	ns	ns	ns

^a SA (2-benzoic acid 99%; 200 mg) or Actigard® (Acibenzolar-S-methyl 50WG;100 mg) powder were injected into tree trunks using an EZ-Ject Lance system. SA (1 mM) and Actigard® (0.1 g L⁻¹) solutions in 0.095% aqueous ethanol were sprayed using a motorized backpack sprayer (Solo) to leaf wetness.

^b Mean incidence of scab and frogeye on foliage was assessed in planta from 50 random leaves on each tree 1, 2, and 3 weeks after the second treatment application.

^c 2015 scab and frogeye incidence on foliage only reflects ‘Cortland’.

^d Standard error of mean.

Table 4. Main effects of salicylic acid (SA) and Actigard® treatments applied to field-established ‘Honeycrisp’ and ‘Cortland’ trees on incidences of apple scab and black rot on fruit ^a.

	Percentage of infected fruit at harvest ^b
	Scab			Black rot
Treatment	2014	2015	2016	2014	2015	2016
Control	42.81	43.89	23.29	17.83	4.23	1.60
SA	44.71	41.21	29.12	4.55	4.71	0.86
Actigard®	39.30	38.47	24.95	17.42	14.21	2.28
Grand mean	42.79	41.61	25.79	13.48	7.81	1.58
SEM ^c	4.93	4.01	2.09	7.99	3.39	0.46
	ns	ns	ns		ns	ns

^a SA (2-benzoic acid 99%; 200 mg) or Actigard® (Acibenzolar-S-methyl 50WG; 100 mg) powder were injected into tree trunks using an EZ-Ject Lance system. Aqueous ethanol (0.095%) solutions of SA (1 mM) and Actigard® (0.1 g L⁻¹) were applied using a motorized backpack sprayer (Solo) to leaf wetness.

^b Fruit showing lesions of scab or black rot at harvest on a whole tree basis.

^c Standard error of mean.

Table 5. Effects of salicylic acid (SA) and Actigard® treatments on the incidences of apple scab and frogeye or black rot on foliage and fruit of field-established ‘Honeycrisp’ and ‘Cortland’ trees during 2014 ^a.

	Mean incidence on foliage ^b		Infected fruit (%) ^c
Cultivar/Treatment	Apple scab	Frogeye	Apple scab	Black rot
‘Cortland’
Control-inject	16.05	25.00	76.13	4.89
SA-inject	17.64	13.00	89.31	1.64
Actigard-inject	20.18	9.50	94.57	1.09
‘Honeycrisp’
Control-inject	3.56	33.37	29.65	29.59
SA-inject	0.67	37.33	9.99	6.99
Actigard-inject	2.22	39.00	14.81	28.52
Control-spray	3.51	32.33	20.98	12.20
SA-spray	2.67	31.00	49.69	4.06
Actigard-spray	3.92	36.33	26.93	17.22
Grand mean	7.64	28.27	42.79	13.48
SEM ^d	1.88	6.33	9.91	13.13
Inject HC vs inject C	P 0.047	P 0.006	P 0.10	ns

^a SA (2-benzoic acid 99%; 200 mg) or Actigard® (Acibenzolar-S-methyl 50WG; 100 mg) powder were injected into tree trunks using an EZ-Ject Lance system. Aqueous ethanol (0.095%) solutions of salicylic acid (1 mM) and Actigard® (0.1 g L⁻¹) were applied using a motorized backpack sprayer (Solo) to leaf wetness.

^b Mean incidence of scab and frogeye on foliage was assessed in planta from 50 random leaves on each tree 1, 2, and 3 weeks after the second treatment application.

^c Fruit showing lesions of scab or black rot at harvest on a whole tree basis.

^d Standard error of mean.

Fig. 2 Effects of salicylic acid (SA) and Actigard® injection treatments on incidence of apple scab and frogeye on foliage (a) and incidence of scab and black rot on fruit (b) of field-established ‘Cortland’ trees during 2015. Means are average of two replicates per treatment and error bars represent standard error of mean. SA (2-benzoic acid 99%; 200 mg) or Actigard® (Acibenzolar-S-methyl 50WG; 100 mg) were injected into tree trunk using an EZ-Ject Lance system. Mean incidence of scab and frogeye on foliage was assessed in planta from 50 random leaves on each tree 1, 2, and 3 weeks after the second treatment application. The incidence of scab and black rot on fruit was determined at harvest on a whole tree basis.

Fig. 3 Effects of salicylic acid (SA) and Actigard® injection treatments on incidence of scab and frogeye on foliage (a), and incidence of scab and black rot on fruit (b) of field-established ‘Cortland’ trees during 2016. Means are average of three replicates per treatment and error bars represent standard error of mean. SA (2-benzoic acid 99%; 200 mg) or Actigard® (Acibenzolar-S-methyl 50WG; 100 mg) were injected into tree trunk using an EZ-Ject Lance system. Mean incidence of scab and frogeye was assessed in planta from 50 random leaves on each tree 1, 2, and 3 weeks after the second treatment application. The incidence of scab and black rot on fruit was determined at harvest on a whole tree basis.

Bock CH, Gottwald TR, Parker PE, Ferrandino F, Welham S, van Den Bosch F, Parnell S. 2010. Some consequences of using the Horsfall-Barratt scale for hypothesis testing. Phytopathology. 100:1030–1041.

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