The Efficacy of Various Preventive Soil-Applied Potassium Phosphite Products Against Foliar Phytophthora Infections on Rhododendron over Time

Caitlin A. Littlejohn; Drew C. Zwart; Matthew A. Borden; Andrew L. Loyd

doi:10.48044/jauf.2025.005

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Figure 1.
Photograph illustrating treatment effects on suppression of Phytophthora infections in detached leaf assays over time where leaves were inoculated and then incubated for 5 days before photographing (Experiment 1).

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Table 1.

Phosphite product manufacturer, formulation, product type, application method, and treatment rates for Experiment 1 and 2. ai (active ingredient).

Treatment	Manufacturer	Formulation	Product type	Application	Rate (g ai/ft²)
Non-treated	—	—	—	—	—
Fortiphite^¹	Plant Food Co., Inc.	Soluble concentrate	Fertilizer	Drench	0.317
Phosphite tablet^²	Doggett Corp.	Slow-release tablet	Fertilizer	Soil-dispersed	0.4
Reliant Dry Phite 28G^³	Quest Products Corp.	Granule	Fertilizer	Broadcast	4.6
Reliant L	Quest Products Corp.	Soluble concentrate	Fungicide	Drench	0.441

↵¹ Generic fertilizer formulation of PolyPhosphite 30 (Plant Food Company, Inc, Cranbury, NJ, USA) 0-0-27.
↵² Proprietary, experimental fertilizer formulation from Doggett Corp. (Lebanon, NJ, USA) with unknown N-P-K breakdown.
↵³ Fertilizer formulation of 0-0-7.

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Table 2.

Treatment effects on detached wounded Rhododendron spp. leaves from the greenhouse study (Experiment 1), following artificial inoculation with an isolate of Phytophthora nicotianae. hpt (hours post-treatment); wpt (weeks post-treatment).

Lesion size (cm²)(standard error of the mean)^¹
Treatment^²	48 hpt	1 wpt	2 wpt	4 wpt	5 wpt	6 wpt	8 wpt	12 wpt
Non-treated	4.7 (1.0)	4.7 (1.3) a	3.8 (0.3) a	4.2 (1.4) a	5.8 (2.0) a	5.8 (1.8) a	3.6 (1.4)	5.4 (2.0) a
Fortiphite	7.1 (0.7)	0.4 (0.1) b	0.5 (0.1) b	0.1 (0.1) b	0.2 (0.1) b	0.3 (0.1) bc	0.6 (0.3)	0.1 (0.1) c
Reliant Dry Phite 28G	6.9 (0.9)	0.5 (0.1) b	0.2 (0.1) b	0.2 (0.0) b	0.3 (0.2) b	0.2 (0.1) c	0.5 (0.2)	1.0 (0.3) bc
Reliant L	3.7 (1.5)	0.8 (0.2) b	0.5 (0.2) b	0.2 (0.1) b	0.3 (0.1) b	0.7 (0.2) b	0.7 (0.1)	4.4 (1.1) ab
P-value³	0.1118	0.0020	< 0.0001	0.0001	0.0011	< 0.0001	0.0580	0.0005

↵¹ Means in a column not followed by the same letter are significantly different (P ≤ 0.05) according to the Tukey’s HSD post-hoc calculation for each time point independently.
↵² Phosphite treatment name.
³ Data collected 5 days after inoculation was first (x + 1) Box-Cox transformed to normalize variance prior to conducting a one-way ANOVA for each time point independently. Bolded P-values indicate significant differences for each time point (P ≤ 0.05) while non-bolded P-values indicate no significance (P > 0.05) across treatments including the non-treated controls.

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Table 3.

Treatment effects on detached wounded Rhododendron spp. leaves from the field study (Experiment 2), following artificial inoculation with an isolate of Phytophthora nicotianae. wpt (weeks post-treatment).

Lesion size cm² (standard error of the mean)^¹
Treatment^²	1 wpt	2 wpt	3 wpt	4 wpt	6 wpt	8 wpt	12 wpt
Non-treated	0.4 (0.2)	6.3 (2.1)	17.4 (3.4) a	16.1 (2.0) a	2.8 (1.0) a	1.2 (0.3) a	0.0 (0.0)
Fortiphite	0.4 (0.3)	2.2 (0.8)	3.1 (1.7) ab	1.9 (1.0) bc	0.1 (0.0) b	0.2 (0.1) ab	0.1 (0.1)
Phosphite tablet	0.2 (0.1)	5.9 (3.2)	7.2 (3.4) ab	8.1 (5.3) ab	0.9 (0.2) a	0.7 (0.1) ab	0.5 (0.3)
Reliant Dryphite 28G	0.3 (0.1)	0.6 (0.4)	1.0 (0.4) b	0.8 (0.3) bc	0.1 (0.0) b	0.1 (0.1) b	0.4 (0.2)
Reliant L	0.5 (0.1)	1.8 (0.7)	2.5 (2.0) b	0.3 (0.3) c	0.0 (0.0) b	0.6 (0.3) ab	0.5 (0.2)
P-value^³	0.7596	0.2378	0.0169	0.0009	< 0.0001	0.0137	0.2646

↵¹ Means in a column not followed by the same letter are significantly different (P ≤ 0.05) according to the Tukey’s HSD post-hoc calculation for each time point independently.
↵² Phosphite treatment name.
↵³ Data collected five days after inoculation was first (x + 1) Box-Cox transformed to normalize variance prior to conducting a one-way ANOVA for each time point independently. Bolded P-values indicate significant differences for each time point (P ≤ 0.05) while non-bolded P-values indicate no significance (P > 0.05) across treatments including the non-treated controls.