Nursery Crop Science
Fungicides That Prevent Phytophthora Root Rot
Mike Benson
Department of Plant Pathology
NC State University
Fungicides are an important component of IPM for Phytophthora root rot in ornamentals. Several new fungicides, including cyzofamid (Ranman), fenamidone (Fenstar), dimethomorph (Stature DM) and phosphites (Biophos and Vital) were evaluated for control of Phytophthora cinnamomi on azalea grown in a pinebark:sand mix. Azaleas ‘Hinodegiri’ were sprayed with phosphite fungicides 5 days before inoculation with rice grains colonized by P. cinnamomi or drenched with the non-phosphite fungicides at inoculation on 24 June 2004. Fungicides were applied three times at 30-day intervals on azalea. There were five replications of each fungicide treatment arranged in a randomized complete block design at the HFL with sprinkler irrigation. Foliar symptoms were rated at 2 week intervals after first symptoms appeared in July. Fresh top weight and root rot were measured and assessed at harvest on 9 September.
Results. Symptoms of Phytophthora root rot developed within about 5 weeks on untreated control plants but not on plants sprayed or drenched with the different fungicides (Table 1). All fungicides tested except Truban and Subdue Maxx continued to protect plants throughout the summer as those in the untreated treatment became more severely diseased.
Table 1.Foliar ratings for azalea treated on a 30-day schedule to control Phytophthora root rot
Treatment |
Rate /100 gal |
Apply |
Foliar rating z |
Foliar rating z 09 Sept |
Top wt (g) (g) |
Root rot (1-5)y |
Untreated control |
-- |
2.4 ax |
3.0 a |
19 e |
3.0 a |
|
Biophos |
2 gal |
Spray |
1.2 b |
1.0 c |
70 ab |
1.0 c |
Vital 4L |
4 pts |
Spray |
1.0 b |
1.0 c |
59 bcd |
1.0 c |
Aliette 80W |
5.0 lb |
Spray |
1.0 b |
1.2 c |
62 abc |
1.2 c |
Cyazofamid 400 SC (Ranman) |
1.5 fl oz |
Drench |
1.0 b |
1.2 c |
56 bcd |
1.8 b |
Fenomen 500 SC (Fenstar) |
14 fl oz |
Drench |
1.0 b |
1.0 c |
80 a |
1.0 c |
Truban 25EC |
8 oz |
Drench |
1.0 b |
2.5 b |
47 cd |
2.6 a |
Aliette 80W |
12.8 oz |
Drench |
1.0 b |
1.0 c |
74 ab |
1.0 c |
Stature-DM 50W |
6.4 oz |
Drench |
1.0 b |
1.0 c |
63 abc |
1.2 c |
Subdue Maxx 2E |
1.0 oz |
Drench |
1.0 b |
2.7 ab |
45 d |
2.8 a |
Non-inoculated control |
-- |
1.2 b |
1.0 c |
65 abc |
1.0 c |
z Foliar rating was 1 = healthy, 2 = chlorotic, 3 = poor growth, small leaves, defoliation, 4 = dead.
y Root rot rating scale was 1= healthy, full root ball, 2= some root rot, less than full root ball, 3= severe root rot, 50% of root system necrotic, 4= very severe root rot, root ball falls apart, and 5 = plant dead, all roots necrotic.
x Means within a column followed by the same letter are not different according to the Waller-Duncan k ratio, t-test, k=100, P=0.05.
Significance to the Industry. Several new fungicides should become available in the near future for nurserymen to use for disease management of Phytophthora root rot. Biophos, Vital and Stature DM have been tested in our program on several crops for the past several years with good success against Phytophthora root rot. Ranman and Fenstar will need more field trials before we know how good these fungicides will be against Phytophthora.
