QBARS - v27n2 Influence of Environment on Incidence of Phytophthora

Influence of Environment on Incidence of Phytophthora Root Rot
in Four Varieties of Rhododendrons in Containers

J. J. McGuire & N. Jackson
College of Resource Development, University of Rhode Island
Kingston, Rhode Island
Contribution Number 1467 of the Rhode Island Agricultural Experiment Station

The rhododendron is one of the major economic crops of commercial nurseries in Rhode Island. Production is increasing, particularly in container culture, but losses largely due to root rot diseases are also increasing. These disorders appear in propagating beds, lining out beds and in three-year-old transplants either in the field or in containers. Losses vary from year to year but have been substantial enough with some hybrids that these hybrids are not now being grown.
Before assistance can be provided growers, more information must be obtained about the diseases. A project at the Ohio Agricultural Experiment Station has shown that there is considerable variation between cultivars in their susceptibility to invasion by Phytophthora sp . Susceptibility was also found to vary with environmental conditions. This experiment was designed to test and extend the Ohio findings.

Phytophthora infection
FIG.25.  Phytophthora infecting
dark 'Nova Zembla' root stock
has not crossed graft union into
more resistant 'English Roseum'
Photo by Dr. J. J. McGuire

Plant pathologists at the Rhode Island Agricultural Experiment Station located rhododendron plants at local nurseries exhibiting wilt symptoms and isolated the associated fungi. The fungus most commonly isolated was Phytophthora cinnamomi and this species was cultured in the laboratory to provide innoculum for subsequent experiments.
Four cultivars of Rhododendron were used in a pathogenicity test: R. X. 'English Roseum', 'Cunningham's White', 'Nova Zembla' and 'Mrs. P. den Ouden'. Of these four, 'Nova Zembla' was known to be susceptible to invasion by P. cinnamomi and 'English Roseum' was known to be resistant. Observations at this station led the investigators to suspect 'Mrs. P. den Ouden' was susceptible while it was thought 'Cunningham's White' was resistant.
There were 270 two-year rooted cuttings of each cultivar used in the experiment. Each cultivar group was divided in half, one half being inoculated with P. cinnamomi and one half being used as a control. White 1 reported rhododendron roots were more susceptible to Phytophthora rot when exposed to temperatures above 70°F and in an alkaline soil reaction. To test the temperature variable, the cultivar groups were further subdivided into three temperature regimes. These were provided by Quonset-type, pipe frame houses, 21 feet wide and 10 feet high in the center, covered with Saran screening. One mesh screen provided 30% shade and one gave 63% shade. The third group were placed in full sunlight. Temperatures were measured by thermocouples placed 1" deep and 1" from the side of the can and recorded four times a day.
Plants were potted in a medium of sphagnum peat moss and horticultural perlite 50% by volume of each ingredient. Three pH ranges [3.5 (low), 5.0 (medium) and 6.6 (high)] were obtained by adjusting the amount of dolomitic limestone added to each container. It was not possible to maintain the pH levels by this method throughout the duration of the experiment but they were maintained up to the time inoculations were made.
The final experimental design was as follows: Shade exposures (3) X Replicates (3) X Samples (5) X Cultivars (4) X pH (3) X inoculations (2) to give a total of 1080 plants in the experiment.
All plants were placed into the container medium in early June 1970. Black plastic containers (2 gallon size) were used and these were placed on a film of black plastic over gravel. Plants were irrigated daily and fertilized twice weekly with dilute amounts of 20-20-20 to provide optimum growth.
Inoculations were made after thirty days by placing a 50 ml. aliquot of mycelial suspension into a hole made in the container medium near the stem base of the plant. A handful of fresh potting medium was placed immediately over the fungus inoculum and after all containers had been treated they were irrigated for one hour.
Plants were observed for wilt symptoms three times during the summer and those exhibiting symptoms were removed. These plants were taken to the laboratory for post mortem examination to determine the cause of death.

On sunny days, temperatures recorded in the media 1" deep and 1" from the south side of the can were 30-40° F lower in both the shaded groups than in the group exposed to full sun. Temperatures in excess of 110° F were not uncommon in the latter during midsummer.
After three months 107 plants had succumbed to Phytophthora root rot (Table 1). Fourteen other plants had also died but the cause of death could not be directly attributed to P. cinnamomi . A few plants (16) were lost to Phytophthora rot in the group that was not inoculated. There seemed to be no correlation between time of symptom development and time of inoculation or differences in the pH of the medium. Two correlations were apparent:

  1. Greatest losses (62 plants) occurred in the block in full sunlight with least losses (16 plants) in the block in 63% shade;
  2. Greatest losses occurred in R. 'Nova Zembla' (49 plants) followed in descending order by R. 'Mrs. P. den Ouden' (26 plants), R. 'Cunningham's White' (19 plants) and R. 'English Roseum' (13 plants).
TABLE 1. Number of container grown rhododendron plants exhibiting wilt symptoms
30, 70 and 95 days after inoculation with Phytophthora cinnamomi .

R. X 'English Roseum' 1(-:-) 1(-:-) 1(-:-) 2(-:-) 0 0 (-:-) 0 0
R. X 'Cunningham's White' 1(-:-) 1(-:-) 2(-: -) 15(-:-) 0 0 2(-:-) 3(-:-) 0
1(-) 2(-) 2(-)
R. X 'Nova Zembla' 4(-:-) 1(-:-) 8(-:-) 17(-:-) 4(-:-) 2(-:-) 6(-:-) 1(-:-)
R. X 'Mrs. P. den Ouden' 3(-) 2(-) 0 9(-:-) 1(-:-) 0 4(-:-) 4(-:-) 0
1(-:-) 2(-)
Total 12 9 13 33 5 2 17 17 15
(-:-) = Innoculated plant.
(-) = Infected non-innoculated plant.
Full Sun
30% Shade
63% Shade
'Eng. Roseum'
'Cunn. White'
'Nova Zembla'
'Mrs. P. den Ouden'
4.8% 7.0% 18.1% 9.6%

In November, flower buds were counted on all surviving plants (Table 2). It can be seen that reduced light also caused a reduction in flower bud production. There seemed to be no influence of pH on flower bud production except in the case of R. 'English Roseum' in full sunlight and in light shade where higher pH seemed to result in fewer buds.

TABLE II. Effect of light on flower bud protection in four clones of rhododendron.
Light Exposure Full Sun 30% Shade 63% Shade
pH H M L H M L H M L


Average Number of Buds per Plant
English Roseum 4.64 5.62 7.28 3.31 4.11 6.32 0.77 0.44 0.53
Nova Zembla 4.27 3.47 3.00 2.27 2.11 2.58 0.25 0.28 0.15
Cunningham's White 2.65 2.88 3.58 2.03 2.62 2.32 1.28 1.00 2.37
Mrs. P. den Ouden 4.42 3.67 4.00 2.38 1.83 5.46 1.46 0.50 1.26
TOTAL 15.98 15.64 17.86 9.99 10.67 16.68 3.76 2.22 2.31

The incidence of Phytophthora root rot seems to show an indirect response to light and/or temperature but not to pH as long as the pH range is within the range 3.5 - 6.5. However, exposure to the higher pH in a medium of peat moss and perlite did result in reduced root growth. Variability in cultivar susceptibility has been demonstrated and would suggest the use of resistant rootstocks for varieties highly susceptible to Phytophthora wilt. It seems plausible to reduce rootball temperatures with light shade but care should be taken not to reduce light intensity to such a degree that flower bud production will be lost. Further work must be done in this area to separate the effect of temperature from that of light and also to explore the possible movement of the fungus across graft unions in resistant varieties.

White, Richard P.,1937, Rhododendron Wilt and Root Rot, New Jersey Agricultural Experiment Station Bulletin No. 615, p. 1-31