JARS v37n1 - P.C., A Case of Underground Terrorism

P.C., A Case of Underground Terrorism
Larry Englander, Ph.D.
University of Rhode Island, Kingston, Rhode Island
Reprinted from The Rosebay

ALTHOUGH the mention of Phytophthora root rot often strikes terror into the heart of rhododendron growers, some knowledge about the nature of this disease and the causal fungus may be helpful in avoiding root rot. Losses from this disease are not very common in Northeast home landscape plantings on sites with reasonably well-drained and relatively cool soils, especially on well established plants. However, the disruptive conditions to which young plants are exposed during greenhouse propagation and subsequent growth in containers or densely planted liner beds is conducive to outbreaks of root rot, with potentially serious losses to the commercial propagator and hobbyist.

Phytophthora cinnamomi (P.c.) is the fungus which causes this root rot. Originally isolated in Sumatra from a cinnamon tree (thus the species name), this pathogenic fungus has been found to have worldwide distribution and to attack a remarkably broad range of plants, ranging from pine to pineapple, Eucalyptus to Erica and avocado to azalea.

Information about the activities of the dreaded P.c. fungus is both interesting and useful in guiding our choice of cultural practices so as not to encourage root rot problems. Like many fungi, P.c. can grow by sending its branching microscopic filaments (hyphae) through the soil, ramifying amongst soil crumbs and bits of organic matter. However, P.c. is not as competitive as many other soil microorganisms, and thus when not attacking a plant it probably spends most of its time in a resting state, either as dormant hyphae or as resting spores (2 kinds), often in bits of root tissue from plants it has previously killed.

While both (sexual and asexual) types of resting spores serve to preserve the fungus in an inactive state between attacks on plants, they have quite different origins and significance.

The first type, the oospore forms after a male structure produced on hyphae from a fungus colony of one mating type comes in contact with a female structure on a colony of the opposite persuasion. In the process of fusion each type contributes genetic information, and the round, thick-walled oospore which then forms will give rise to a colony with a new combination of characteristics, some derived from each parent. This is particularly significant in regard to the occurrence of new strains of a fungus which may then be able to attack previously resistant plant cultivars, or resist toxic action by a previously effective fungicide.

A second and much more prevalent type of resting spore of P.c. is the chlamydospore, produced often in grape-like clusters on the branches of the fungus hyphae. By virtue of its asexual production, the chlamydospore contains genetic coding similar to that in the parent colony.

The term, 'resting spores' may be misleading; rather, these spores are lying in ambush, awaiting the conditions which trigger a resumption of activity and a continuation of the sequence of events in the disease cycle. Thus, with the occurrence of the proper combination of factors in the chemical and physical environment, the fungus hyphae or germinating resting spores produce an egg-shaped structure, the sporangium. When soil moisture content is high, the jelly-like contents of the sporangium cleave into as many as 40 individual segments, each of which becomes a zoospore. The zoospores begin vibrating inside the sporangium, and within moments are violently discharged through a small opening in the tip of the sporangium. These newly released, tiny zoospores are bean-shaped with two delicate flagella, the whip-like projections which vibrate and propel the zoospores in a spiraling path through the soil water. Most of the zoospores long distance travel (inches, feet or miles) is passive; they are caught up in currents of ground or surface water. The ability to swim, however, becomes very important when the zoospores approach roots of a susceptible plant. Apparently, a chemical or electrical signal inadvertently is given off by plant roots from a region just behind the root tips and from wounds, and zoospores in the vicinity respond to this homing signal, swimming towards the surface of the root. Once there, the zoospores lose their flagella, become spherical, and after a short while, germinate. Also responding to the stimulus from the plant, the germ tubes grow right to the root surface and penetrate the root cells, thus initiating infection. These infected roots are soon rendered nonfunctional by the invading hyphae, turn brown and become rotten.

When sufficient numbers of the small 'feeder' roots become infected the root system no longer can supply the foliage with sufficient water and minerals. This deficiency is especially pronounced during periods of environmentally induced stress, such as hot, dry weather. First the new foliage wilts, its normal color changing to olive-drab. After these leaves droop, dry, and turn brown, this same sequence progresses to the mature foliage. A symptom diagnostic of Phytophthora root rot may be found at the base of the stem when above-ground symptoms are apparent. If one nicks the bark with thumbnail or knife just above the soil line, exposing the cambium and the outermost layer of woody xylem, this exposed tissue will be brown or reddish-brown rather than white as in a healthy plant. The discoloration must be observed immediately, since in a short time even healthy tissue will darken upon exposure due to oxidation of plant phenolic materials. Naturally, this test is not recommended for every plant with a drooping leaf, because healthy plants may become girdled when wounded in this way in a location so close to the microorganism-rich soil, thus becoming vulnerable to infection. Prior to resorting to stem slicing, water should be applied to wilted plants and these observed for recovery when the temperatures moderate. Wilting of foliage on severely infected plants can not be reversed by watering. Upon discovery of a rhododendron with Phytophthora root rot, one has a strong desire to have at hand a fungicide which might cure the infected plant. Unfortunately, there is no currently available fungicide with significant therapeutic ability. Studies on two fungicides registered for use against Phytophthora root rot on ornamentals, Truban' (Mallinckrodt Chem.) and ‘Subdue’ (Ciba-Geigy Corp.), showed each can reduce mortality due to P.c. on some rhododendron and azalea cultivars (1, 3). However we found that neither material is effective if applied to rhododendrons in as short a time as one week after the initial root infection occurs. Since infection may occur at any time after roots form, a preventive program should begin when cuttings are stuck. Also, these fungicides, applied as a root and soil drench, do not persist in the root zone, thus must be reapplied approximately once a month, and with some doses on certain cultivars the plants may have an adverse (phytotoxic) reaction. Considering the inadequacy of these fungicides and curing existing infections, and their lack of residual activity, their use may not be considered practical in many situations. Rather, cultural practices should be chosen which both discourage the introduction and inhibit the activities of the pathogenic fungus.

It is wise to remove and destroy diseased plants to prevent further spore production and dispersal by the fungus. If the diseased plant is in its own container, disposal can be accomplished simply and neatly. However, much more care must be given to removal of plants from greenhouse beds or field plantings. Casually yanking or digging these plants can scatter infested soil and rotten roots all along the path of disposal. It is necessary to place the plant and associated soil in a container right at the site of removal. In densely planted beds or greenhouse benches, it is advisable to remove a buffer zone of apparently healthy plants. We found in propagation benches that young plants in the immediate vicinity of wilted rhododendron cuttings had deceptively healthy looking foliage and roots, even months after the adjacent plant died (3). However, many of these symptomless plants were indeed infected, and a large proportion died the following summer.

Since zoospores are readily disseminated in moving water, dump-piles of dead plants and soil may be a prime source of the fungus, especially if drainage water from the infested plants moves towards planted areas or the irrigation water supply.

In the rhododendron propagation process there are several measures which will reduce the likelihood of P.c. problems. Work surfaces, benches, flats and containers should be cleaned between crops by removing all old growing mix and plant material, then liberally applying liquid laundry bleach (diluted no more than 1:20). Propagation mixes should not be re-used because populations of pathogens may increase with each crop. Cuttings should be selected from healthy stock plants free of stem and leaf blemishes, making certain that containers and work benches on which these are held and processed for sticking are extremely clean. Under no circumstance should cuttings be placed on the ground, nor should cutting material be taken from an area on the stock plant which is subjected to rain splashing from the soil surface. Cuttings of most rhododendron cultivars should be taken after some maturation has occurred (semi-hardened), thereby allowing a good complement of carbohydrates to have accumulated naturally in the tissue, rather than resorting to the microbiologically risky practice of applying sugar solutions to stems. Damage to new roots of young plants should be minimized. This may be accomplished by preventing desiccation of the mix, refraining from unnecessary checking on root development by lifting plants, and transplanting before root systems have become so inter-meshed that extensive damage is inflicted in the process of separating plants.

Container mixes should be selected with good drainage as a major consideration. Also, shading of plants is an important consideration, and in container cultures, especially so. Ideally, a level of exposure to sunlight should be tailored for each cultivar which would not result in root damage from heat on the soil or container surface, yet would allow good plant color and bud set. Polyethylene ground tarps, used for weed control, collect drainage water from potentially infected plants and allow its redistribution to other plants, thus the plastic should be amply perforated or covered with gravel. Rhododendrons forced into rapid growth rates by excessive application of fertilizer seem to be more susceptible to P.c.

In established stock-plant or landscape plantings, new rhododendrons of unknown health should be isolated for at least one season by planting at a substantial distance, or at least downhill, from valuable rhododendrons. In heavy soil it is advisable either to plant in raised beds or to prepared soil in an area large enough for years of root system expansion, and to dig several radiating trenches to accommodate excess water. Soil amended to increase porosity and drainage should be used in the root zone and trenches.

Root damage, and thus susceptibility to P.c, may be reduced by proper mulching, preferably with a coarse material such as pine needles, oak leaves or bark chips. This can significantly reduce root temperatures during summer months and help stabilize the soil moisture content. It is best to postpone the use of deep mulches on roots until winter, since if applied in the autumn this might insulate roots from the natural warning signals that stimulate acclimation to winter temperatures. Also, some of the deep winter mulch should be pulled away in early spring to allow thawing of the ground so that roots can absorb moisture to meet the demands by the transpiring foliage. I believe that, unlike recommendations for deep and infrequent waterings for lawn grasses, rhododendrons should be watered gently and as frequently as necessary to maintain the soil in a moist (but not wet) condition to the full depth of the roots. Here mulches become very valuable. The predominantly superficial and fibrous root system of a rhododendron is sensitive to drying out, yet excessive free water around the roots encourages P.c. There is an ideal moisture balance which is both delicate and difficult to achieve, but the experience of the grower and the relative resistance to disease of older plants can account for the remarkable success of rhododendrons as popular landscape plants.

Concerning the potential of one day removing Phytophthora root rot from the category of one of our most serious ornamentals diseases, I believe my optimism is justified. Several areas of research show great promise in this regard. A better understanding of the mechanism of resistance to root rot, exhibited by certain rhododendron cultivars, may allow this factor for resistance to be incorporated into breeding programs. Likewise, studies on the characteristics of the fungus and the zoospore "attraction" phenomenon may allow us to manipulate the environment in such a way as to break the disease cycle, New experimental fungicides are periodically developed, and our chemical arsenal is thus improving. Exciting results have come from work in Ohio; by incorporating composted bark in the growing medium, not only were favorable drainage characteristics achieved but also chemical and biological agents extremely suppressive to P.c. were conferred to the mix (4). Studies on the mechanism and value of mycorrhizae on rhododendrons will at some point determine whether the beneficial fungus provides protection to the plant from invasion by the root rot fungus (2). Although Phytophthora root rot is a serious threat, there is every reason to believe that research will continue to provide methods by which this disease can be kept under control.

Literature Cited:
1.  Benson, D.M. 1979. Efficacy and in vitro activity of two systemic acylalines and ethazole for control of Phytophthora cinnamomi root rot of azalea. Phyto-pathology 69: 174-178
2.  Englander, L. 1980 Mycorrhizae and Rhododendron. A.R.S. Quarterly Bull. 34 (1): 8-12.
3.  Englander, L., J.A. Merlino, and J.J. McGuire. 1980. Efficacy of two new systemic fungicides and ethazole for control of Phytophthora root rot of Rhododendron, and spread of Phytopthora cinnamomi in propagation benches. Phytopathology 80; (in press).
4.  Hoitink, H.A.J. 1980. Composted bark, a lightweight growth medium with fungicidal properties. Plant Disease 64: 142-147