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Diagnosis and Control of Phytophthora Diseases
The first step in controlling any of the several diseases caused by Phytophthora spp. is to obtain an accurate diagnosis. Although Phytophthora is a recognized disease problem in the Pacific Northwest, it has been misdiagnosed half the time in Oregon.10 A wide variety of cultural and chemical controls can be implemented for Phytophthora problems. Time spent collecting all the information for an accurate diagnosis will aid control efforts in the long run.
Diagnosing a Phytophthora Problem
Certain lines of evidence can quickly lead you toward (or away from) a Phytophthora disease diagnosis. The evidence is in the field characteristics, field history, and symptoms of the affected plants. Knowing the biology of the Phytophthora fungus and conditions that favor its development also helps the diagnosis.2, 3 Plant samples can also be taken to a laboratory for traditional or “high tech” tests to confirm the presence of this fungus-like organism.
Field Characteristics and History
The two main factors to look for when diagnosing Phytophthora-caused plant problems are the pattern of diseased plants and water drainage. These factors can be observed in flat open areas, undulating fields, sloped nursery container yards, or even in the greenhouse. Phytophthora organisms are commonly referred to as water molds. They thrive, grow, reproduce, and infect plant roots in water, saturated soil, and along river banks and ponds. Flooded or saturated soils are not favorable to plant growth and can predispose plants to infection. Anywhere significant quantities of water collect, accumulate, or flow, Phytophthora spp. can be found.
Phytophthora diseases can be found on cranberries growing in low bogs near the coast or in the mountains on trees growing several thousand feet above sea level. The common factor in all cases is water. Excess water or poor drainage allows roots to become flooded for extended periods. During such events, this organism can produce a swimming spore (zoospores). Zoospores are attracted to the roots and begin the infection process.
Diseased plants generally are in the lower areas of a field, where excess rain or irrigation water drains off. Hill crests or steeply sloped areas are unlikely places to find Phytophthora diseases. A few exceptions include Phytophthora root rot problems on noble fir in Christmas tree plantations or Port-Orford-cedar forest trees. Diseased trees can be found in sites where water flows, either in naturally occurring ravines and ditches or along drainage from logging roads.
Puddled water or poor drainage can be in areas where growers think they have taken corrective measures. Container nurseries have used various means to collect and recycle excess water. Plastic-covered ground is notorious for producing puddles under container-grown plants, allowing Phytophthora to increase rapidly.6 Even weed cloth can be a problem. With age, sloped gravel beds can settle, forming pockets that collect water or do not rapidly drain.
Many growers tile poorly drained fields to drain off excess water. Tiles generally empty out in a nearby ditch, but if not inspected regularly they can become clogged with debris. Nearby trees such as poplar or willow can fill and clog tiles with their roots. Regular maintenance can help prevent a flooding condition.
Unusual situations also can occur in the greenhouse. Leaks from the roof or a cooling system or internal condensation that drips in the same bench location can start Phytophthora problems. Watering hoses that are left on the ground or stuck down drainpipes also can pick up and spread propagules of Phytophthora.
Other patterns can include dying plants around irrigation lines and equipment. Leaks, large-volume nozzle sprinkler heads, main line blowouts, or even an oversized drip emitter can result in too much water and problems with Phytophthora. Some container nurseries may have sloped, well-drained beds, but drains can clog, back up, and leave plants sitting in several inches of water. Flooded plants may not show symptoms for several months. By that time containers in the bed may have been moved to a different location, masking the original pattern. The plants originally grouped near the drain now may be scattered through a different bed.
Some of the best evidence indicating a Phytophthora disease are symptoms shown by the plant itself. Knowledge of the plant’s susceptibility is also helpful (see the table Plants Susceptible to Phytophthora Diseases, below).
It is necessary to thoroughly examine above- and belowground parts of the plant. Aboveground symptoms are useful but not completely diagnostic. Many different problems can result in the same aboveground symptoms as Phytophthora root rots. Anything that girdles or cuts off water and nutrients to the top of a plant results in wilting, leaf chlorosis (yellowing), leaf necrosis (browning), and premature leaf fall and plant death. Causes include feeding by root weevil larvae, winter injury, mechanical injury, wire or plastic used to keep trees straight after planting, nursery tags, lack of water, and other fungal root rots as well as Phytophthora root rot.
In Oregon, many plants with Phytophthora root rot do not show aboveground symptoms until summer. As hot, dry weather sets in, the plant does not have enough functional roots left to keep up with transpiration. Plants frequently wilt and collapse within a week. Because of the wilting, many people water plants even more than usual, flooding their roots, encouraging the pathogen, and potentially spreading the disease even more.
Belowground symptoms of Phytophthora root rot include the plant’s having few if any feeder roots while the remaining roots are dark and in some stage of decay. Symptoms will be most severe at the root tips and least severe near the root crown. Decaying roots are generally due to other microorganisms’ feeding on the roots after being killed by Phytophthora. There are some exceptions, which will be discussed shortly.
Near the advancing margin of a Phytophthora-infected root, the roots generally are firm. Use a pocket knife on larger roots (fingernails are okay on smaller roots) to expose the vascular cambium. The area between the bark (phloem) and inner wood (xylem) is highly discolored where the Phytophthora organism has been actively colonizing the root or root crown. Many times the cambium has a dull red to reddish brown color. Above this area, the cambium will be the normal color for the plant, generally some shade of white to light green. The transition between the discolored area and the healthy area may be sharp, with a distinctive margin.
Phytophthora root rots generally start low and work up the plant. Exceptions include:
- Apple—cambium discolored by root crown and collar rot may be above the graft union (collar rot) or below the graft union (crown rot) depending on the susceptibility of the scion and/or rootstock.
- Dogwood and Madrone—also gets a canker developing near the soil surface. Trunk may become flat or sunken in the cankered area.
- True fir (Abies spp.)—in addition to root rot, narrow stem cankers may run straight up or spiral up the tree. Bark may split, with abundant pitch flow.
- Strawberry—the outer root cortex remains white while the inner core or stele turns a pinkish red color, which is why the disease is commonly called red stele. (Note that after a few minutes’ exposure, even healthy root tissue turns reddish.)
Symptoms of Phytophthora diseases are not restricted to root rots because some species attack only aboveground parts of the plant. Large necrotic leaf or stem blotches characterize late blight of potato and tomato. Under the leaf, along the leading edge of the necrotic tissue, a downy, white growth may develop. Defoliation and a dieback starting at branch tips characterize holly tip blight. Rhododendrons develop a leaf spot and blight in winter caused by P. syringae. Crabapple or flowering pear nursery stock can develop a black, sunken canker several inches above the soil due to the same fungus.
Phytophthora ramorum causes different symptoms on different hosts. ‘Sudden oak death’ on tree species is characterized by ‘bleeding’ cankers that girdle the trunk of tanoaks and some other oak species. On rhododendron, Pieris, Viburnum, Camellia and evergreen huckleberry, the disease is characterized by leaf blights and shoot diebacks. Symptoms on rhododendron may be indistinguishable from those caused by other Phytophthora species. The leaf petiole and midrib may be discolored, or the leaf tip or entire leaf blade may be necrotic. Leaf spots can occur where water accumulates on the leaf margins. Shoot dieback occurs when disease is severe. On Viburnum, infected leaves may die and fall off, leaving dark leafless stems. In more severe infections, Viburnum can be killed. On Pieris, infected leaves turn a dark brown. Young shoots and leaves are very susceptible to infection. Other hosts such as camellia may be infected but have only subtle symptoms, such as small leaf lesions on the lower leaves. Infected leaves on these hosts often fall off.
Detecting the Organism
Another good piece of evidence for an accurate diagnosis is the presence of the organism in the diseased plant. Traditional methods of detecting Phytophthora sp. require laboratory procedures and an experienced person to recognize the various organisms one can see or obtain from roots. Some technology is available to growers to test rotted roots.
When a grower, consultant, or county Extension agent sends a sample into a plant disease clinic, any of several procedures might be used to detect Phytophthora sp. Sometimes infected plant tissue such as roots can be sectioned, stained, and observed under a compound microscope. The presence of many thick-walled spores (oospores) unique to these organisms indicates recent colonization of the plant tissue. However, organisms related to Phytophthora may produce similar structures.
Several pieces of root (from near the advancing margin of discolored cambium) can be placed on a shallow jellylike surface called agar in a petri plate. If Phytophthora is present, it may grow out of the root onto the agar. An experienced person can then recognize the growth habit, pattern, colony color, and other characteristics and determine whether it is Phytophthora or not.
The problem with this method is that the organism may not grow out of the diseased plant material. Plant material must be fairly fresh, and the organism must be actively growing. Even then, chances are only 50% that Phytophthora will be detected from any one piece.2 Many times the plant sample is dried out or completely dead. The chance of obtaining it alive from this material is much lower than 50%, if not zero.
A way around this problem is to “bait” out the organism by placing the rotted material (or soil) in a glass of water and floating a healthy piece of plant material (the bait) on top. Commonly used baits include leaf disks, needles, or mature pear fruit. The Phytophthora-infected material produces the swimming spores (zoospores) which infect the bait. Chances of obtaining a culture from this freshly colonized material are very high. In any case, these procedures take time, several days or weeks if further identification is needed.
There are serological based test kits to detect Phytophthora directly from infected plant material.1, 9, 10 Some kits are designed for growers to use in the field in a few minutes. The kits are very useful and can be a good option for samples that normally would be rejected by a plant disease clinic as unsuitable for Phytophthora isolation.10 One limitation of the kit is that a positive result can be obtained only from colonized plant tissue. A false negative result can be obtained from an infected plant if using nondiscolored tissue or tissue that has not been colonized by the organism.
There are very sensitive laboratory techniques (PCR) that can detect the DNA of Phytophthora even when it is present in small amounts. Plant or water samples are frozen in liquid nitrogen and then ground up. The small amount of DNA present in the sample is extracted using chemicals such as chloroform and isopropanol. A unique segment of the Phytophthora DNA we are looking for is added. If DNA of the suspected Phytophthora is present in the sample, then, under the right conditions, the unique reference DNA will match up with the Phytophthora DNA. These matched-up segments of DNA are copied many times so they can be readily detected. Although the test is very sensitive and can quickly find the DNA of a specific Phytophthora species, it takes special laboratories to do the work. Also these techniques cannot tell us if the fungus is alive or dead.
The success or failure of a chemical control program also can aid diagnosis. Usually, the objective of the diagnostic process is to ultimately determine a management course of action. A history of repeatedly using phenylamine (Subdue or Ridomil) or phosphonate (Aliette, Agri-Fos, Fosphite) fungicides with no result may indicate that the problem is not caused by Phytophthora.
No one piece of information alone is enough to conclusively diagnose a Phytophthora disease. Several lines of evidence from the field, the sick plants, and the lab must add up to be certain of the diagnosis. Even isolating the organism alone is not enough; it may only indicate a broader or deeper problem. In some situations, one may obtain one negative test result while other test results and observations suggest that Phytophthora is the problem.
The best way to control a Phytophthora disease is before it starts. Water regulation, clean stock, crop rotation, using composted bark, sanitation, chemicals, and host resistance are among the controls that can be implemented.8
Regulating water is an important way to control Phytophthora diseases. This includes both the amount and duration of water coming to the plants and the way water is conducted away from plants. Phytophthora species generally require free water for a long duration to infect plants. These organisms are not active until the soil is at or above field capacity. In other words, when water does not move down through the soil with the force of gravity.
Water can be regulated easily where irrigation is used. Methods include longer times between irrigations, shorter irrigations, using the correct size of nozzle or drip emitter, and preventing irrigation water from constantly contacting tree trunks. In Oregon nurseries, we have noticed that many of the same crops categorized as “low water users” also tend to be the ones with the most problems with Phytophthora diseases. The reason is, they tend to be grown with plants that require more water and therefore receive too much irrigation during the production cycle. Arranging plants according to water use may help alleviate some of the chronic problems nurseries have with these diseases.
Many times, the amount of water coming into the production system cannot be controlled. In these situations there are some simple techniques to conduct water away from root crowns and roots to prevent the kind of environment that favors Phytophthora. Methods include planting on raised beds or mounds, planting in permeable, well-drained soils, using highly porous potting mixes, tiling poorly drained fields, and sloping container beds. In each case, excess water drains away from root crowns and roots before Phytophthora can become a problem. In New York, planting raspberries on raised beds was as effective as chemical control of Phytophthora root rot.7
Soil layers such as hardpans impede drainage and often allow free water to accumulate above the hardpan. This sets up a favorable environment for Phytophthora infection. Preventing excess soil compaction or ripping or subsoiling these areas can help increase water drainage.
Drought predisposes safflower plants to be more severely affected by P. drechsleri when the soil is subsequently flooded. Plants growing at excess soil moisture for long periods or under salty conditions also are more susceptible to Phytophthora infection.
Purchasing and planting clean, disease-free stock is very important in many production systems. Stock should be accompanied by an official tag or similar documentation since saying something is tested does not mean it is clean.8 Planting in clean or sterile soil or potting media is also important. Using soilless media has helped solve many problems associated with contaminated soil. Avoiding wounds on many tree species is a good way to control P. cactorum.
Cultural methods to eradicate the organism have been used successfully in many crop production systems. Phytophthora does not compete well in soil without a host. The propagules are sensitive to many organisms that commonly inhabit soil. Therefore, especially for annuals, crop rotation is effective if the alternate crop is not susceptible to the pathogen.
High temperatures have been used to control Phytophthora in many ways. Steam heat is effective to kill Phytophthora in contaminated soil, media or on planting containers such as pots. If you re-use pots you can soak pre-cleaned pots in hot (180°F) water for at least 30 min or use aerated steam (140°F) for 30 min. Solar heating in the field by laying out clear polyethylene tarps helps pasteurize the soil. This method has been useful in places with a large proportion of cloudless days such as Israel, California, and Arizona. In Oregon also, the technique has worked for many soilborne pathogens and may also be useful against Phytophthora.
Some fertilizing regimes have been used against Phytophthora spp. They include using organic materials that release ammonia and nitrous acid, using sulfur-based fertilizers and amendments that reduce the pH to less than 4 for acid-tolerant plants, reducing pH to less than 5 in high-aluminum soils (for plants with a tolerance for aluminum), applying foliar nutrients to make up for rotting fibrous roots’ loss of uptake, and avoiding excessive nitrogen fertilization which makes the resulting succulent foliage more susceptible.
Adding composted hardwood or fir bark in potting mixes has increased Phytophthora control in container-grown plants.4, 5 Composted bark increases the air-filled porosity of the media, releases inhibitors as it decomposes, and allows antagonistic soil fungi such as Trichoderma sp. to build up.
Avoiding contaminated ground is also useful. Growing susceptible crops in containers on raised benches can help roots avoid contact with contaminated ground. Staking susceptible plants, which are raised in pots too small for the size of the plant, helps prevent them from being easily blown over by the wind. Leaves can quickly become infected while the plant is lying on the ground or in nearby puddles.
Host resistance to Phytophthora diseases is an effective control and can be used in several situations. For example, several hybrids and species of rhododendron are resistant to Phytophthora root rot. Several cultivars and species of Chamaecyparis are resistant to Phytophthora lateralis. Many cultivars of potato also carry resistance to late blight. Markets, however, may not accept the resistant types or may demand named (susceptible) hybrids.
Susceptible tree species have been grafted onto resistant rootstocks and used successfully against Phytophthora root rot.
Also, within a species there may be many individuals (called races) with varied abilities to attack each cultivar. For example, in strawberry, race 1 of Phytophthora fragariae var. fragariae is capable of infecting cultivars ‘Climax’ and ‘Del Norte’. Cultivar ‘Surecrop’ is not infected; however, races 2 and 5 will infect this cultivar. As soon as resistant cultivars are identified, the organism may undergo sexual recombination and produce offspring capable of infected the new cultivar. Despite this, resistant cultivars are very useful and can be used to produce an acceptable crop.
Using nonhosts is uncommon but effective. For example, Douglas-fir can be planted in low areas or where water drains from a true fir Christmas tree plantation. The Douglas-fir does not become infected with Phytophthora as the true fir would. The species of Phytophthora and its host range dictate the kind of plants that can be replanted on a site from which a diseased individual has been removed.
Chemicals are used to help supplement all the other management techniques. Chemicals are used to eradicate Phytophthora from production equipment, water, and soil. Other chemicals are used to protect plant tissues from infection or to inhibit further growth of the fungus in plant tissues.
Starting with clean tools and production areas will help prevent many diseases. Several kinds of disinfectants are used to treat greenhouse benches, pots, tools, and equipment used for planting and harvesting. Not only will they kill Phytophthora but many other disease-causing organisms. Although they are useful as disinfectants, these products have a very short residual time and will not be effective as long term fungicides or bactericides. These materials include products with peroxide, quaternary ammonium, and/or sodium hypochlorite (bleach) bases. Some, such as dilute bleach solutions, are highly corrosive to metal tools or surfaces. To get the best value from these products, soil or plant debris must be cleaned from objects and surfaces. Increasing contact time with the disinfectant will also improve the products’ efficacy. Since they are quickly tied up by organic mater do not use on soil or gravel.
Many Pacific Northwest growers treat irrigation water to reduce Phytophthora inoculum from suspect water sources. Fruit growers have used copper sulfate, introduced near the irrigation water intake, in an effort to reduce losses from sprinkler rot. Container nurseries recycle irrigation water and use a variety of chemical methods to disinfect it; chlorine gas, sodium hypochlorite, and ozone systems have been installed but with variable efficacy.
When fumigating soil or media for control of weeds and insects you will also get control of many fungi and Phytophthora. Soil fumigation using methyl bromide with or without chloropicrin is effective but was too expensive for Phytophthora control alone. It was phased out after being classified as an ozone depleter under the Clean Air Act. Other effective soil fumigants include metham sodium and dazomet products which break down in soil to isothiocyanate. Metham sodium can be applied with irrigation water.
Several contact fungicides are commonly used to protect plants against foliar infection by various Phytophthora spp. These fungicides inhibit germination and/or penetration of the sporangia, zoospore, or chlamydospore into plant tissues. Since the chemicals are not systemic they are ineffective once the pathogen enters plant tissues. They are best used before spores are dispersed onto healthy roots or leaves and before that inoculum tries to infect the plant. Several forecasting programs can help time applications. The most notable is Blitecast, which monitors temperature, rain, and relative humidity to help time fungicides against late blight of potato.
Copper-based compounds such as bordeaux mixture have been used for a long time and can still be effective. Other copper-based protectant fungicides include copper hydroxide, copper oxide, basic copper sulfate, copper oxychloride, and copper ammonium carbonate. In each case, the active agent against Phytophthora is the Cu++ ion. Some of these may leave residues on plant foliage. They are generally used during the dormant season. Acidic conditions, such as tank mixing with phosphorous acids, will make too many copper ions available and cause plant injury.
The ethylene bis-dithiocarbamate fungicides such as maneb, mancozeb, and zineb are also contact fungicides. Chlorothalonil products, such as Bravo, have also been used to effectively control foliar Phytophthora diseases. Organic tin compounds, such as TPTH, are effective but somewhat more phytotoxic. Etridiazole is also an effective chemical but since it is sensitive to UV irradiation it is used primarily as a soil drench.
There are four groups of chemicals that are taken up and moved around in plant tissues. These include the phenylamide, phosphonate, cinnamic acid, and quinone outside inhibitor (QoI) groups. There are slight differences in the way each group moves into and within the plant which has a bearing on how and when they are used to manage Phytophthora diseases. Some chemicals move up from roots to shoots (apoplastic or xylem movement only) or both up and down in the vascular system (symplastic or both xylem and phloem movement).
This uptake and movement in the tissues provides both protectant and suppressive activity. They must be applied when there is active plant growth for this movement to occur. All of these chemicals are used extensively to control Phytophthora and related diseases. Products that contain these chemicals are used as seed treatments (for damping-off diseases), soil drenches (for root and crown rots), or foliar sprays.
The phenylamide group has xylem movement only (from roots to shoots) and includes metalaxyl, oxadixyl and mefenoxam. When trying to prevent a root rot, these chemicals must be applied or incorporated into the soil or media. It is best to time application in the spring just before or when early root growth occurs. When trying to control a foliar disease it must be applied to the foliage. Why? Although root-applied chemical will move up into the leaves it will not be at a high enough concentration to achieve disease control.
The phenylamide group is active only against oomycete organisms which include Phytophthora, Pythium, and the downy mildews. It suppresses sporangial formation, mycelial growth, and establishment of new infections. It does not inhibit zoospore release, zoospore encystment, or initial penetration of the host.
The phosphonate group moves both up and down in the vascular system (both xylem and phloem movement) and includes fosetyl-Al and phosphorous acid. A plant takes up these chemicals through roots, leaves, and stems and then moves it to other parts. Trunk, soil, or foliar applications can effectively control Phytophthora root diseases. Soil microorganisms can degrade some of these chemicals quickly, so foliar applications are preferred. Application can be made any time during active plant growth.
The mode of action of this group of chemicals is slightly different. There is direct activity on Phytophthora itself, however, they also stimulate host plant defense responses. There is controversy as to whether they control Phytophthora by direct activity on the organism itself or by a combination of direct activity and the enhancement of natural host resistance.
The phenylamide and phosphonate fungicides do not kill Phytophthora. They can only prevent establishment of the organism before it gets into the plant. They can also prevent continued growth if the organism is already inside the plant. The result is that they can delay symptoms that might have developed. Once chemical activity has subsided over time, Phytophthora can resume growth within infected plants.
For this reason you may choose to, or be directed to, AVOID use of these chemicals when producing susceptible plants. It would allow you to identify infected plants that need to be discarded. It also would avoid the shipping of infected nursery stock to places where the disease is not yet present.
The cinnamic acid chemical group, dimethomorph, has similar properties as described for the phenylamide group. It also moves from roots to shoots but is less effective overall. It is active on Phytophthora diseases but does not control diseases caused by Pythium.
Some of the fungicides in the QoI group have activity on Phytophthora and many fungi. Although these fungicides are systemic they do not move as much within plant tissues. By comparison, these chemicals move slowly into green leaves and stems. This is called translaminar movement. Once in the tissue there is limited movement within the xylem. These chemicals are used to control foliar infections.
The mode of action of each of these groups is so specific that many Phytophthora species have developed resistance to them. This means the organism can grow and cause disease at chemical concentrations that used to prevent them from growing. Tank mixing with contact fungicides will help prevent the development of these resistant types.
Integrating several controls should help prevent losses due to Phytophthora diseases. Fighting these diseases after they have become established is difficult if not impossible. Emphasis should be on recognizing the potential for disease and taking preventive steps before losses occur.
Parke’s top 10 Management Tips for Nurseries:
- Ensure good water drainage.
- Purchase clean stock.
- Keep propagation area as clean as possible.
- Use new or clean pots!
- Never put containers on bare soil.
- Clean up leafy debris.
- Keep container plants from tipping over.
- Destroy sick plants.
- Use only clean water for irrigation.
- Scout regularly for disease symptoms.
1 Benson, D.M. 1991. Detection of Phytophthora cinnamomi in azalea with commercial serological assay kits. Plant Disease 75:478-482.
2 Erwin, D.C., Bartnicki-Garcia, S., and Tsao, P.H. 1983. Phytophthora: Its biology, taxonomy, ecology and pathology. St. Paul, MN: American Phytopathological Society.
3 Erwin, D.C., and Ribeiro, O.K. 1996. Phytophthora Diseases Worldwide. St. Paul, MN: APS Press.
4 Hoitink, H.A.J., and Powell, C.C. 1990. Fighting Phytophthora: A guide to combating Phytophthora root rot and dieback in ericaceious crops. American Nurseryman, May 15, 171:67-73.
5 Hoitink, H.A.J., Inbar, I., and Boehm, M.J. 1991. Status of compost-amended potting mixes naturally suppressive to soilborne diseases of floricultural crops. Plant Disease 75:869-873.
6 Linderman, R.G. and Zeitoun, F. 1977. Phytophthora cinnamomi causing root rot and wilt of nursery-grown native western azalea and salal. Plant Disease Reporter 61:1045-1048.
7 Maloney, K.E., Wilcox, W.F., and Sanford, J.C. 1993. Raised beds and metalaxyl for controlling Phytophthora root rot of raspberry. HortScience 28:1106-1108.
8 Parke, J., Pscheidt, J.W., Regan, R., Hedberg, J. and Grunwald, N. 2008. Phytophthora Online Course: Training for Nursery Growers. Oregon State University Extended Campus. Currently available at http://oregonstate.edu/instruct/dce/phytophthora/
9 Peterson, F.P., Miller, S.A., and Grothaus, G.D. 1990. Monoclonal antibody-based immunoassays for detection of Phytophthora spp. in plants. (abstr) Phytopathology 80:962.
10 Pscheidt, J.W., Burket, J.Z., Fischer, S.L., and Hamm, P.B. 1992. Sensitivity and clinical use of Phytophthora-specific immunoassay kits. Plant Disease 76:928-932.