Because Phytophthora species can be difficult to isolate from diseased plants, plant pathologists often use baits to detect Phytophthora. These include seedlings, leaves, or fruits of various host plants (Erwin and Ribeiro 2005). Different baits vary in their susceptibility to the various Phytophthora species, and no single bait can detect all species. Among baits, green pears are readily available, are susceptible to many common and uncommon Phytophthora species, and are relatively easy to interpret. Pears can be used to detect Phytophthora in soil samples, water samples, and root samples. A false negative result using pear baits may result if the Phytophthora species present does not readily infect pears.
Baits need to be added to samples when actively swimming zoospores are likely to be present (description of the disease cycle of Phytophthora root rots). For this reason, it is necessary to add the bait at the very beginning of whatever test procedure you are using. If zoospores encyst before baits are added, baiting may provide a false negative result. Depending on temperature, a given zoospore will generally swim no more than 8 to 24 hours. Mechanical agitation and rapid changes in temperature or the salt concentration of the water can induce zoospores to encyst.
It is important that pears be as free of wounds and as green as possible. Smooth, green-skinned varieties (e.g., Bartlett, D' Anjou, Packham) are all acceptable, but green D’Anjou pears tend to be slower to ripen, which is desirable. Phytophthora species are among the few organisms that can infect green, unwounded pears. Ripe pears and those with nicks and scratches are susceptible to invasion by organisms other than Phytophthora, especially Pythium species. Don’t use pears with large wounds and avoid pears with many wounds, especially fresh wounds. If there are small surface wounds or discoloration, you can use a permanent marker to make a light dotted line around the affected areas, which can help you interpret what you see when pears are removed after baiting.
Rinse pears thoroughly before using. You can also wash pears using a little dish soap, but be sure to rinse thoroughly. Don't try to peel off the grocery sticker, as it will generally tear the pear skin. If pears will be used to bait water samples, test each pear in a container of water to be sure they will float. Pear baits in water samples should float at the water surface as shown in Figure 1 upper left. If a pear does not float, make a pear floatation device (PFD) for it from a small piece of closed cell foam (e.g. a packing peanut) and a clean rubber band. Using a permanent marker, number the pear with the sample number prior to placing in the sample.
Bags should remain open to allow for air exchange, but close bags while transporting to avoid spills. Make sure that the bag is in a container (e.g. cut-off 1 gal plastic bottle) that will not allow the sides to collapse and spill water. The container should be large enough to prevent any accidental spills or leakage if the bag is punctured. Place these containers a secondary container (e.g., a plastic tub) if necessary to contain spills.
Incubate pears in water samples at moderate room temperatures. A fluctuating day/night temperature regime ranging between about 18-24°C (65-75°F) is generally suitable for detecting a variety of Phytophthora species using pear baits. Daily cycling of water temperature can promote zoospore release from sporangia that may be in the water.
Start checking pear baits for symptoms 2 days after the pears were added to the water. Pears should be removed from water as soon as symptoms appear, and should be removed after 3 days even if no lesions have appeared. Infected pears that are mostly submerged in water sometimes only develop subtle light discolored patches. When removed from the water, the lesions will darken quickly, commonly within an hour (Figure 1). Lesions may also appear up to 5 days (or rarely longer) after pears have been removed from the water.
There is no advantage to leaving the pear in the water once lesions appear, as these lesions will become more vulnerable to infection by other organisms, which can complicate diagnosis. If water chemistry is unusual (e.g., highly acid), pears may develop a network of splits or cracks, sometimes starting at wounds. Cracking also develops on some pears for other reasons that are not clear, but are likely related to the physiology of the pears. Pears should be removed from water immediately if substantial cracking develops, as they will only become more degraded if they remain in water.
Use clean waterproof gloves to remove pears from water samples and change gloves or wash them between samples. Use soap and water or alcohol (70% isopropanol) to clean the gloves and then rinse thoroughly with water. Rinse each pear individually with running tap water over a sink when you remove it from a sample bag. Pears may develop a slippery biofilm on the surface; this does not have to be rinsed off completely. Be careful to avoid cross-contamination of other samples from splashing water and handling wet pears.
Set pears to dry on racks or clean paper towels so they do not touch each other. Keep the pears indoors at temperatures between 18-24°C (65-75°F). Lesions will continue to grow in size and coalesce. For many Phytophthora species, this process will occur faster at warmer temperatures.
In floating pears, lesions may develop on any portion of the pear that has been in the water (Figure 1). Phytophthora lesions may occur either in nonwounded areas of the pears or may be associated with wounds. They range from dark to light brown and are normally somewhat to quite firm initially, though they may become soft in the center. Symptom intensity and the number of lesions increases with the amount of inoculum in the water. Positive samples may have only one or two spots at low inoculum levels or may be covered with lesions under high inoculum levels. However, Phytophthora species vary in their aggressiveness to pear baits, so the number of spots does not always correlate with inoculum level.
Lesions associated with Pythium infections or true fungi are always associated with wounds. They normally start out soft and watersoaked and are commonly sunken. “Classic” firm, dark brown lesions developing independent of wounds (Figure 1) are rarely, if ever, anything other than Phytophthora. Softer, less typical lesions associated with wounds can also be caused by some Phytophthora species. See Using green pears to bait for Phytophthora/Assessing baits for additional images of Phytophthora infections on pear baits and information on how to confirm lesions as caused by Phytophthora .
Pear baits that are used for baiting should be heat-treated to kill Phytophthora before disposal. Heating pears in a microwave in a heat-resistant plastic bag to a temperature of 95 C (203 F) for 30 seconds is sufficient to kill these pathogens in infected pears. An alternative standard is 85 C (185 F) for 3 minutes. Allow the bag to cool slowly to maximize the duration of the high temperature treatment.
Baiting with green pears can also be used to detect Phytophthora in root/soil samples or in water collected from water bodies, watercourses, or runoff. See 3.3. Individual plant sampling/baiting and Using green pears to bait for Phytophthora for details on these techniques and more photos of pear symptoms.