Diagnosing Phytophthora diseases can be challenging, as other factors including drought, nutrient deficiency, sunburn, and other pathogens can cause similar symptoms to the ones caused by Phytophthora.  In addition, many infected plants remain asymptomatic, and disease symptoms start appearing only after transplanting.  This is one of the main reasons why regular testing for Phytophthora is needed. 

Plant health monitoring In the nursery should occur on an ongoing basis. Keeping records on environmental conditions and cultural practices associated with various plant batches in the nursery can help with the diagnostic process if symptoms develop. Plants with symptoms of sudden or chronic water stress, including wilting, leaf necrosis (browning), chlorosis (leaf yellowing), dieback, and sudden plant death are of particular concern.  Both Phytophthora root rot and inadequate irrigation can cause these water stress-related symptoms, so irrigation practices should be checked. In particular, check whether irrigation is adequately wetting the entire root zone of the affected plants. Irrigation water may run off without wetting the whole root zone if head space at the top of the container is inadequate. Also look at the distribution of affected plants. In general, both Phytophthora root rot and irrigation problems may tend to show some clustering, with patches of adjacent plants being affected. However, at an early stage of symptom development in a Phytophthora-infected batch, symptomatic plants could be widely scattered with no obvious pattern.   

If container soil seems to be excessively wet, Phytophthora root rot should be considered a possibility. Rotted roots will not absorb water and the container media may remain wet. Wet soil conditions also favor Phytophthora root rot. High moisture alone seldom, if ever, causes health problems in nursery container plants because of the extended length of time that soil needs to remain saturated for flooding to cause damaging anoxia. Most problems attributed to overwatering in nurseries are actually associated with Phytophthora root rot.

If symptoms do not appear to be related to inadequate irrigation, roots should be inspected for the presence of root rot (brown discoloration, affected roots often soft, outer root cortex may easily come off). Also examine total root ball size and vigor. Infected plants tend to have fewer roots than healthy ones and often fail to develop many new roots when transplanted into a larger container. Also check whether dead or discolored areas (cankers) are visible on the stem near the soil line. Many soilborne Phytophthora species can cause these crown rot symptoms.

Set symptomatic plants aside, in a quarantine area, away from the healthy plant stock. Set symptomatic plants aside in a quarantine area away from healthy plant stock. Leave gaps on the bench where symptomatic plants had been and monitor the nearby plants for symptom development. If symptomatic plants will be culled, root/soil baiting can be used to test for the presence of Phytophthora. You can also use the nondestructive bench leachate test in your facility to test live symptomatic plants and surrounding live plants for Phytophthora.  Symptomatic green pear baits from these tests can be submitted to the CDFA Plant Disease Diagnostic Laboratory in Sacramento for confirmation and further Phytophthora identification. You can also submit plant samples to the greenhouse pathology lab at UC Davis for general plant diagnostics.  When submitting a sample, it is important to submit an adequate and informative sample and ensure that it arrives quickly and in good condition. Further details about sample collection and submission are at these links. For a reliable Phytophthora diagnosis, symptomatic plants and baits should be analyzed in a laboratory by a plant pathologist.