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How to Manage Pests

UC Pest Management Guidelines


Floriculture and Ornamental Nurseries

Sudden Oak Death

Pathogens: Phytophthora ramorum

(Reviewed 3/09, updated 3/09)

In this Guideline:


SYMPTOMS

Although hosts of P. ramorum show a range of symptoms, the disease is generally characterized by irregular, necrotic leaf lesions, instead of distinct leaf spots. Leaf infections can develop down into the petiole and twigs. Often, infected leaves fall before the lesion develops to the petiole. Sometimes infections occur initially on stems or develop into stems and cause blights, where stems and associated leaves wilt, become necrotic, and die. A distinct dark line can mark the advance of the infection on some species such as California bay laurel. Under natural conditions, California bay laurel tends to get infected on the tip of the leaf, where the leaf hangs down and water accumulates. This characteristic is seen in some nursery hosts as well. Infection often occurs on leaf tissue where free water remains on leaves for long periods, such as deep in the leaf canopy, near or touching the soil, and between overlapping or cupped leaves.

While not commonly seen on nursery hosts, bark cankers on the trunks of trees are also associated with this disease. Cankers have red-brown to black discoloration, seep dark black to reddish colored sap, and often develop 3 to 6 feet above the ground, although they can be higher or lower. Symptoms caused by fertilizer burn, chemical injury, drought injury, freeze damage, sunburn, and root damage can look similar to P. ramorum infection. Abiotic injury is often found distributed over the entire plant, while P. ramorum leaf spots are often found on only a few leaves or on one portion of the plant.

COMMENTS ON THE DISEASE

Tens of thousands of tanoaks (Lithocarpus densiflorus), California black oaks (Quercus kelloggii) and coast live oaks (Quercus agrifolia) have died as a result of P. ramorum infection. In Marin, Santa Cruz and Monterey Counties, portions of the wildland-urban interface forest changed dramatically; tree crowns turned brown in a few weeks, giving the impression of instantaneous mortality, giving rise to the name sudden oak death.

Camellias, rhododendrons and other popular ornamental plants are susceptible to P. ramorum infection, and the pathogen can move long distances through shipments of infected nursery stock. Federal and state quarantines are in effect that require nursery inspections, and if the pathogen is found, affected nursery stock must be destroyed as a means of eradication. For a current host list and additional information, see the California Oak Mortality Task Force Web site (http://nature.berkeley.edu/comtf/).

BIOLOGY

Phytophthora ramorum, while having many features in common with fungal organisms, is not a true fungus. Phytophthora species are Oomycetes or "water molds" and require a moist environment to actively grow and reproduce. Phytophthora ramorum produces several reproductive structures important for pathogen spread and survival, including sporangia, zoospores and chlamydospores. Sporangia give rise to the zoospores, which can swim in water. Chlamydospores are resting spores that help the pathogen survive extreme temperatures, dryness and other harsh conditions. Phytophthora ramorum can grow within a temperature range of 36 to 80° F with an optimum temperature of 68° F.

Spore structures commonly form on leaf surfaces of susceptible leaves and twigs following prolonged wetting. They are moved in contaminated soil, from plant to plant via windblown rain, or by direct contact of infected leaves. In California forests the pathogen sporulates prolifically on California bay laurel trees (Umbellularia californica) that serve as reservoirs for inoculum. Infected California bay laurel can also be an important source of inoculum when in close proximity to nursery stock.

MANAGEMENT

For most nurseries, the foremost objective of pest management programs is to prevent the introduction of the pathogen into the nursery via infected plant material or other means. This can be partly accomplished by careful inspection of all incoming host propagative material and stock.

Monitor the outside source stock weekly, and intensify monitoring a few weeks after bud break, especially in rainy periods when environmental conditions are highly conducive to pathogen infection and development.

Disease symptoms might take weeks to several months to develop and become apparent, so plants may appear healthy. Fungicides that have activity on Phytophthora might prevent new infections and therefore, interfere with detection of this pathogen; it is best not to apply fungicides while evaluating the disease status.

For nurseries surrounded by native host trees and shrubs and in the vicinity where P. ramorum is found, monitor areas surrounding the nursery, especially wet areas, near puddles, or rain runoff zones. It is very important to detect the pathogen early, while it is still at very low levels.

  • Periodically inspect nearby native hosts for disease symptoms. Infected California bay laurel trees near the perimeter of nurseries may produce inoculum that can spread and cause infection of nearby host plants, so removal of these trees may be warranted.
  • Consider building berms to prevent water and soil movement into production areas from surrounding hillsides the nursery that may contain infected hosts.
  • Irrigation water pumped from streams and ponds in areas of infected native hosts may be contaminated with P. ramorum. Consider having this water periodically tested to detect P. ramorum. If it is found to be present, use alternative irrigation sources, such as well water, or disinfection treatments.

For more information on developing a detection and monitoring program, see Nursery Guide for Diseases Caused by Phytophthora ramorum on Ornamentals: Diagnosis and Management, UC ANR Publication 8156.

Cultural Practices
Cultural practices that can be useful to reduce disease risk:

  • Avoid irrigation practices where the foliage is wet for prolonged periods. If sprinklers are used, irrigate in the morning to allow for thorough and quick drying of foliage.
  • Monitor and maintain irrigation systems to insure the most uniform application of water to the crop. Correct low spots, areas of poor drainage, and clogged or leaking irrigation heads.
  • Monitor irrigation-water sources, other than well water, for P. ramorum. Use disinfection systems if using recycled water.
  • Wounded leaves (even tiny wounds or scratches) are much more susceptible to infection. Avoid handling host plants if they might be wounded when environmental conditions favor disease.
  • Avoid splashing water from soil to foliage, as well as from foliage to soil or container soil. Use raised benches, gravel, or other means to elevate susceptible plants above soil. Transplants, even on gravel beds, appear to be very susceptible to disease because of the close proximity of foliage to soil, runoff water, or rain splash. Raised benches may be warranted for transplants.
  • Remove plants or plant parts that are suffering from poor vigor, disorders, or other serious problems from production areas and destroy them. A small number of plants or plant parts could be bagged and disposed. If a cull pile is needed temporarily, cover it with a clear polyethylene sheet until the culls can be destroyed or composted.
  • Propagate cuttings only from disease-free hosts.
  • Use only new or disinfested containers and soil. Place potting soil piles as far from infected native hosts or cull piles as possible and covered with clear polyethylene sheeting. Do not mix potting soil components on bare soil.
  • Disinfect tools and shoes that have been in contact with contaminated plants or soil.

Fungicides. Fungicides used to protect nursery stock from P. ramorum function are preventive treatments only; currently, even the most active fungicides do not stop the development of P. ramorum once foliar lesions are present. Whether or not to use Phytophthora-specific fungicides for the sole control of P. ramorum is a complex question and should be reserved for special cases, such as where a nursery is exposed to local inoculum sources from surrounding infected native hosts or when the nursery has no choice but to use irrigation water that could contain P. ramorum inoculum. When fungicide applications are made to control other Phytophthora species, it may be appropriate to select fungicides and application intervals that can also control P. ramorum. Regardless of the reasons to use fungicides, these chemicals should only be used after other management strategies and preventive steps have been fully implemented.

Although fungicides might help prevent the spread of the disease if infested plants are introduced in the nursery, they might also hinder the detection of the disease. The masking of symptoms with fungicide use might eventually lead to the sale and movement of infected plants to nurseries or customers in noninfested areas. Such an occurrence might circumvent the quarantine program that intends to limit the spread of sudden oak death. Finally, Phytophthora-specific fungicides used continuously may lead to fungicide resistance.

When applying fungicides, good coverage over the foliage is important; add a wetting agent to prevent significant run-off and loss of fungicides on the hard-to-wet leaves of certain plant species. Apply treatments before environmental conditions favor pathogen infection; for example, spraying before a period of rainy weather will allow water to linger on leaf surfaces for many hours.

Some fungicides applied to the foliage move into leaves and are not washed off by rain or sprinkler irrigation, while others provide a protective layer of chemical on the leaf surface. Some can be applied to the soil, where they are adsorbed and moved upward to the leaves, to protect them from infection. Some have residual activity that can last for several weeks after they are applied. Read fungicide labels and technical information provided by the fungicide manufacturer to learn how the fungicide can be used most effectively.

Resistance management. To help reduce the potential for the development of resistance by P. ramorum to fungicides, alternate or tank mix fungicides with different mode-of-action Group numbers. Fungicides active on P. ramorum may already be used in the nursery to control other foliar or soil-inhabiting Phytophthora species or related pathogens (such as downy mildews), and their use should be considered in planning the overall fungicide resistance management program.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside
S. A. Tjosvold, UC Cooperative Extension Santa Cruz County

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