Phytophthora Diseases Affecting Ornamentals and the Potential for Foliar Outbreaks

Phytophthora diseases affecting ornamentals and the potential for foliar outbreaks results from various ornamental disease surveys conducted over the past decade. Communication with diagnosticians who routinely receive ornamental disease samples indicate foliar outbreaks of Phytophthora remain a persistent issue affecting our industry. It seems species of Phytophthora are present in all ornamental nurseries with the potential to cause significant disease of great economic importance. Phytophthora is routinely introduced into greenhouses and nurseries through contaminated stock plants contaminated media and contaminated irrigation water. Once introduced, inoculum of the pathogen can spread by splashing irrigation water and through cultural practices such as potting, pruning and the movement of plant material. 

The discovery of Phytophthora ramorum causing a serious disease on oak and other tree species in Western U.S. nurseries set the stage for numerous Phytophthora studies. A review of the literature on Phytophthora species associated with ornamental plants, reveals that numerous species are reported. With an increase of scientists developing more specific and sensitive detection methods, we will likely see the reports of Phytophthoraspecies affecting ornamental production continue to rise. Based on crop type (i.e. tropical foliage, bedding plants, woody ornamentals) and geographic location, the Phytophthora species present can widely vary. For example, there are six species of Phytophthorareported on tropical foliage with P. nicotianae among the most common to cause foliar outbreaks. More than a dozen Phytophthora species have been reported associated with foliar diseases of woody ornamentals with P. citricola among the most common.

It’s crucial growers understand the detection of Phytophthora in and among their plants. The inoculum of this pathogen, capable of causing root rot as well as foliar diseases, increases from low (even levels so low they are undetectable) to high levels within a few days depending on the conditions. The increase of inoculum is caused by the rapid production of sporangia and zoospores from infected plant tissues when free moisture is present. Phytophthorahas a short generation time and great reproductive capacity, often leading to disease outbreaks in a short period of time. Sometimes it seems the disease can turn on literally overnight. The presence of free moisture and the leaf wetness period are the most important environmental factors associated with foliar outbreaks of Phytophthora. Leaf wetness is the presence of free water on the surface of a plant leaf that results from overhead irrigation, rainfall, fog or dew, which can form on any surface of the plant, mainly on the top and bottom sides of the leaves. Thus, irrigation practices to minimize leaf wetness and proper plant spacing to allow for air movement through the canopy are important disease management strategies for controlling foliar outbreaks of Phytophthora.