Can Phytophthora ramorum be controlled in our nurseries? - An overview

Sharon L. von Broembsen
Entomology & Plant Pathology, Oklahoma State University, Stillwater, Oklahoma

Phytophthora spp. occur in most ornamental nurseries in the USA on a wide range of hosts. The aboveground symptoms caused by some of the most commonly occurring Phytophthora spp. are similar to those caused by P. ramorum. Nursery diseases caused by Phytophthora spp. are notoriously difficult to control, and much attention has been given to how to accomplish this. (See the other papers in this session on specific ways to control Phytophthora spp. in nurseries.) Nursery managers have had to learn how to minimize the effects of Phytophthora spp. to economically acceptable threshold levels. However, a new threshold (incorporating zero tolerance for P. ramorum) is now required; this necessitates new ways of thinking and tighter controls.

It is not clear at this point whether P. ramorum will be able to become established in nurseries in Sudden Oak Death (SOD) affected areas of the USA or whether nursery stock will serve as a vector to areas outside the current distribution. Yet we definitely need to be prepared for both these possibilities. The current management strategies being employed to control P. ramorum in nurseries in quarantined areas in the USA appear to be working. This gives us hope that the nursery facet of SOD is indeed manageable and that P. ramorum can be prevented from establishing in nurseries. The success of these strategies comes at great cost to all involved in their implementation, not least the nursery industry in these areas, but the cost is borne because there is no other choice. It remains to be seen if the high cost of controlling P. ramorum forces some of these nurseries out of business.

But what is the potential for visually disease-free nursery material infected with P. ramorum to move the pathogen to other regions where infected plants might provide opportunity for cross infection of new hosts? Based on the extremely low incidence and lack of spread in nurseries in the affected areas in the USA and assuming that the more nursery based strains occurring in Europe are not introduced to the USA, the chances seem low. This is the difficult part of the overall SOD management strategy: deciding how much of our public and private resources should be devoted to this small but significant possibility. As a first step, public funds have already been provided for preliminary surveys of hosts of P. ramorum in nurseries outside the affected areas, especially those that have received material from affected areas both within and outside the USA or those located in areas where native forest systems are deemed vulnerable. This is a solid precautionary measure. As more information becomes available, the decision to continue and expand such surveys can be taken. The surveys provide an excellent opportunity to help nursery managers understand the serious implications of the SOD situation and the need to raise the level of attention given to controlling Phytophthora spp. The nursery industry outside the currently affected areas must be kept up to date with the latest scientific information as soon as it becomes known. Intensified control measures will increase operational costs, but a fully informed nursery industry will respond positively to this situation, because nursery managers will understand that it is in their best interest to do so.

Can Phytophthora ramorum be controlled in our nurseries? It can and must be controlled.