Powdery Scab and Early Blight

Combating diseases affecting the Intermountain West

Published in the February 2013 Issue Published online: Feb 18, 2013 Robert Davidson, CSU Extension Seed Potato Special
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The research program for Dr. Robert Davidson, the 2012 Potato Grower magazine Researcher of the Year is composed of several different aspects with a decided applied focus.

Rob, along with research associate and colleague Andrew Houser, has spent the last several years working with disease issues that affect not only Colorado, but much of the Intermountain West. Their program has evolved based upon the needs of the industry and grower input.

Much emphasis has been placed on dealing with seed-certification-related issues like controlling PVY and Potato leafroll virus in the early generation seed stocks as well as working toward better production methods for producing mini-tubers in the greenhouse and first-year field stocks. The growers in Colorado's seed certification program produce in excess of 150 cultivars and/or clones annually, so there is a very real need to match research efforts for diseases with the cultivars being produced.

Evaluation of Advanced Clones for Suseptibility to Powdery Scab Root Galls - Inoculated Controls, San Luis Valley, Colorado To do this, Davidson and Houser utilize both on-site (at the San Luis Valley Research Center in Center, Colo.) field plots and greenhouses as well as grower cooperator fields to conduct their research program. Additional emphasis has been placed on how the cultivars respond to the San Luis Valley environment, especially where water is concerned. Growers are looking for ways to maintain the highest yields of quality potatoes under difficult field circumstances. Because of this, Davidson and Houser work closely with the Colorado cultivar development program, the potato physiologist and the post-harvest storage specialist (all stationed at the SLV Research Center) as well as the growers to bring the best information about each cultivar to the industry.

Three aspects of their research efforts will be discussed. For example, they have spent years trying to clarify the control or management of a soil-borne pest that is growing in importance. This disease, powdery scab, caused by the protist Spongospora subterranea, has wreaked havoc on many of the fresh-market, thin-skinned cultivars. Specialty cultivars seem to be especially susceptible to this problem. Their work has focused on understanding how the pathogen infects the crop, which environmental conditions are most problematic, especially in terms of increasing the soil population of the spore balls and how the average grower can manage the crop to avoid the subsequent scab lesions that are commonly associated with this disease.

Much of their research has been highlighted before in other publications and during the potato schools held around the country. Control options starting with a good soil test prior to planting, which predicts the population of the sporeballs, coupled with appropriate planting of the best-suited cultivars for the soil situation, and use of Omega (a.i., fluazinam) in-furrow, have been quite successful in keeping the problems from this pest to a minimum.

Additionally, work is currently focused on identifying good resistance in newer cultivars for all phases of the disease. This includes the root-galling phase, where much inoculum increase in the soil can take place with little-to-no evidence for the producer that this is occurring, if the tuber shows few or no symptoms. Also, there is the tuber phase where scab lesions can easily cause a lot to be out of grade or even rejected for sale.

Evaluation of Advanced Clones for Tuber Susceptibility to Powdery Scab based upon Severity Index, San Luis Valley, ColoradoFigures 1 and 2 illustrate just how far we have come for many of the major categories of potatoes. Russets are typically resistant to the tuber symptoms, but often have root-galling problems. Reds and yellows tend to be quite susceptible to the tuber phase of the disease, but may be less susceptible to the root-galling phase. Specialty cultivars tend to be susceptible to both phases. We have found that many of the newer clones range between moderately resistant to moderately susceptible to the different phases of powdery scab. Other work indicates that growing resistant cultivars in infested soil can lead to reduced inoculum in the soil after only one season.

Another focus of the program has been on the use of fungicides to control early blight (Alternaria solani). While this may seem routine, growers often apply fungicides without taking into account the mode of action and the potential for resistance to the active ingredient by the fungus.

Davidson continually encourages the growers to do a good job of rotating active ingredients and making certain applications are timely and cover the foliage.

Early Blight Degree Days for the San Luis Valley. Based on the model by Franc, et al. 1988. Plant Disease 72: 851-854Overall cost of the fungicides when correctly applied rarely exceeds $40-60/acre for the three-to-four applications necessary to control early blight in the San Luis Valley. The program stresses the need for individual growers to understand that they are part of a larger community of producers and need to appropriately handle their crop. One interesting finding is that the greatest yield advantage in the crop was seen when a strobilurin product was used first in the rotation at or around the time of initial infection. In addition, other chemistries are compared in the field and/or greenhouse for efficacy against pink rot, powdery scab, insects, Rhizoctonia and other issues as they become problems.

Finally, Davidson and Houser have been screening the newest cultivars/clones for the timing and symptom expression of several key diseases including bacterial ring rot, PVY, Potato leafroll virus, dry rot, blackleg and soft rot, powdery scab and pink rot. Several clones have been identified with good resistance to some of these diseases and are currently being listed on any cultivar management information available to the producers. Also, symptoms to bacterial ring rot are well documented for each cultivar. Each year the commonly grown cultivars are evaluated to verify the symptom picture for growers and the seed certification program.

Davidson indicates that through these research efforts, he believes growers are better able to manage many of the more-difficult disease problems seen in potato in Colorado and, it is hoped, producers in other parts of the U.S. can take advantage of these results.