OSPUD Participatory Organic Potato Project

OSPUD: Organic Late Blight (LB) Management Report 2007

For diagnostics and conventional management recommendations see the Michigan State University Late Blight Extension Bulletin at www.lateblight.org/lateblight-bulletin.pdf

 Ospud Potato Late Blight Spray Trial, Summer 2007

 INTRODUCTION: 

Most organic potato farmers in western WA and northwestern OR have experienced late blight epidemics.  In 2006 Ospud emphasized cultural management strategies and identified and trialed LB resistant clones.  However, farmers west of the Cascades can experience epidemics in spring or fall despite practicing best cultural management and growing resistant clones.  For this reason, Ospud farmers requested an LB spray trial in 2007.  Requested materials included coppers, oxidizers, compost teas, and biologicals.

MATERIALS AND METHODS: 

Treatments:

¹Cuprofix is not currently OMRI or WSDA listed for use on certified organic farms

²Kocide 3000 was OMRI-listed in October 2006 but may no longer be listed.

³Nordox is currently listed by OMRI and WSDA for use on certified organic farms. It is regulated, and must be used in a manner that minimizes accumulation of copper in the soil [205.601(i)(1)]

⁴Josephine Porter Institute for Applied Biodynamics, PO Box 133, Woolwine, VA 24185                      276-930-2463 www.jpbiodynamics.org. Materials were applied as recommended by JPI.

⁵Wilt Farms, Hwy 99W, Corvallis, OR. 541 752-0460. Tea was applied as received from Wilt Farm on dates that Wilt Farm produced tea. On two of the application dates, tea was stored overnight and applied the following day.

Treatments were applied with a hand-pump backpack sprayer. Spray volume was very high, approximately 600 gallons per acre, to ensure coverage of all foliar surfaces, including the undersides of leaves where Phytophthora infestans sporulates.

The randomized complete block experiment (9 treatments, 4 blocks) was conducted at the Lewis Brown Horticulture Farm in Corvallis, Oregon. Plots consisted of 3, 15-ft rows of Yukon Gold potatoes planted on 12” in-row and 34” between-row spacing. Potatoes were planted on a conventionally managed field. A conventional aphicide was applied at planting but no other pest management materials were applied other than the treatments. Plots were separated by a 10 ft no-plant gap in the row direction. Contiguous plots were separated by a continuous buffer row of potatoes that also served as an inoculation row. Potatoes were planted on June 19 to ensure that the potatoes matured during the fall when shorter days, dew, and rain increase the likelihood of a LB epidemic. Establishment in this field was poor; approximately 20% of the seed did not sprout. Sprouted seed was transplanted from extra rows and the 10 ft between-plot border rows to replace the unsprouted seed; after transplanting, plot stands were close to 100%.

Inoculation: Late blight did not occur in this field naturally. A natural late blight epidemic did occur at Persephone Farm. Diseased, sporulating foliage was collected from the Persephone field in the morning on August 28 and stored in plastic bags in a cooler during the day. The Lewis Brown trial was irrigated in the afternoon to wet the foliage. Approximately 2 stems per 15 ft inoculation row were distributed in inoculation rows in the field trial just before sunset.

Disease evaluation: The central 12-ft section of the center row of each plot was evaluated for disease severity. This 12-ft row section was evaluated as three 4-ft sections, resulting in three disease ratings per plot. These ratings were averaged to generate one mean disease severity rating per plot. Disease severity was evaluated on a 10 point scale: 1 = 0% necrosis, 2 = 0.1-2.5% (mean = 1.25), 3 = 2.5 – 10% (mean = 6.25), 4 = 10-25% (mean = 17.5), 5 = 25-50% (mean = 37.5), 6 = 50-75% (mean = 62.5), 7 = 75-90% (mean = 82.5), 8 = 90-97.5% (mean = 93.75), 9 = 97.5-99.9% (mean = 98.75). 10 = 100%.  AUDPC was calculated for the period 9/12/07 – 9/23/07. Analysis of variance and mean separation by the LSD procedure were performed using SAS statistical software.

• The three copper products, all applied at the highest labeled rate, reduced overall late blight severity over time (AUDPC, Table 1) by 88% with no significant difference amongst copper products. In epidemics initiated early in tuber bulking, this level of disease control would likely increase potato yield.
• There was no water control applied on the same dates as the Wilt Farm tea; therefore, Wilt Farm tea and other treatments should not be directly compared. Nonetheless, potatoes sprayed with Wilt Farm compost tea exhibited 23% less late blight severity over time (AUDPC, Table 1) than the water control (applied on the same dates as the other treatments). Disease severity was 60% and 28% lower at the 2^nd and 3^rd evaluation dates in the Wilt Farm tea treatment than in the water control treatment, respectively (data not shown). In this late-initiated ongoing epidemic, this reduction in disease severity would not likely be effective in increasing yield. However, in a spring epidemic ended by hot dry weather (as was experienced at Spring Hill Farm in 2006, where mean disease severity was approximately 30% when the epidemic ended), this reduction in disease severity could have a significant impact. This data is only suggestive; the efficacy of Wilt Farm tea requires further investigation in replicated field trials including a water control.
• Oxidate significantly reduced disease severity by 42% at the 2^nd evaluation date but not on any other date. No other treatments significantly reduced disease severity.

  

Table 1. Weighted mean disease severity over time (AUDPC).

Treatment	Area under the disease progress curve1
Sonata	596 a2
Horsetail tea	578 a
Water control	571 a
Maria Thun compost tea	507 ab
Oxidate	500 ab
Wilt Farm tea	435 (b)3
Cuprofix	71 c
Nordox	60 c
Kocide	58 c
LSD (P=0.05)	112

¹ AUDPC was calculated for the period 9/13/07 – 9/23/07

²AUDPC values followed by different letters are significantly different (P=0.05)

³Wilt Farm tea was not applied on the same dates as the other treatments, including the water control; for this reason, Wilt Farm tea should not be directly compared to the other treatments.