More on Downy Mildew on Pickling Cucumbers – Ed Kee, Extension Vegetable Crops Specialist; kee@udel.edu

 

Downy mildew continues to be a major challenge to pickling cucumbers, as well as other cucurbit crops.  At this point at least 95% of our fields are demonstrating some level of infection.  While we and at least one crop consultant have seen fungicide applications of Ridomil Gold Bravo made during the fruit stage at least “hold the vines,” the frustration and anxiety level is justifiably high among growers and processors.   Please refer to last week’s Update for specific recommendations, but here are some additional observations and possible strategies:

 

1. We have been asked about resistance of downy mildew to Ridomil (mefenoxam) formulations, especially about the possibility of resistant strains of downy mildew “blowing up” from North Carolina.  Resistant strains of downy mildew have not been detected in either region, Delmarva or North Carolina.  However,there is a significant risk of resistance development.  Ridomil formulations should be used in a rotation with other products that have a different mode of action (Tanos, Cabrio or Gavel).

 

2. Ridomil Gold 4/E is labeled at 1-2 pts/acre and Ultra Flourish 2/E at 2-4 pts/acre for application in a 7-inch band to control Pythium.  This application also will likely provide early season control of downy mildew.  The Vegetable Production Recommendation Guide has the information for converting this to a per/acre basis.  See the other article on downy mildew for additional information.

 

3. Are there variety differences in the reaction to Downy mildew?  Yes.  There are differences in the relative resistance of varieties.  Seed company ratings indicate Palomino and Sassy have intermediate resistance, while Vlaspic has more resistance that Palomino.  Another factor in all of this is the resistance level of the pollinator varieties.  Female hybrid varieties that have resistance may be mixed with older pollinator varieties that have little or no resistance.  This situation can be negative in two ways.  First, the non-resistant pollinators encourage the development of the disease.  Secondly, the failure of the pollinators to produce blossoms as a result of the disease would increase the amount of non-payable product, i.e., crooks & nubs.  Finally, while there is a lot of anecdotal evidence circulating about varieties, there is the possibility that as the disease hit our region, much of our acreage happened to be in one variety or another, thus skewing what we’re looking at.  Having said that, however, there are differences in resistance level among varieties, as noted above.

 

As was stated last week, there are several materials listed in the Commercial Vegetable Production Guide.  The example programs that include Bravo or Ridomil were recommended based on performance data from other states, our experience, and the goal of addressing both Downy mildew and Phytophthora fruit rot.  With the situation as it is, trying other materials alone, or in combination is justified.

 

Growers are asking about performance of various fungicides.  It is important to remember, that up to now, all fields were treated with fungicides after infection has occurred.  The real evaluation of the performance and effectiveness will begin when our region begins harvest of fields that have had fungicide applications before major infection occurred.   We should be in that mode in seven to ten days.

 

We are evaluating all of this and will keep the industry posted.

 

Vegetable Crop Diseases - Bob Mulrooney, Extension Plant Pathologist, bobmul@udel.edu

 

Late Blight Advisory.

 

Disease Severity Value (DSV) Accumulation as of July 21, 2004 is as follows:

Location: Joe Jackewicz Farm, Magnolia, DE. Greenrow: April 25, 2004

Date	Daily DSV	Total DSV	Spray Recommendation
4/25- 5/18	4	18	7-day
5/19	4	22	7-day
5/20	2	24	7-day
5/21	2	26	7-day
5/25	5	31	7-day
5/27	3	34	7-day
5/30	8	42	7-day
5/31	1	43	7-day
6/1	1	44	7-day
6/4	17	61	5-day
6/7	2	63	5-day
6/8	1	64	5-day
6/10-6/13	9	73	7-day
6/14-6/15	3	76	7-day
6/16	3	79	7-day
6/17	3	82	7-day
6/22	1	83	7-day
6/23	1	84	10-day
6/25	3	87	7-day
6/26-30	0	87	10-day
7/1-7/11	7	94	10-day
7/12	5	9199	7-day
7/13	2	101	7-day
7/14	2	103	7-day
7/17	10	113	5-day
7/19	1