NEWSLETTER ARTICLES

Vegetables and Plant Pathology

DOWNY MILDEW OF SPINACH

Many growers experienced difficulty keeping downy mildew disease of spinach under control during the fall and winter of 1996/97. Spinach cultivars that had been resistant to attack were found infected by Peronospora farinosa f. sp. spinaciae, the cause of spinach downy mildew. Also spray programs that had previously kept downy mildew in check had to increase several-fold to achieve the same control. These events initiated a research project by Steve Koike, Farm Advisor, Monterey County, and research personnel at the University of Arkansas, to determine what had changed in the host (spinach)/pathogen (P. farinosa f. sp. spinaciae) interaction. A research paper, presenting some initial results, was recently presented at the annual meeting of the American Phytopathological Society. The following information was taken, in part, from this presentation.

Table 1. Differential spinach genotypes used to characterize races of the downy mildew pathogen.

Table 2. Disease reactions on commercial spinach lines to isolates CA-1 (race 5) and SP-1 (race 6) of Peronospora farinosa f. sp. spinaciae.

^a Disease reactions were based on the percentage of cotyledons infected. "+" = < 10% infected.
* Spicer and Spinnaker received a metalaxyl seed treatment, and consequently the reactions are likely not a reflection of resistance.

The results of research presented in Tables 1 and 2 indicate that two new races of spinach downy mildew are now present in California. These have been designated race 5 and race 6. Some existing spinach cultivars have resistance to race 5, but no commercially available cultivars were shown to have resistance to race 6.

The origins of race 5 and 6 are unknown. A new race of P. farinosa f. sp. spinaciae found in Europe appears to be similar to race 6 (USA). However, tests to compare the identity of these races have not been conducted.

The appearance of two new races of this spinach pathogen in California does not bode well for the spinach industry. If we look at the appearance of new races of the pathogen (Table 3), it is readily apparent that new races are emerging with alarming frequency.

Table 3. History of the occurrence of races of P. farinosa f. sp. spinaciae.

^a Year observed or reported.

It took 134 years for a second race to emerge from the time race 1 was identified in 1824. Race 3 emerged 18 years later. Race 4 appeared in California in 1991, 12 years after race 3. Now race 5 and 6 have appeared in a short 6- to 7-year span, indicating that the pathogen is very quickly evolving mechanisms to overcome the host resistance developed by plant breeders. Breeding to develop resistance to races 5 and 6 is underway. Contact your seed supplier for information on the availability of resistant cultivars, if any. In the meantime, the judicious use of metalaxyl and fosetyl-A1 will have to be the mainstay of downy mildew suppression in spinach.

SANTA BARBARA COUNTY WELL WATER REMAINS HIGH QUALITY
Franklin Laemmlen

The Department of Pesticide Regulation (DPR) has recently released its 1997 report on ground water testing for pesticides. Statewide 2,508 wells were sampled. In Santa Barbara County, water from 44 wells was tested. The water was analyzed for 56 different pesticides. All 44 wells showed no detectable levels of pesticides. Farmers and ranchers of Santa Barbara County are to be congratulated for their continued wise use of agricultural chemicals of all types, and their careful stewardship of this important and valuable resource. If you would like a complete report of all the results from the statewide testing program, call 805/934-6240 or send an e-mail with your request.

SOIL COMPACTION - PAY ATTENTION TO HOW YOU WORK YOUR LAND
Franklin Laemmlen

Compaction occurs when pores, or small open cavities, are "squeezed" out of the soil. Soil pores may be reduced in size or eliminated altogether, usually due to pressure applied to the soil surface by equipment, foot traffic, or hard rain on bare soil. Wet soils, especially those with moisture levels below field saturation and above optimum tillage moisture, have the greatest potential to incur compaction.

Without enough soil pore space, plant roots find it difficult or impossible to grow and penetrate through soil. Air, water and fertilizer cannot infiltrate through compacted soils, thus robbing the roots of essential nutrients, water and oxygen. Plants growing in compacted soils are more susceptible to nutrient deficiencies, drought stress disease, and drowning because water cannot move through the soil to the roots. Soil drainage is also greatly impaired.

The best time to test soil for compaction is when soil moisture is high. Soil moisture content can greatly affect soil compaction readings. As soils dry out, resistance to penetration increases, and misleading readings may result. Soil moisture should be at or somewhat above field capacity to get the most true soil compaction readings from test equipment.

Soil compaction tools range from manual soil compaction rods to digital recording compaction meters. With a manual compaction rod, you can tell when you hit a hard compacted layer. Dial compaction probes tell you how much pressure (in pounds per square inch) it takes to penetrate the soil via a needle moving across a scale. Digital compaction meters are similar to the dial probes, but they give you a digital reading and have the ability to record the depth and pressure for each test. No matter which tool you choose, make sure that the probe has depth markings, so if you hit a compacted layer, you can record its depth for later corrective actions.

Soil compaction occurs below the surface, so it's hard to spot. The best way to test is to probe the soil with a compaction tester to a depth of 36" to 48". The testing rod should move down through the soil with steady, even pressure. Hard, compacted soils resist penetration with the rod. Often penetration abruptly stops at a fairly uniform depth across a field. This is referred to as "plowpan." Digging up plants and examining root growth in various locations in the field is another way to tell if compaction is a problem. If you find evidence of compaction, dig to the depth indicated and check for abnormal root growth and abnormal soil structure.

Soil compaction can be avoided or reduced by never working wet soil. Also vary the depth of tillage equipment. Use floatation tires, dual tires or track equipment. Always inflate tires to the lowest safe psi. Restrict the number of drive rows through a field as much as possible and reduce tillage to the minimum necessary to keep the field in production.

Cultivation practices that help correct compact soil include cultivating, subsoiling or ripping to the proper depth to break up the plowpan. Also apply and incorporate mulch, compost, and/or green manure. Reduce tillage operations to the minimum necessary for residue incorporation, seedbed preparation and pest control (weeds, insects, and diseases).

WHAT DO YOU DO WHEN FEDERAL OR STATE INSPECTORS VISIT YOUR FARM - Franklin Laemmlen

• Be open and friendly.
• Be professional. Ask to see their credentials, what department they are from, if they have ever worked in this industry before, what their education (training) is.
• Ask their reason for visiting you (inspection, observation or complaint?).
• Let them know you are in charge, you know what is going on, that this is your place.
• Ask what is the scope of their inspection.
• Ask them to show you chapter and verse of anything they require you to do.
• Ask for a copy of the report they produce.
• Get the name and number of their supervisor.
• They are there to help you - communicate this to them and make sure they do.
• Make a thorough memo of the visit, the date, what they looked at, and for how long.