NEWSLETTER ARTICLES

Small Farms and Specialty Crops

CUCUMBER MOSAIC VIRUS IN SUGAR PEAS
by Mark Gaskell

Cucumber mosaic virus (CMV) was found in samples of sugar peas taken in September from fields bordering Arroyo Grande Creek off of Lopez Drive in San Luis Obispo County. Dr. Rebecca Creamer, Plant Pathologist at UC Riverside, isolated the cucumber mosaic virus and also observed a second unidentified virus in the samples.

The fields where the plant samples were taken showed symptoms of widespread and generalized poor growth and vigor with yellow, spindly plants. Several fields on both sides of Arroyo Grande Creek showed similar symptoms, and although samples with the virus were taken from one field, presumably the other fields were also infected with the virus.

Cucumber mosaic virus is rare in sugar peas, which are not considered a primary host for this virus. CMV is one of several so-called minor mosaic viruses of peas, which include lettuce mosaic virus (LMV), watermelon mosaic virus (WMV), turnip mosaic virus (TuMV), and Plantago mosaic virus (PIMV). These diseases are more often reported in peas in the Eastern US and are rare in the mid-West and West. All are aphid-transmitted except PIMV. Genetic resistance in peas has only been identified for the PIMV.

It is likely that the occurrence of the CMV in sugar peas is related to the increase in CMV observed in bell peppers and squash along the coast in the past two years. Peppers have been affected by CMV in other areas of California, also.

There is no control of CMV in peas once they are infected. Control consists of preventing infection. Chemical control with registered insecticides has not shown efficacy in the control of CMV.

The pea plantings most likely to be affected will be those during the summer months planted in proximity to native vegetation or other crops harboring the CMV and aphids. Isolating sugar pea plantings planned for the summer months from areas with history of CMV outbreaks, may also offer some protection from the disease.

MAKING A CASE FOR "CENTRAL COAST SWEETS"
by Mark Gaskell

Initial results from field trials conducted in 1995/96 indicate that sweet onions may be a promising new crop for Central Coast growers. Sweet onions are traditionally available primarily in spring and early summer from growing areas as diverse as Vidalia, Georgia, South Texas, New Mexico, California's Imperial Valley, and Maui in Hawaii. The famed onions from Vidalia or Maui have old, established reputations in the marketplace and consistently command premium prices.

There have been many new developments over the past ten years to breed new varieties of sweeter onions and to extend sweet onion production to new growing areas. Growers in Vidalia, Georgia, are now successfully using modified atmosphere storage to extend the marketing season of their onions. These and other short day onions have notoriously short storage lives with traditional onion storage practices. Producers in Peru, Chile, Uruguay, Nicaragua, and Mexico are also producing sweet onion varieties to export to the US during the winter off-season, in advance of the first US onions from Georgia, South Texas, or the Imperial Valley in March or April.

Onions marketed as sweet onions are typically short day onion varieties which have relatively low solids and high moisture content. The Walla Walla sweet onions from Washington state are one of the rare instances of attempting to use intermediate daylength onions for sale as "sweets." The term "sweet onion" is primarily a marketing tool, however, as little relationship is consistently found between sugar content and acceptability to consumers. The primary factor which relates to consumer acceptance of sweet onions is rather "mildness" or lack of pungency or "bite" in the onions. The short day onions are milder because they have higher water content, but there is also great variability for pungency among short day varieties and even within a variety depending upon growing conditions.

Researchers have also recently developed an additional tool for documenting differences among onions for mildness. A laboratory analysis of pyruvic acid development (PAD) by the onion tissue has been statistically related to consumer taste panel acceptance as a sweet onion. The PAD measurements are gaining acceptance within the industry as a clearer index of onion mildness. Short day onions marketed as sweet onions will typically have PAD values ranging from 1 to 5 (the units are µmoles/gram fresh wgt). There are on-going discussions within the industry about the actual limits for sale as a sweet onion, and some producers or shippers would prefer to set the upper limit at 4.0. But, it is still common to see onions which are successfully marketed as sweet onions with PADs of 5 and even above 5.0. University of Georgia researchers claim that the average for the Vidalia onion is 3.9, which means that PADs above 4.0 in individual onions are not uncommon. A long day onion coming out of storage during the winter months will have a PAD in the 9-12 range and will be quite pungent.

Marketing is as important as growing in measuring success with sweet onions, and pungency is not the only factor. Some buyers actually prefer the older granex type flat onion traditionally associated with Vidalia onions or Maui Sweets. Some buyers are reluctant to take rounder, grano types of sweet onions, even with acceptably low pungency. They often sell these round onions at lower average wholesale prices.

We initiated field trials with short day varieties on the Central Coast in Fall of 1995. Short day cultivars have not traditionally been thought to be adapted at this latitude (34°N) in California because of the daylength relationship to bulbing and because of the potential for bolting during the winter period. Temperature interacts with daylength to influence onion bulbing, and it is possible that the mild coastal temperatures may permit some short day onion varieties to be grown in this area. The premium prices received for sweet onions justify an effort to determine if sweet onions may be successfully grown on the Central Coast.

We initiated the trials to see if it might be possible to combine variety, management, and growing conditions to successfully produce sweet onions. We seeded sixteen short day onion cultivars in seed beds in September and October for transplanting in November and December in field trials in Buellton, Arroyo Grande, and Paso Robles. All of the cultivars produced commercially acceptable yields at both transplant dates in Buellton. All of the onions from both transplant dates bolted in Arroyo Grande. In Paso Robles, the November transplanted plots produced well. It is possible that transplant size was a confounding factor in Arroyo Grande as average transplant size was consistently larger in Arroyo Grande than in Buellton. It is known from earlier research that bolting tends to be higher in larger transplants in overwintering plantings.

The yield data from the Buellton plots is presented in Table 1. Yields ranged from 350 to 700 50-lb cartons per acre. Although many plots produced adequate yields in Paso Robles, the field variability was too great to document statistical differences between varieties.

Table 1. Yield of commercial cultivars of "sweet" onions, transplanted at two different transplant dates. Buellton (Santa Barbara County), CA - 1995-96

CULTIVAR	YIELD (kg/plot)	Jumbo	Percent Lg. Med.	Prepack
Nov. 1 Transplant
PSX1190	12.30 a 1/	6	88	6
Mercedes	10.32 b	9	86	5
Sunup	9.52 bc	14	77	9
Granex 33	9.44 bc	0	89	11
Pegasus	8.94 cd	11	80	9
Vidalia	8.13 de	8	73	19
Primavera	8.02 de	0	85	15
Success	7.46 ef	4	72	24
Yellow Granex	7.16 ef	7	75	18
Equanex	6.25 f	6	70	24
Dec. 1 Transplant
PSX8589	10.54 a	41	49	10
Granex 429	9.89 ab	42	53	5
PSX13589	9.65 ab	9	80	11
PSX1190	9.28 abc	15	78	7
Mercedes	9.20 abc	19	76	5
Sunsweet	9.10 abc	4	86	14
Granex 33	8.66 abc	0	83	17
Sweet Dixie	8.41 bcd	1	81	18
Sweet Success	8.41 bcd	0	91	9
Sunup	8.13 bcd	0	86	14
Savannah Sweet	8.12 bcd	0	85	15
Equanex	8.10 bcd	14	66	20
Pegasus	7.87 bcd	4	82	14
Primavera	7.79 bcd	0	80	20
Sweet Vidalia	7.49 cde	5	72	23
Mr. Max	7.42 cde	0	86	14
Yellow Granex	6.92 de	0	79	21
Rio Bravo	5.87 e	31	57	12

^1/ Numbers followed by the same letter are not significantly different at the 5% confidence level.

We sent samples to the laboratory of Dr. Marita Cantwell at UC Davis for PAD analysis, sugar content, and a visual quality scoring. Dr. Cantwell then stored the onions at 32°F (0°C) for 90 days and again measured these parameters to determine changes during storage.

Table 2. Pungency and sugar concentration of potential "sweet" onion cultivars. Transplanted early December 1995, harvested May/June 1996.

Cultivar	Pungency (µmole pyruvate/g FM)		Sugar Concentration (mg/g FM) 1/
	Buellton	Paso Robles	Buellton	Paso Robles
PSX9589	1.73	1.95	51.3	-
Granex 429	3.87	4.56	45.3	-
PSX13589	3.09	2.19	65.6	-
PSX1190	3.20	4.22	60.7	57.1
Mercedes	3.99	2.19	73.0	68.7
Sunsweet	2.71	1.95	49.8	58.7
Granex 33	3.82	4.31	55.2	69.0
Sweet Dixie	4.97	5.61	42.8	69.3
Sweet Success	3.39	-	71.5	-
Sunup	4.19	4.06	44.9	77.0
Savannah Sweet	2.32	4.04	52.4	67.7
Equanex	2.62	-	41.8	-
Pegasus	2.11	2.79	51.9	79.0
Primavera	3.26	3.15	41.0	69.7
Sweet Vidalia	3.04	-	54.5	-
Mr. Max	3.91	4.56	72.6	69.5
Yellow Granex	3.64	-	47.9	-
Rio Bravo	4.16	5.18	57.2	67.7
LSD 5%	1.55	0.75	14.5	12.0

^1/ milligrams of sugar per gram of fresh weight

All of the cultivars, except three, had lower pungency, and all cultivars had lower percent sugars following storage. Visual ratings of a number of cultivars indicated acceptable appearance following storage. For copies of detailed data on these trials, please contact our office.

There appears to be promise in growing sweet onions on the Central Coast, but additional trials are needed. We need to determine the optimum combination of transplant data and transplant size for short day cultivars. We also need to determine optimum management to increase the percentage of Jumbo sizes. Trials are continuing in a more limited geographical area this season to continue to evaluate the potential for sweet onion production on the Central Coast. The same group of onion cultivars has also been planted in the Imperial Valley as a comparison.