Phacelia, Lana Woollypod Vetch, And Austrian Winter Pea: Three New Cover Crop Hosts Of Sclerotinia Minor In California

Steven T. Koike¹, Richard F. Smith, Louise E. Jackson, Lisa J. Wyland, John I. Inman, and William E. Chaney ¹UCCE Monterey Cnty, 1432 Abbott St, Salinas CA 93901

Because cover crops contribute soil-plant nutrition and possible pest management benefits to cropping systems, farmers in the Salinas Valley have recently shown renewed interest in alternative cover crops for both conventional and organic vegetable production and have begun considering newly utilized cover crop species such as phacelia (Phacelia tanacetifolia) and oilseed radish (Raphanus sativus).

However, the impact of phacelia, oil seed radish, and other cover crops on populations of Sclerotinia minor, the causal agent of lettuce drop, is not known. Because of the extensive lettuce industry in the Salinas Valley, information was needed on the interaction of new cover crop introductions and S. minor. The purpose of this study was to test cover crop species for susceptibility to S. minor and to assess the effect of cover crop plantings on lettuce drop incidence in field situations.

Procedures

Our two-year field study and greenhouse inoculation experiments identified three cover crops as new hosts of S. minor. Pathogenicity was established by planting 4-wk-old transplants of six cover crops and lettuce into sand amended with sclerotia (35 sclerotia/100-cm³ sand). After 4 wk incubation in a greenhouse, phacelia, Lana woollypod vetch (Vicia dasycarpa), and Austrian winter pea (Pisum sativum L. ssp. arvense) became infected in addition to lettuce. S. minor was reisolated from the diseased cover crop plants. To assess susceptibility in a field situation, seven cover crop species, lettuce, and fallow control treatments were planted for two consecutive years into randomized, replicated field plots infested with sclerotia.

Results

In both 1993 and 1994 experiments (see Table 1), phacelia, Lana woollypod vetch, purple vetch (Vicia benghalensis), Austrian winter pea, and lettuce became infected and the pathogen was isolated from field samples. Numbers of sclerotia in soil samples from cover crop plots were not

significantly higher than those from fallow plots. When lettuce was planted after cover crop incorporation, phacelia, Lana woollypod vetch, and Austrian winter pea plots had significantly higher lettuce drop incidence than fallow plots in the first year. In the second year, only phacelia plots had significantly more lettuce drop. This is the first report of S. minor as a pathogen of phacelia, Lana woollypod vetch, and Austrian winter pea cover crops in California.

Conversely, greenhouse and field inoculations failed to result in S. minor infections of oilseed radish, barley, and fava bean cover crops. For both 1993 and 1994 field experiments, lettuce drop incidence in these three cover crop treatments was not significantly different than that for fallow treatment plots.

Funding for this research was provided by the Sustainable Agriculture Research and Education Program, University of California, Davis, and by the USDA-EPA A.C.E. Project 91-COOP-1-6590. We thank H. Agamalian, S. Dacuyan, T. G. Gonzales, E. D. Oakes, J. Taylor, M. Vidauri, and Hartnell College.

Table 1. Disease incidence caused by Sclerotinia minor, sclerotia per 100 gram soil, and lettuce drop incidence for field trials in 1993-1994
	Percent disease x		Mean sclerotia/ 100 g soil y		Percent lettuce drop disease z
Cover crop	1993	1994	1993	1994	1993	1994
Phacelia	13.9	21.4	3.4	7.2	20.6	39.4
Oil seed radish	0.0	0.0	0.0	2.8	14.8	24.3
Barley	0.0	0.0	0.6	1.4	11.2	18.0
Lana woollypod vetch	18.4	27.4	5.0	8.0	22.6	31.9
Purple vetch	17.2	19.0	2.6	7.2	17.2	26.5
Fava bean	0.6	0.0	6.2	2.8	8.4	18.6
Austrian winter pea	30.3	36.5	3.2	4.6	32.4	27.7
Romaine lettuce	96.9	82.0	29.0	6.6	23.4	25.4
Fallow control	0.0	0.0	1.8	3.0	11.4	18.9
L.S.D. (P = 0.05)	5.5	8.0	11.9	N.S.	9.2	13.1

x Ratio of the number of plants infected with S. minor to the total number of plants evaluated, expressed as percentages. In each replication, plants were evaluated in four 1-m² sections and the values averaged.

y Mean number of sclerotia per 100 g soil sample. Samples were collected just prior to the planting of the lettuce crop. Eight soil cores were taken per plot and bulked into a composite sample. Samples were processed and assayed for S. minor sclerotia.

z Ratio of the number of lettuce plants infected with S. minor to the total number of plants evaluated, expressed as percentages. All lettuce plants in the 5-m X 2-m bed plots were evaluated and the values averaged.

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