Psyllids are plant sap-feeding insects that harbor prokaryotic endosymbionts in specialized cells within the body cavity. Four-kilobase DNA fragments containing 16S and 23S ribosomal DNA (rDNA) were amplified from the primary (P) endosymbiont of 32 species of psyllids representing three psyllid families and eight subfamilies. In addition, 0.54-kb fragments of the psyllid nuclear gene wingless were also amplified from 26 species. Phylogenetic trees derived from 16S-23S rDNA and from the host wingless gene are very similar, and tests of compatibility of the data sets show no significant conflict between host and endosymbiont phylogenies. This result is consistent with a single infection of a shared psyllid ancestor and subsequent cospeciation of the host and the endosymbiont. In addition, the phylogenies based on DNA sequences generally agreed with psyllid taxonomy based on morphology. The 3 end of the 16S rDNA of the P endosymbionts differs from that of other members of the domain Bacteria in the lack of a sequence complementary to the mRNA ribosome binding site. The rate of sequence change in the 16S-23S rDNA of the psyllid P endosymbiont was considerably higher than that of other bacteria, including other fast-evolving insect endosymbionts. The lineage consisting of the P endosymbionts of psyllids was given the designation Candidatus Carsonella (gen. nov.) with a single species, Candidatus Carsonella ruddii (sp. nov.).Insects within the families Aphididae (aphids), Psyllidae (psyllids), Aleyrodidae (whiteflies), and Pseudococcidae (mealybugs) feed predominantly or exclusively on plant phloem sap. These insects have a number of common structural properties (8) and constitute separate lineages within the suborder Sternorrhyncha (15, 54). The utilization of plant sap necessitates the penetration of tissue by flexible mouth parts (stylets) and the ingestion of the fluid. This mode of feeding is conducive to the transmission of viruses and other infectious agents, and members of these families are vectors of pathogens of agriculturally important plants (7,28,51). In addition, these insects may reach enormous populations, causing plant nutrient deprivation, leaf curling, and gall formation (8). Plant phloem sap, the diet of these insects, is rich in carbohydrates but deficient in nitrogenous compounds (21,46). Due to this deficiency, a large amount of plant sap is consumed and is excreted as honeydew. This sticky material may cover the plant and serve as a substrate for fungal growth (8).A common feature of organisms that live on diets containing an excess of one class of compounds and a deficiency in essential nutrients is the presence of prokaryotic intracellular symbionts (endosymbionts) that may provide the missing essential nutrients (6, 21, 37). Early histological studies indicated that these endosymbionts are housed within specialized cells (bacteriocytes) that form an aggregate (bacteriome) found within the body cavity (5, 10, 37). Typically, insects of a particular family or superfamily were found to have a ...

Winter cover crops are increasingly common on organic and conventional vegetable farms on the central coast of California between periods of intensive vegetable production. A 2-yr study was conducted in Salinas, California, to quantify (1) cover crop and weed biomass production during cover cropping, (2) early-season canopy development of cover crops, (3) weed seed production by burning nettle during cover cropping, and (4) weed emergence following cover crop incorporation. The cover crops included oats, a mustard mix, and a legume/oats mix that were planted in October and soil-incorporated in February. Weed and cover crop densities, early-season cover crop canopy development, above-ground weed and cover crop biomass production, seed production by the burning nettle, and postincorporation weed emergence was evaluated. Mustard produced more early-season biomass than oats and the legume/oats mix. There were no differences in above ground biomass production by the cover crops at the end of their growth period. Suppression of weed biomass and seed production of burning nettle was greatest in mustard, and least in oats and the legume/oats mix. The weed suppressive ability of each cover crop was affected by early-season canopy development and was highly correlated with cover crop plant density. Weed emergence following cover crop incorporation was in order of legume/oats mix > oats > mustard in yr 1, but was not different in yr 2. This study provides initial information on cover crop effects on weed management in irrigated and tilled vegetable production systems in the central coast of California. The results suggest that the legume/oats mix could exacerbate weed problems in subsequent vegetable crops.

Previous studies have established that psyllids (Hemiptera, Psylloidea) contain primary endosymbionts, designated as Carsonella ruddii, which cospeciate with the psyllid host. This association appears to be the consequence of a single infection of a psyllid ancestor with a bacterium. Some psyllids may have additional secondary (S-) endosymbionts. We have cloned and sequenced the 16S-23S ribosomal RNA genes of seven representative psyllid S-endosymbionts. Comparison of the S-endosymbiont phylogenetic trees with those of C. ruddii indicates a lack of congruence, a finding consistent with multiple infections of psyllids with different precursors of the S-endosymbionts and/or possible horizontal transmission. Additional comparisons indicate that the S-endosymbionts are related to members of the Enterobacteriaceae as well as to several other endosymbionts and insect-associated bacteria.

Long-term research on cover crops (CC) is needed to design optimal rotations. Winter CC shoot dry matter (DM) of rye (Secale cereale L.), legume-rye, and mustard was determined in December to February or March during the fi rst 8 yr of the Salinas Organic Cropping Systems trial focused on high-value crops in Salinas, CA. By seed weight, legume-rye included 10% rye, 35% faba (Vicia faba L.), 25% pea (Pisum sativum L.), and 15% each of common vetch (V. sativa L.) and purple vetch (V. benghalensis L.); mustard included 61% Sinapis alba L. and 39% Brassica juncea Czern. Cover crops were fall-planted at 1x and 3x seeding rates (SR); 1x SR were 90 (rye), 11 (mustard), and 140 (legume-rye) kg ha -1 . Vegetables followed CC annually. Cover crop densities ranged from 131 to 854 plants m -2 and varied by CC, SR, and year. Year, CC, and SR aff ected DM production, however, the eff ects varied across the season and interactions occurred. Averaged across years, fi nal DM was greater in rye and legume-rye (7 Mg ha -1 ) than mustard (5.6 Mg ha -1 ), and increased with SR through January. Dry matter production through the season was correlated signifi cantly with growing degree days (GDD). Legumes contributed 27% of fi nal legume-rye DM. Season-end legume DM was negatively correlated with GDD at 30 d, and legume DM in the 3x SR increased during years with frequent late-season rainfall. Seed costs per Mg of fi nal CC DM at 1x SR were approximately three times higher for legume-rye than rye and mustard.

A gronomy J our n al • Volume 101, I s sue 1 • 2 0 0 9 47