Several different strawberry pathogens can be transmitted from the nursery to fruiting fields in infected transplants. These include viruses, phytoplasmas, foliar and root-knot nematodes, and various fungi that cause plant diseases. In addition, weed seeds and vegetative reproductive structures of perennial weeds can be spread on contaminated transplants.
Use certified transplants to prevent virus diseases and foliar nematode in fruit production fields. Certified transplants also play an important role in reducing the spread of other strawberry pathogens into fruit production fields and help reduce spread of weed problems because certified fields generally are kept weed free.
One source of virus-tested plants for propagation by nurseries is the Strawberry Program of the University of California's Foundation Plant Services (FPS). At FPS, plants to be propagated by nurseries are tested to ensure that they are free of the virus diseases considered important in California strawberry production. Buyers of these virus-indexed plants from FPS must be licensed with UC Davis InnovationAccess as a propagator of university-patented strawberry material. Royalties are collected by UC upon the sale of nursery stock. More information can be obtained by contacting the Strawberry Licensing Program.
Mother plants are tested for these pathogens by the Strawberry Program of University of California's Foundation Plant Services:
|Arabis mosaic virus||HI|
|Raspberry ringspot virus||HI|
|Strawberry crinkle virus||SI, PCR|
|Strawberry feather leaf virus||SI|
|Strawberry latent C virus||SI|
|Strawberry latent ringspot virus||HI, PCR|
|Strawberry leaf roll disease||SI|
|Strawberry mild yellow edge virus||SI, PCR|
|Strawberry mottle virus||SI, PCR|
|Strawberry pallidosis associated virus||SI, PCR|
|Strawberry vein banding virus||HI, PCR|
|Tobacco necrosis virus||HI|
|Tobacco ringspot virus||HI|
|Tobacco streak virus= strawberry necrotic shock virus||SI, PCR|
|Tomato black ring virus||HI|
|Tomato bushy stunt virus||HI|
|Tomato ringspot virus||HI|
|Xanthomonas fragariae (angular leafspot)||PCR|
|Phytophthora fragariae var. fragariae (Red steel root rot)||VI|
|Aphelenchoides besseyi (Strawberry crimp nematode)||VI|
|1||SI = strawberry indicator plants; PCR = polymerase chain reaction; HI = herbaceous indicator plants; VI = visual inspection|
Once plants reach the nursery stage, the California Department of Food and Agriculture administers a Strawberry Certification Program of virus testing and nursery inspections designed to ensure that certified transplants are as free as possible from potentially harmful pathogens and weeds. All strawberry plants that receive certification tags must meet the requirements of this program.
The first nursery generation of certified strawberry plants is produced by allowing virus-tested meristem plants to multiply in screenhouses, where they are protected from virus vectors. The certification program tests one daughter plant from each meristem plant for the viruses mentioned above.
Next, three or more generations may be produced in the field. The packaged plants of each generation that passes certification inspection receive a tag of a different color.
- The first generation is called Foundation and receives a white tag,
- The second generation is called Registered and receives a purple tag, and
- The third is called Certified and receives a blue tag.
- Subsequent generations are noncertified and receive no tag.
Soil fumigation is supervised by the county agricultural commissioner and fields are inspected at planting, during the season, at harvest, and during trimming and packing operations for signs of pests. Inspectors look for diseases, insects, weeds, nematodes, correct number of plants per package, and off-type plants. Plants representing at least 1% of all plants in each foundation block clone are tested for viruses using the leaf grafting technique.
Certified transplants or plants of an equivalent generation usually are used for fruit production, but the more expensive Registered plants may also be used if an additional level of cleanliness is desired.
To achieve optimum plant vigor, it is essential to follow recommended procedures for handling and planting the cultivars. Good plant vigor increases yields and reduces the impact of pests.
The best time to plant depends on cultivar and location. Yield, quality, and earliness of production are affected by planting date. The correct planting date helps ensure vigorous growth, improves yields, and reduces pest problems. Planting too early reduces vigor and yield, may increase mite problems, and may result in a higher incidence of misshapen, small fruit. Planting too late causes excessive growth of foliage and runners, delays fruit production, and decreases yield.
- Summer planting is used in the Central Valley.
- Planting in spring and summer (both usually called "summer" plantings) is used for fall-harvest, day-neutral cultivars in the Santa Maria Valley and Southern California areas.
- Fall planting (sometimes called "winter" planting) is used in all areas except the Central Valley and is the most widely used planting timing in the state. (Successful fall planting requires mild autumn weather that allows crown production during the shortest days of the year.)
Keep transplants protected during planting operations to prevent drying, and place them into beds with soil moisture near field capacity. In fields fumigated before beds are formed, transplants usually are placed into holes punched into the tops of formed beds by machine either before or after polyethylene mulch has been applied.
Proper placement of transplants is critical. For this reason, all transplants are planted by hand. Make sure that the crown is properly exposed after the soil is closed around the transplants. Plants die if placed too deep and grow poorly if placed too shallow. Roots should be vertical in the planting slot and not be allowed to form a 'J'. If excessive root length interferes with proper planting, roots may be pruned but should never be less than 4 inches in length. Place transplants no farther than 6 to 7 inches from a drip line.
Recommended planting densities vary with cultivar, nursery source and planting date, nitrogen fertilizer management, soil type, field location, and bed width. In all plantings, plants are staggered, placing them as far apart from each other as practical on the bed. This practice minimizes plant competition and improves spray coverage. Planting too closely reduces fruit size, makes picking inefficient, and increases problems with diseases, especially gray mold, and uneven coloring. Planting too far apart reduces yields unnecessarily.
- In 4-row beds of the Santa Maria Valley and Southern California, planting densities vary from 22,000 to 28,000 plants per acre, with the most common plant spacings using about 24,000 plants per acre.
- Planting densities for 2-row beds in the Watsonville and Salinas area vary between 16,000 and 20,000 plants per acre.
- Higher plant densities are designed to give higher yields early, while lower plant densities are designed for a longer season where plant crowding is minimized later in the growing season.
Sprinkler irrigation immediately after planting is the best way to establish transplants. Newly developing roots need good soil-root contact with adequate moisture and are sensitive to dryness and salinity. Careful placement of fertilizer and frequent irrigation is needed to prevent reductions in growth and yield. Frequent irrigation is critical during the first 4 weeks after planting. Irrigate often enough to keep soil in the beds near field capacity (tensiometer readings of 5 to 10 centibars) but avoid standing water. Excessive irrigation encourages diseases. If drip irrigation is used to get plants established, place the drip lines and mulch, then irrigate long enough to bring beds to field capacity before planting. Winter irrigation may be needed if the soil dries out.
Occasionally, strawberry plants form runners in fall after planting. Runners must be removed to encourage the formation of large, high-yielding plants. Remove runners as soon as enough are present to justify sending crews through the field. Runner removal may be combined with hand-weeding operations. Cultivars differ widely in their production of runners and this should be considered for cultivar selection alongside chilling requirements.