Day-neutral cultivars of the cultivated strawberry are very important in California's commercial strawberry industry. Since the introduction of the first successful day-neutral cultivar Selva in 1983, the cultivated area of day-neutral cultivars has grown rapidly in both the Watsonville/Salinas district and the Santa Maria district along the coast of California, replacing and expanding upon the summer-planted production acreage of short-day cultivars. Plants of these day-neutral cultivars are produced in high-elevation nurseries in northern California and southern Oregon and are transplanted as fresh-dug plants into the fields in late autumn. In the Watsonville/Salinas district, the state's largest commercial production area, the day-neutral cultivars Albion and Diamante now account for nearly 50% of the surface area or ≈2500 ha (CSC, 2006). In addition, production in the Santa Maria district from fresh-dug plants of these day-neutral cultivars is ≈500 ha (CSC, 2006). The major reason for the increase in production area of day-neutral cultivars is the extension of the flower-initiation period of day-neutral genotypes relative to short-day genotypes. This extension of flower initiation results in greater season production (Bringhurst et al., 1989), reduces peak volumes as a percentage of total fresh shipments, and extends the fresh market season later in the year, each of which contributes to higher overall returns to the producers (Sjulin, 2003).
A more recent application of day-neutral cultivars, using both public and proprietary cultivars, is midsummer planting of cold-stored plants along the southern California coast in both the Santa Maria and Oxnard districts. These plants are primarily produced in low-elevation nurseries located in the Central Valley of California. The production in this application is relatively low yielding but is timed to higher-average pricing periods from mid-September through mid-December. The average returns to the producers have been high, and they have been re-invested into rapid growth of the cultivated area since the system's introduction in the early 1990s. The area cultivated in this system now exceeds 2000 ha (CSC, 2006).
Significant amounts of strawberry day-neutral plants are supplied to producers from various nurseries. About 150 million fresh-dug plants of day-neutral cultivars are needed annually to supply the 3000 total ha in the Watsonville/Salinas and Santa Maria districts, assuming an average plant density of 50,000 plants/ha. An additional 130 million plants of cold-stored plants of day-neutral cultivars are needed annually for the 2000 ha planted at a higher average density (65,000 plants/ha) in the summer along the southern California coast. Some California strawberry nurseries also produce additional plants for foundation nursery stock and provide cold-stored plants to nurseries and producers in other areas of North America and abroad.
Anthracnose crown and fruit rot, incited by Colletotrichum species, is a serious disease of strawberries and many other crops worldwide (Peres et al., 2005). Strawberry nurseries in the western United States that supply the California fruit production industry are particularly concerned with C. acutatum J.H. Simmonds, which can be introduced into fruiting fields by infected planting stock (Eastburn and Gubler, 1990). Plants sold either domestically or abroad should be free of detectable levels of infection by pathogens such as C. acutatum, and new sensitive diagnostic techniques (Parikka and Lemmetty, 2004) will require even higher levels of disease control in the nursery fields.
The defining characteristic of a day-neutral cultivar is the ability to produce flowers following lengthy cold storage on the mother plant and daughter plants, even under the relatively warm conditions of California's Central Valley (Bringhurst et al., 1989). This flowering response is the principle reason why Colletotrichum-susceptible day-neutral cultivars are much more problematic in the nursery than comparable susceptible short-day cultivars. The presence of readily infected flower and fruit tissue, combined with the warm, wet conditions typical of strawberry nursery production, create an ideal situation for a disastrous infection by Colletotrichum fungi. Measures will be discussed that reduce the likelihood that such an infection will occur. These measures are elimination of potential sources of inoculum of the pathogen, cultural practices that reduce the susceptibility of the host plant to infection, and chemical and biological control agents.
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