- Agronomic: barley, corn, flax, millet, oats, potatoes, rapeseed, rye, sunflower, wheat, grass (misc. perennial), hay
- Fruits: berries (other), berries (strawberries)
- Vegetables: asparagus, beans, beets, broccoli, carrots, cauliflower, garlic, onions, peas (culinary), peppers, sweet corn, tomatoes
- Additional Plants: herbs, native plants, ornamentals
- Animal Production: feed/forage
- Crop Production: cover crops, fallow
- Farm Business Management: whole farm planning
- Pest Management: compost extracts, cultural control, integrated pest management, prevention, trap crops
- Sustainable Communities: sustainability measures
Our results show the fungus can colonize nearly all plants tested thus far. This includes a number of the common strawberry nursery cover crops such as triticale, Sudangrass, Austrian winter pea and bell bean. We can isolate the fungus from both living and dead tissue of most of the species inoculated. We are also able to induce sporulation from surface sterilized plant debris. We have not confirmed sporulation from tissue on living plants.
Extracts from various plants did not inhibit the fungus. However, essential oils from several plants did show inhibitory properties.
Strawberry fruit production in California is dependent upon the annual production of clean transplants from certified nurseries in Northern California and Southern Oregon. Nurseries located in Oregon’s Klamath Co., and California’s Lassen, Siskiyou, Modoc, Shasta, Tehama, Glenn, San Joaquin, and Merced Cos. provide roughly 100% of the nursery stock required for California’s annual strawberry production system, and a significant portion of the stock used as the foundation for strawberries in other states and countries.
Strawberry nurseries rely heavily on chemical fumigants and fungicides for controlling pathogens and other pests. If the strawberry industry is to reduce its heavy dependence on these chemical control practices, effective alternatives for managing diseases and pests must be developed.
One method by which growers have attempted to avoid disease/pest problems is to adopt the practice of a multi-year crop rotation period between strawberry plantings. This can serve to break the disease cycle of various pathogens and pests by providing host-free periods. During these rotation years, nurserymen generally plant a cereal and/or legume crop.
Colletotrichum acutatum is representative of a large group of opportunistic fungal pathogens that affects a wide range of crops (Wharton and Diéguez-Uribeondo, 2004) such as strawberries (Smith and Black, 1990; Howard et al, 1992; Curry et al, 2002), blueberries (Smith et al 1996; Yoshida and Tsukiboshi, 2002), almond (Adaskaveg and Hartin, 1997; Forster and Adaskaveg, 1999), avocado (Freeman, 2000), peach (Adaskaveg and Hartin, 1997; Zaitlin et al, 2000), citrus (Zulfiqar et al, 1996; Timmer and Brown, 2000), mango (Arauz, 2000), and olive (Martin and Garcia-Figueres, 1999). Anthracnose is one of the most destructive diseases of strawberry in plant production nurseries and in fruit production fields. C. acutatum can infect all parts of strawberry plant causing crown collapse, leaf and petiole lesions, flower blight, and fruit rot. If infected plants are brought from nurseries to fruit production fields, crown rot generally will occur causing complete plant collapse. However, plant collapse seems to also require predisposing stress such as exposure to heat after digging the plants. Thus, plant collapse is generally not a problem in most years. Under conditions of rainfall, severe fruit rot occurs from inoculum brought into the field by transplants. Growers are dependent on fungicides for control of fruit rot. Crown rot has been reduced by simply washing soil away from the plants before planting or using fungicide dips of strobilurins (Daugovish and Gubler, 2006). However, complete control is dependent on clean plant production.
Colletotrichum spp. generally enter susceptible tissue of a primary host directly through the host cuticle by a penetration peg that emerges from a melanized appressorium. Appressorial development has been shown to be affected by numerous factors including temperature, light intensity, nutrient stress, and host surface characteristics (Emmett and Parbery 1975, Parbery 1981). Studies have shown that appressorial development is generally favored by cooler temperatures and severely impaired at temperatures of 35°C and above (Leandro et al., 2003a).
Experiments have shown that when C. acutatum is inoculated onto non-hosts such as pepper, tomato, garden bean, and eggplant, melanized appressoria are formed but do not appear to function (Horowitz et al., 2002). Some colonization of the tissue does occur but not via direct penetration from the appressoria. The fungus remains in a quiescent state in the epidermal cell layers without causing damage to the tissue. Its growth is restricted to the upper cellular layer beneath the cuticle where it is able to reach a balance with the plant, obtaining enough nutrients from the apoplast to survive but not taking so much that it harms the plant. The result is that the fungus is able to survive asymptomatically for extended periods (Horowitz et al., 2002). Whether or not these latent infections can be triggered to become active infections, and whether or not such infections will result in sporulation remains to be determined.
The fungus can survive in soil for at least nine months (Eastburn and Gubler, 1990). Though soil fumigation prior to planting the next strawberry nursery crop greatly reduced inoculum density, the fact that the pathogen is there during the fall-spring period indicates that various plants might become infected during this time. Moisture has also been shown to play a role in survival of the pathogen in soil (Eastburn and Gubler, 1992; Feil et al, 2003). Survival of C. acutatum has been found to be highest under cool, dry soil conditions and decreases with increases in both temperature and moisture.
At one point it was thought that crop rotation might be the answer to preventing inoculum carryover in strawberry plant production. Unlike Verticillium dahliae, C. acutatum lacks an identified resting structure. A several year host free period seemed like a sensible practice for reducing inoculum levels. However these rotation fields were noted to have been contaminated or infested by the presence of infected, volunteer strawberry plants (Gubler et al., 2006). Ideally, there would be no strawberry plants or plant material remaining after harvest. This may be an unrealistic goal.
To reduce inoculum density or prevent the possibility of pathogen increase in rotation fields in the vicinity of new plantations, it would be ideal to plant crops that suppress C. acutatum or at least are non-hosts to this pathogen.
It has been shown that some crops contain substances that have an antifungal effect. Mustard or pepper extracts suppress Verticillium dahliae (Bowers and Locke, 1998), Fusarium oxysporum (Bowers and Locke, 2000), and Phytophthora nicotianae (Bowers and Locke, 2004), and some Chinese herbs inhibit the growth of powdery mildew (Chu, et al., 2006). Cassia and clove treatments reduced soil populations of Fusarium oxysporum in controlled experiments (Bowers and Locke, 2000). Formulations of cassia extract and synthetic cinnamon oil reduced pathogen populations of Phytophthora nicotianeae (Bowers and Locke, 2004). Garlic extracts have been shown to inhibit growth of a wide range of soilborne fungal organisms (Wilson et al 1997; Sealy et al, 2007).
Palmarosa (Cymbopogon martini), thyme (Thymus zygis), cinnamon leaf (Cinnamomum zeylanicum), and clove buds (Eugenia caryophyllata) showed high levels of antifungal activity against Botrytis cinerea (Wilson et al 1997). Essential oils of cinnamon (Beg and Ahmad, 2002; Ranasinghe et al, 2002), clove (Beg and Ahmad, 2002; Ranasinghe et al, 2002), lemongrass (Paranagama et al, 2003), palmarosa (Velluti et al, 2004) thyme (Faleiro et al, 2003) and butterfly pea (Osborn et al, 1995) are known to be antifungal.
Compounds from several members of the family Rutaceae have been shown to have growth inhibitory components against Colletotrichum fragariae, Colletotrichum gloeosporioides, Colletotrichum acutatum, Phomopsis obscurans, Botrytis cinerea, and Fusarium oxysporum (Cantrell et al, 2005). In contrast, strawberry plants contain substances that stimulate sporulation of C. acutatum (Leandro et al., 2003).
1. Determine whether or not the commonly used strawberry nursery cover crops are a host for C. acutatum or if not a primary host, determine what role they may play in the survival of this fungus during the rotation period.
2. Screen crops that have possible inhibitory effects on the germination and growth of C. acutatum. Evaluate their potential as novel rotation crops.