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Peer Reviewed

© 2002 Plant Management Network.
Accepted for publication 24 June 2002. Published 19 July 2002.

Seed Transmission of Fusarium oxysporum in Common Naranjilla (Solanum quitoense) in Ecuador

J. B. Ochoa, Instituto Nacional Autonomo de Investigaciones Agropecuarrias, Casilla 340, Quito Ecuador; and M. A. Ellis, Department of Plant Pathology, OARDC, The Ohio State University, Wooster, OH 44691

Corresponding author: M. A. Ellis.

Ochoa, J. B., and Ellis, M. A. 2002. Seed transmission of Fusarium oxysporum in common naranjilla (Solanum quitoense) in Ecuador. Online. Plant Health Progress doi:10.1094/PHP-2002-0719-01-HN.

Fig. 1. Fruit of “common naranjilla.” Fruit turn orange as they mature (click image for larger view).

Naranjilla vascular wilt (NVW), caused by Fusarium oxysporum (2) is a major constraint to production of common naranjilla (Solanum quitoense) (Fig. 1) in Ecuador. Farmers have generally abandoned production of common naranjilla in many areas primarily due to uncontrollable epidemics of NVW. In regions where common naranjilla is still commercially grown, mortality due to NVW may reach 80%. Common naranjilla is generally planted after and rotated with primary or secondary forests on both sides of the Andean Mountains. Recently, it has been observed that as the crop is introduced into new areas where it has not been previously grown, NVW rapidly becomes established resulting in serious losses. Naranjilla is a perennial crop that is traditionally produced from seed. The rapid spread of NVW within established production areas and into new production areas suggests that F. oxysporum could be transmitted on naranjilla seeds. Studies were conducted in 2001 and 2002 in order to determine if F. oxysporum is borne on naranjilla seeds.

Naranjilla branches with attached fruits were collected from plants with typical symptoms of NVW (Fig. 2) in a commercial planting in the Pastaza Valley, Ecuador. In order to verify vascular colonization by F. oxysporum in infected plants, isolations were made from discolored vascular tissues from 10 branches bearing fruit (one each from 10 infected plants). Tissue sections (0.125 cm3) from each branch were surface disinfested by soaking for 3 minutes in a 3% solution of sodium hypochlorite, then rinsed 3 times in sterile water. These tissue sections were then placed on petri dishes containing potato dextrose agar (PDA) and incubated at 20°C for 5 days. Fusarium oxysporum (1) was isolated from all tissue sections.

Fig. 2. Advanced symptoms of naranjilla vascular wilt (click image for larger view).

Fig. 3. Fusarium oxysporum growing from surface disinfested naranjilla seeds on potato dextrose agar (click image for larger view).

Seeds were removed from mature fruits collected from the same branches used for pathogen isolation. Seeds were also collected from mature fruits of apparently healthy plants. Seeds were dried on a greenhouse bench and then stored in the laboratory at room temperature prior to use. In order to determine if F. oxysporum was seedborne, seeds were surface disinfested as described above, then placed on PDA in petri dishes and incubated at 26°C for 2 weeks. There were 10 seeds from infected and apparently healthy plants per each of 10 petri dishes. Fusarium oxysporum was recovered from 90% of the seeds collected from infected plants (Fig. 3). Fusarium was not recovered from seeds collected from apparently healthy plants. Koch’s postulates were completed as previously described on naranjilla plants (2) using a F. oxysporum isolate recovered from seed.

In a second experiment, seeds from the diseased plants were germinated in petri dishes containing sterilized sand. Petri dishes were incubated at 26°C for up to 3 weeks. As seeds germinated, 100 apparently healthy seedlings were aseptically transplanted to pots containing sterile potting mix (40% organic soil, 40% compost and 20% vermiculite). Plants were observed daily and the development of NVW symptoms was recorded. Within 94 days after transplanting (plants 5 to 10 cm tall) 20% of the plants developed typical symptoms of NVW (Fig. 2), and F. oxysporum was isolated from discolored vascular tissue of all diseased plants as previously described. The remaining non-symptomatic plants may have been colonized by the fungus; however, symptoms were not apparent.

Results from these studies indicate that F. oxysporum is seedborne in common naranjilla. Seed transmission is probably responsible for the rapid spread of NVW in Ecuador. At present, there is no program for production of disease-free naranjilla seed in Ecuador. The production and use of disease-free seed should aid in preventing the introduction of the pathogen into new production areas as well as reduce disease spread in established production areas. In addition, we intend to evaluate the use of systemic fungicides with efficacy against F. oxysporum as seed treatments for control of NVW.

Literature Cited

1. Nelson, P. E., Toussoun, T. A., and Marasas, W. F. O. 1983. Fusarium Species: An Illustrated Manual for Identification. Pennsylvania State University Press. University Park, PA.

2. Ochoa, J. C., Yangari, B., Galarza, V., Fiallos, J., and Ellis, M. A. 2001. Vascular wilt of common naranjilla (Solanum quitoense) caused by Fusarium oxysporum in Ecuador. Online. Plant Health Progress doi:10.1094/PHP-2001-0918-01-HN.