Naranjilla (Solanum quitoense Lamarck) produces fruit that are consumed primarily as juice (1) in Colombia and Ecuador (Fig. 1). Naranjilla is a perennial crop that is traditionally reproduced by seed; however, asexual reproduction using stem cuttings is becoming a common practice. Plants flower at 3 to 4 months of age and begin bearing fruit at 6 to 12 months. In Ecuador, naranjilla is cultivated on approximately 12000 ha, and is mainly rotated with primary or secondary forests on both sides of the Andean mountains at 500 to 1500 meters above sea level. Although little variation exists in the domesticated naranjilla, generally referred to as “common naranjilla,” two horticultural varieties have been developed: S. quitoense var. quitoense and S. quitoense var. septentrionale (2).
Farmers have generally abandoned production of “common naranjilla” in many areas mainly due to uncontrollable epidemics of an apparent vascular wilt disease, which was first noted in the early 1970s. The disease is referred to as naranjilla vascular wilt (NVW), and is currently the major constraint to the production of naranjilla in Ecuador. In regions where “common naranjilla” is still grown commercially, losses due to NVW may reach up to 80%.
Diseased plants are readily distinguishable due to their flaccid and chlorotic appearance. Flaccidity and chlorosis start in the lower part of the plant and progressively move upwards causing wilt of the entire plant (Fig. 2). In later stages of disease development, progressive defoliation occurs, which is followed by a descending necrosis. Discoloration of the vascular system is a characteristic symptom. Vascular discoloration is visible as the vascular system becomes exposed following leaf and flower abscission in defoliation. Vascular discoloration is clearly observed when longitudinal (Fig. 3) or transverse cuts are made on the main roots, stems, leaf petioles, fruit peduncles, and fruits.
Isolations were made from discolored vascular tissues from roots, stems, and fruits of symptomatic plants. Tissue sections (0.125 cm3) were surface disinfected 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 plates containing potato dextrose agar (PDA) and incubated at 20ºC for 5 days.
A Fusarium sp. was uniformly and consistently isolated from discolored vascular tissue of all plant parts from affected plants. Based on morphology of the fungal colony and conidia from monosporic isolations, the fungus was identified as Fusarium oxysporum Schlechtend: FR (3).
Pure cultures (single spored) of the fungus were used to conduct pathogenicity tests in order to complete Kock’s postulates. Inoculum was grown on PDA for 5 to 7 days and adjusted to 5 x 106 condia/40 ml of water. Pathogenicity tests were conducted on 2-month-old plants of S. quitoense var. quitoense. Five wounded and five non-wounded plants were inoculated with 5 x 106 conidia of the test fungus suspended in 40 ml of water. Wounds were made on the main roots at 3 cm below the soil line. Soil was removed from the crown region of the plant, and a 0.5 cm cut, approximately 0.25 cm deep was made in each of three main roots of each plant, then the soil was replaced. The conidial suspension was then applied to the soil around the base of the plant. Non-wounded plants received the 40 ml of inoculum only. Five wounded and five non-wounded control plants each received 40 ml of sterile water only.
All wounded and non-wounded inoculated plants developed symptoms typical of NVW. Flaccidity and chlorosis developed on the upper leaves at 39 and 48 days after inoculation on wounded and non-wounded plants, respectively. No symptoms were observed on control plants. Fusarium oxysporum was re-isolated from infected vascular tissues of inoculated plants as previously described, thus completing Koch’s postulates.
Future attempts to control the disease in Ecuador will focus on developing new varieties with resistance to F. oxysporum.
1. Heiser, C., and Anderson, G. 1999. “New” Solanums. Pages 379-383 in: Perspectives on New Crops and New Uses: Proceedings of the Fourth National Symposium New Crops and New Uses. J. Janick, ed. ASHS Press. Alexandria, VA.
2. National Research Council. 1989. Lost Crops of the Incas: Little Known Plants of the Andes with Promise for Worldwide Cultivation. National Academy Press. Washington, D.C.
3. 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.