Richters InfoSheet D1273-300  


Fusarium Disease in Basil and the Resistant Nufar Variety

Fusarium is a disease that causes sudden wilting and death in basil. It is caused by the fungus, Fusarium oxysporum f. sp. basilicum. The fungus, which spreads via contaminated seeds, and persists in soils for years, attacks the xylem in the stem, blocking water uptake and leading to a characteristic sudden leaf wilt that does not respond to watering. Light brown striations from the base of the stem is a common symptom.

The disease was first noticed in the United States in 1991, and has since been found in basil crops throughout Canada and the United States. It is assumed that the disease spread to North America by infected seeds from Italy.

Early attempts at controlling the disease included treatment of seeds with hot water and bleach, inoculation of soils with competitive fungi, and testing and removal of contaminated seed lots from the market. None of these approaches succeeded in eliminating the disease, and as of 1998, fusarium infected seeds were still widely prevalent in the market.

Most, if not all, available sweet basil (Ocimum basilicum) varieties are susceptible to fusarium, including the Genovese types popular for pesto. Other species such as camphor (O. kilamandscharicum) or lemon (O. americanum) apparently do not show symptoms of the disease, but they may act as hosts for the disease. In fact, other members of the mint family to which basils belong, such as rosemary and thyme, act as asymptomatic hosts of the disease also. This is significant because it is likely that the disease can spread from symptomless herbs to sweet basil.

The commercial practice of offering fusarium-free seeds serves only to prevent new contamination via seeds. If the fungus is present in other herbs nearby, or in contaminated soil, or on contaminated equipment, then even plants grown from fusarium-free s eeds could be infected causing significant losses. Hydroponic growers have reported repeated infestation of crops grown with fusarium-free seeds, likely caused by spores surviving equipment cleanup between crops.

In the absence of an effective means of controlling the disease, it became necessary to search for sweet basil strains that are resistant to it, or at least tolerant of it. To this end, symptomless plants growing on naturally infested soils were selected and stabilized for resistance, as well as for essential oil content, yield and other horticultural characteristics. This work has yielded the first resistant cultivar named ‘Nufar’.

To verify resistance of the Nufar cultivar, Richters commissioned Dr. Robert Wick of the University of Massachusetts, the leading U.S. researcher on fusarium on basil, to test seedlings grown from Nufar basil seeds. Wick inoculated 50 seedlings at approxi mately 5 cm in height with a high dose of Fusarium oxysporum f. sp. basilicum spores. Two of the 50 seedlings died due to undetermined causes. The remaining plants were allowed to grow to the flowering stage. None of the 48 Nufar plants showed symptoms of fusarium wilt, while 50% of a susceptible strain developed wilt.

Wick checked the asymptomatic Nufar plants for the presence of the fusarium fungus and found 55% of plants had the fungus. The presence of the fungus on asymptomatic plants shows that Nufar does not eliminate fusarium, but instead tolerates it. The tolerance mechanism is not clear, but it would be expected to be similar to that of other species of basil known to be resistant to the disease.

Wick concluded in his report to Richters that, “I would have a high degree of confidence in claiming that this line of basil is resistant.”

While the development of fusarium-resistant sweet basil is a breakthrough for the commercial basil industry, it is important to realize that fusarium resistance may degrade in the future if the fusarium fungus mutates and overcomes the resistance mechansism. So far there is no indication that this is happening.

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