Tracking down Typhoid Mary: Rotation crops and weeds as hidden hosts of Fusarium oxysporum f. sp lycopersici race 3, the cause of Fusarium wilt in tomato

With recent spread of the new Fusarium wilt race, Fusarium oxysporum f. sp. lycopersici (Fol) race 3, all major processing tomato regions in California are suffering severe losses. Factors influencing build up and persistence of Fol race 3 in California are poorly understood; alternate hosts are not known, the potential for cryptic colonization of resistant hosts has not been examined, and effects of environment on persistence are not well characterized. Practices are needed which discourage inoculum accumulation and persistence at high levels, reducing the chances of a new resistance-breaking race emerging in the state. We are initiating several concurrent research lines to address these issues.

Research questions:

Are all rotation crops created equal? The relative impact of rotation cropping vs. extended fallow in managing persistence of Fol in infested soils.

It is common practice for processing and fresh tomato producers to rotate tomato and non-tomato crops in an effort to manage Fusarium wilt. In result, a large suite of vegetable, grain and cover crops are grown in fields that harbor Fusarium oxysporum f. sp. lycopersici (Fol). In the absence of crop residues, Fusarium oxysporum typically does not persist at economically significant levels for more than 6 months in California fields. However, in preliminary studies, Fol race 3 was able to infect root tissue of most plant species and previous studies indicate that some crops, such as eggplants and other Solanaceous hosts, may become systemically infected, sometimes without showing symptoms.

We are interested in determining whether alternate crops can allow inoculum loads to persist for longer periods compared to an extended fallow, and whether all crops are equally conducive to inoculum persistence. Based on this, we can provide both processing and fresh market producers with more precise recommendations for soil management strategies to suppress persistence of Fol.

Understanding the basis for wilt resistance: True non-hosts or cryptically colonized? 

Although many processing tomato producers rotate out of tomato in an effort to manage Fusarium wilt, with current irrigation infrastructure it is most economically efficient to continuously crop to tomato for three to five years before rotating to a different crop. During this period, it is common to rotate less productive resistant cultivars with the more productive susceptible and “tolerant” lines to reduce inoculum build up. Although it is assumed that resistant cultivars, and to some extent, tolerant lines, contribute less inoculum to the field since they are less affected by wilt, all tolerant cultivars and many resistant ones develop less severe wilt symptoms in the field. In consequence, it is possible that many of these cultivars allow inoculum to persist at high levels, increasing the chance of resistance-breaking race emergence.

We are interested in better understanding the nature of the tolerant and resistant cultivar interactions with Fol race 3; these studies will lend insight into relative inoculum contributions of different cultivars over time, and can be used in crop breeding programs to select for cultivars which do not allow inoculum to accumulate.