Introduction

The current process of increasing yield by the use of soil fumigation was first demonstrated in 1953 with the use of chloropicrin.1 However, broad spectrum soil fumigants are costly to apply and reduce the profitability of the harvest.9 The use of preventative fumigants may no longer be an option for the control of soil borne disease due to the possibility of legal restrictions. 4 Soil fumigants require treated soil to be covered with gas-proof sheeting (polyethylene or vinyl) for at least 24 to 48 hours after treatment. Planting cannot take place for an additional 2 to 3 weeks.  Even when fields are sprayed with a fumigant mixture of methyl bromide and chloropicrin (MBC) V. dahliaereemerges in the following year. 7 Due to the above factors, research experiments have been carried out at the University of California Davis (UC Davis) with the ultimate goal to develop highly productive commercial lines of resistant strawberry varieties that no longer depend on fumigant application of the soil. 4

An example of how V. dahilae disrupts productivity can be seen when comparing the healthy strawberry plant in Figure 1 with Figure 2 that shows  a field infested with Verticillium wilt disease.

Microsclerotia are the reproductive spores of V. dahilae   that can live in the soil for 25 years making crop rotation useless. V. dahliae can spread quickly to new areas by seed, tools, machinery, and by infected roots.9  The fungus causes the outer leaves of strawberries to become necrotic and the growth of new leaves are greatly reduced. 1 A variety of weeds can act as carriers of V. dahliae  making eradication difficult.

Fig. 2  A strawberry field with a high concentration of V. dahliae at 30 ms/gram. The field shows various levels of resistance from little injury to death. (Image used with permission from Mark Bolda,, UCCE).

Quiz

Question

Question: Microsclerotia can live in the soil for how many years

Looks Good! Correct: Microsclerotia can live in the soil for 25 years.
Question

Question: Verticillium dahliae can cause what percent of crop loss?

Looks Good! Correct: Verticillim dahliae can cause 50% of a strawberry crop loss.
Question

Question: How is Verticillium dahliae transferred from one area to another?

Looks Good! Correct: All of the above; seed, infected roots, tools.
Question

Question: How long does a farmer have to wait after fumigant application?

Looks Good! Correct: Farmers should wait at least 2 to 3 weeks before planting the strawberry crop in a field that has had fumigant application.

Fig. 3 Strawberry plants showing various levels of resistance to Verticillium wilt. The two plants on the left are susceptible to the disease as indicated by the black roots. The plant in the center shows moderate resistance to the disease. The two plants to the far right are strawberry plants that show a high level of resistance (UIUC, Image used with permission from  J.L. Maas).