New strawberry diseases reach Central Coast growing areas

Two potentially serious soil-borne strawberry diseases that had been confined to Ventura and Orange counties moved north in 2010 to strawberry fields in Monterey, Santa Cruz and southern Santa Clara counties.

Dieback disease has been increasing in Ventura and Orange County strawberry fields over the past five years because macrophomina and fusarium have developed in ground that is no longer fumigated with methyl bromide.

In 2010, fusarium was discovered for the first time ever in a Monterey County strawberry field, and macrophomina was discovered for the first time ever in Santa Cruz and southern Santa Clara County strawberry fields.

"We have some new problems that have reached the Central Coast area. We have had soil-borne problems in Southern California since 2005, and they have reached the Central Coast," said Steve Koike, University of California Cooperative Extension plant pathology farm advisor in Salinas.

The symptoms of macrophomina and fusarium are virtually identical, and only a laboratory analysis can tell which disease is causing the problems.

The two new disease threats are both soil-borne, spread via the soil and able to persist for a very long time. Symptoms that are common to both macrophomina and fusarium include poor growth, stunting, die back, plant collapse, extreme stress reactions and discolored crowns. Verticillium, a more familiar soil-borne strawberry disease, has all the same symptoms. And phytophthora has the same symptoms except stress reactions.

"The macrophomina and fusarium symptoms are identical. We can't tell the difference without laboratory tests," Koike said.

Koike recalled seeing what he thought was yet another case of macrophomina in a Ventura County strawberry field and only reluctantly taking a sample back to the lab in Salinas, which is where he learned the field had fusarium rather than macrophomina.

"We know very well what to do on paper to manage these diseases. If you have some flexibility, don't go with strawberries to fields that have a history of these diseases," Koike said.

If a grower doesn't have flexibility in choosing fields, fumigation of troubled fields before planting strawberries is another option.

"In 2006 Ventura County was the only area with fusarium in strawberries. This year we found it in one small area of one field in Monterey County. We have not found it anywhere else in the Central Coast, and hopefully it was cleaned up. The grower fumigated the area with methyl bromide at high rates," Koike said.

Another option is to plant tolerant varieties, but that requires a laboratory analysis to learn which disease is in the field because some varieties are tolerant of macrophomina, others are tolerant of fusarium but no variety is tolerant of both diseases.

"Resistant or tolerant cultivars exist and breeders are working to improve them, but we don't have a variety that is tolerant of both of these diseases," Koike said.

Monterey, San Andreas and Ventana strawberries are relatively tolerant of fusarium, but are susceptible to macrophomina. Chandler and Seascape strawberries, on the other hand, are relatively tolerant of macrophomina, but susceptible to fusarium.

Another management tool is to avoid stressing the plants, because stress makes the disease symptoms much worse.

"Avoid stressing the plants. Fields with macrophomina can look fine early in the season and then collapse when they get a heavy crop load that stresses the plants," Koike advised.

Long-term rotation out of strawberries should help manage fusarium, but it is not yet known which rotation crops would do well in ground infested with macrophomina.

"Fusarium is specific to strawberries, but we're trying to confirm which other crops macrophomina goes to. We suspect it has a fairly large host range," Koike said.

It is crucial in managing both of these diseases to avoid carrying them on equipment to new fields, or new areas of infested fields, because that is the most common way these diseases spread.

Both macrophomina and fusarium figure to become more troublesome in the major strawberry growing areas outside Watsonville and Salinas as methyl bromide use is phased out.

"In almost all the cases, it is fields that are no longer receiving methyl bromide and chloropicrin fumigation. The alternatives do not have the same efficacy," Koike said.

Macrophomina has recently become an increasingly important strawberry disease in many places, including Egypt, France, India, Israel, Spain, California, Florida and Illinois. In almost all cases the disease developed after fumigation with methyl bromide for strawberries was discontinued.

"We have macrophomina in Santa Cruz and Santa Clara counties, and this could become a problem for strawberry production in our area. There are also small areas of Sacramento County with macrophomina," Koike said.

So far strawberry fusarium has remained far more limited.

"Fusarium is not as broadly distributed. It is only in Ventura and Monterey counties," Koike said.

But the day of reckoning could come after methyl bromide use is eliminated in Monterey and Santa Cruz counties, as it was years ago in Ventura County.

"We now have these two pathogens in the Central Coast area, and it's likely to be a persistent problem. The year we don't use methyl bromide anymore, look for these two problems," Koike said.

(Bob Johnson is a reporter in Magalia. He may be contacted at bjohn11135@aol.com.)