Lab diagnoses more critical than ever with look-a-like plant disease symptoms

To the human eye the vascular and cortical discoloration in strawberries caused by Macrophomina crown rot, Verticillium wilt, Phytophthora crown rot, and Fusarium wilt appear very similar. Only laboratory analyses can accurately diagnose the culprit and provide the right direction for treatment.

“No pest control advisor would even think about making disease decisions by just looking through a windshield, right?” asked Steven Koike, plant pathology farm advisor with the University of California Cooperative Extension (UCCE), Monterey County.

Koike spoke to pest control advisors, growers, and other industry representatives at the 34th annual California Association of Pest Control Advisers conference in Anaheim, Calif.

“Since some diseases have similar symptoms, it’s always best to conduct a lab analysis so the correct prescription is written for treatment.”

Koike operates the UCCE plant pathology diagnostic lab in Salinas, Calif. He highlighted several diseases of growing importance on the Central Coast.


Tospoviruses are carried only by thrips. Koike says the tospovirus virus called impatiens necrotic spot virus (INSV) was first found in California lettuce in 2006. Since then INSV has spread through lettuce fields in Salinas, Chualar, Soledad, Greenfield, Castroville (Salinas County), plus parts of San Benito County.

The Salinas Valley is the leafy green production capital of the world. INSV is now found in a significant number of lettuce fields.

“In some fields, the INSV disease in lettuce has reached as high as 50 percent; this is severe crop loss,” Koike said. INSV is typically found in ornamentals including bedding plants.

When INSV was first found in the Salinas Valley, several pest control advisors (PCAs) brought bags of suspect lettuce to the Salinas lab for analysis. Growers had accused the PCAs of burning the lettuce with pesticides.

Koike’s lab analysis proved that the cause of the damage was due to a virus. The culprit was INSV, which like tomato spotted wilt virus (TSWV) in lettuce, causes brown necrosis and spotting. Spots can cover the entire plant, resembling chemical burn.

“PCAs, crop consultants, and growers need to do their homework and get crop problems diagnosed,” Koike said.

INSV occurs in all lettuce types on the Central Coast. TSWV, not INSV, is found in lettuce grown in the San Joaquin Valley.

Tospoviruses are difficult to control and will likely remain a long-term issue in lettuce, tomatoes, and other crops. Koike asked, why after 50 to 60 years of lettuce growing in the Salinas Valley is INSV now showing up in lettuce? INSV is rarely found in lettuce elsewhere in the world.

“We really don’t know the answer yet, but we’re conducting research to find out,” Koike said. “We think it’s vector pressure since thrips populations have increased over the last few years worldwide.”

Soil fumigant impact

Soil fumigants are used to disinfest fields of pathogens and pests prior to the planting of crops including strawberries. Yet the reduced use of the highly effective fumigant favorites methyl bromide (MeBr) and chloropicrin (Pic) due to California regulations is changing disease pressures.

When strawberry fields were fumigated with MeBr and Pic, the soil-borne fungus Macrophomina crown rot, which causes plant collapse, wasn’t a problem. When the fumigants were removed and other products were substituted, Inline and Vapam for example, Macrophomina was found in several Ventura and Orange county strawberry fields and a single field in Santa Barbara County.

When this disease first enters a field, it’s usually found in small patches in a field corner and many infected plants die. The diseased patch grows larger each year.

“Pathologists predicted this would happen once methyl bromide and chloropicrin were removed,” Koike said. “We believed that moving away from standard fumigation practices could create a change in disease control in strawberry. Macrophomina is likely the first case of this change.”

Macrophomina symptoms include vascular and cortical discoloration similar to Phytophthora.

“I think Macrophomina will be a continuing story, unfortunately,” Koike said. The pathogen is spread by plowing and moving dirt and equipment.”

Fumigation changes have also resulted in the introduction of Fusarium wilt fungus in strawberries. While only a handful of cases have been confirmed worldwide, the fungus which causes plant collapse just recently reared its ugly head in coastal strawberry fields.

“There was no Fusarium disease on strawberry in California until the change in fumigation practices away from methyl bromide and chloropicrin,” Koike said.

Fusarium wilt symptoms are similar to Phytophthora, Verticillium, and Macrophomina. Correct diagnosis is essential.

Koike also discussed the soil-borne fungus Verticillium which has caused losses in coastal lettuce for almost a decade. Verticillium causes dark discoloration of the plant’s center. Crop losses can range from 10 percent to 40 percent. The fungus, spread by dirt and mud on equipment, is a long-term problem for the lettuce industry, Koike says.

Verticillium wilt has become a “political football” spawning intense debate in California among lettuce growers, crop consultants, and the seed industry. While the exact source of most Verticillium wilt is uncertain, Koike says Verticillium wilt has been found at low levels on lettuce seed.

“This is very rare,” Koike said. “Commercially available and planted lettuce seed very infrequently carries Verticillium. It’s possible that the lettuce seed itself is one source of Verticillium wilt.”

Verticillium wilt also occurs in spinach, but only in the seed crop, Koike says. Since disease only occurs after plants bolt and start to form seed, Verticillium wilt is not found on California spinach grown for market.

“In commercially planted spinach seed in the Salinas Valley, we have found infected spinach seed rates as high as 40 percent,” Koike said. In some fields 3.5 to 4 million spinach seeds are planted per acre in the Valley.

“If you plant something that has 40 percent Verticillium wilt, you do the math,” Koike explained.

“There’s a lot of potential Verticillium that’s coming in on the seed. That is why lettuce and spinach growers, the seed industry, and PCAs are involved in ongoing discussions.”

Koike stopped short of passing total blame on any one factor. Preliminary tests show some Verticillium strains in spinach seed can infect lettuce while other strains don’t. Second, millions of lettuce seeds are planted per acre and a thorough survey of lettuce seed hasn’t been conducted. That needs to occur before the blame is placed on spinach seeds.

Finally, many fields in the Salinas Valley have contained active Verticillium wilt populations for many years. The wilt may not be related to the seed at all, Koike says. The fungus may have been in the soil for a long time.

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