Dear Sweet Basil, You Have Problems...Basially Yours, The U of I Plant Clinic

After working on projects and presentations, it was very, exciting to receive a sweet basil sample at the U of I Plant Clinic.  It arrived on my birthday, which made this sample even sweeter. 

This basil had been growing in a greenhouse system of 12 foot channels and 4 inch offsets with water-flow, similar to a hydroponic operation; however the root plugs of the plants appeared to be encased in a soil mixture.  These growers had first experienced problems 4 weeks ago.  Luckily, this submitter had included information with his plant sample that provided some much needed clues as to the demise of their sweet basil. 

Unfortunately, the temperature in the greenhouse had dipped down below 50 degrees F overnight, at least once.  Basil is very, sensitive to cold temperatures and should be grown at temperatures of around 55 to 60 degrees F.  We suspected that most of the necrotic areas seen on the leaves were due to cold injury; however we still needed to keep an open mind; as other issues could be lurking.

As routine, I try to rule out problems on ALL parts of the plant, even if they don't appear to be an issue at first glance.  I scanned some of the plant parts under the dissecting scope and checked for bacterial ooze under the microscope.  There were no signs of bacterial disease.

Dr. Babadoost, U of I Fruit and Vegetable Pathologist happen to peek his head into the lab and I asked him to take a look at the basil sample.  He was concerned with a fungal disease on basil, which has recently been a major issue in Illinois, called downy mildew.   For more information on downy mildew in basil, you can go to the following links:
Downy mildew wants to ruin your summer
Downy mildew poses a threat to Illinois basil crop
 

Luckily, we found no sign of any fungal diseases, such as Downy mildew, on this sample.  Dr. Babadoost and I agreed that the sample had signs of abiotic injury (most likely cold injury) as well as some insect and possible virus symptoms.  The plants had also been without water for about 15 hours and the heat was turned up to 80 degrees F (I assume to warm the greenhouse).  These factors could have also caused some plant stress symptoms.

My next stop was the office of Dr. Weinzierl, U of I Extension, Fruit and Vegetable Entomologist.  He looked at the sample and found a few leaves with possible thrip feeding.  When these leaves were examined under the scope, there were still a few thrips present.  By the way, thrips are very, small and can be very, tricky to find!  Dr. Weinzerl said that he did not recommend any treatment, as the thrip injury to the basil did not appear to be threatening the crop at this time.  Thrips can be one of those pests you don't want to have in the greenhouse, because infestations can be very, difficult to control.

The small, tan circles on leaves are thrip feeding.  The darker areas are suspected cold injury.

But, the plot thickens.......I still needed to test suspect plant tissue from this sample for a virus.  I happen to have an Agdia Inc. test strip that would test for the presence of INSV, impatiens necrotic spot virus.  This test was requested by the grower, but also, one of the few viruses that I could find know to be a problem in basil.

Two red lines on the test strip indicated that some of the plants in this sample tested positive for INSV (impatiens necrotic spot virus).  The recommendation was to rogue all of the plants that appeared to have viral symptoms immediately.  But, guess the insect vector of INSV?  THRIPS!  This may change the previous recommendation of no treatment for thrips.

The moral of the story is there could be multiple problems with a sample.  Many questions come to my mind.  How long were some of these plants infected with INSV?  Did the cold, heat, or water stress bring on the virus symptoms?  The fact of the matter is that these plants should "grow out of abiotic injury"; however, if they do not, it is possible that the plants may face further complications from the virus infection or other possible factors not seen in the sample submitted.