Growing media temperatures soar in plants on black landscape cloth during heat waves like the one we are experiencing. Garden mums may exhibit signs of wilting during extended periods of 90 plus degree temperatures. Monitor root health and test growing media of garden mums now. Take random samples of plants out of their pots and examine roots carefully. The roots tell a lot about overall plants health, often before the top growth shows symptoms. Signs of poor root health are blackened or rotted roots or the lack of roots, especially young feeder roots.
Downy mildew was found on field-grown basil in New Jersey the week of June 13, 2013. This is a very early outbreak and suggests contaminated seed as the source. Basil Downy mildew is always present in Florida and California and has been reported in Virginia. To date, no outbreaks in Massachusetts have been observed.
Tospovirus is a genus that includes Tomato Spotted Wilt Virus (TSWV) and Impatiens Necrotic Spot Virus (INSV). Western flower thrips is the predominant insect vector. Thrips acquire the disease as a first instar larvae as they feed on virus-infected plants (including infected weeds showing few, if any symptoms). Once mature, the winged adult thrips are primarily responsible for viral spread. An infected thrips is able to transmit tospoviruses to at least one plant per day until its death. Adults do not transmit the virus to their young.
This tomato plant tested positive for Tomato Spotted Wilt Virus (TSWV) using an on-site test kit. Note the blackened color of the foliage and markings. It was difficult to see the ringspot markings of the virus in the sample. Holding the leaf sample up to the light can help to see the lines of ringspot.
Impatiens Downy Mildew was found in greenhouses in Michigan. Earlier this spring, growers in three nearby states reported outbreaks of downy mildew on impatiens in their greenhouses. In the last two weeks, downy mildew on impatiens has been confirmed in Michigan greenhouses spanning four counties. In each case, the disease was widespread within the greenhouse and the losses were nearly 100 percent. The diseased plants were disposed of immediately and were not purchased by consumers.
Powdery mildew was found on calibrachoa and rosemary this week.
Impatiens Necrotic Spot Virus (INSV) was diagnosed on Lobelia 'Hot Arctic Blue'. INSV is spread from plant to plant exclusively by western flower thrips. The virus can arrive in incoming plant material and virus infected thrips. Symptoms include distorted leaves and brown spots that look like a thumb print. Thrips cannot transmit the virus unless they acquire it as first instar larvae. First instar larvae must feed on virus-infected plants to acquire the virus.
Powdery mildew was diagnosed on calibrachoa ‘Noa Tangerine’ and on dahlias. Symptoms of powdery mildew are easy to recognize when the disease is severe but at first you may only see a few dead or dying lower leaves. Powdery mildew growth may be sparse and difficult to detect even with a microscope.
INSV and TSWV (Tomato Spotted Wilt Virus) are two members of the Tospoviruses which are vectored exclusively by Western flower thrips (Frankliniella occidentalis). These viruses have an extremely wide host range (more than 600 species of plants are susceptible). There is no cure or chemical treatment for plant viruses. Discard affected plants, eradicate weeds that may be hosts, and control thrips populations. Inspect plant material at arrival or quarantine new shipments. Do not grow vegetable transplants and ornamental bedding plants in the same greenhouse.
Jim Mussoni, IPM scout in Massachusetts reports that chilli thrips have exploded with the warm weather this past week. Carefully monitor Sweet potato vine, Osteospermum and Thunbergia. Chili thrips are much smaller than western flower thrips. For links to photos of Chilli thrips and their damage see: University of Florida - Chilli Thrips Photos and Research. Pylon (consult label for plant safety info), Conserve, Avid, Overture, or the Group 4A products are some options for management.