Wednesday, September 21, 2016

Tomato Spotted Wilt Virus Strikes Again

A chile pepper plant with leaf deformity, spotting and yellowing caused by a virus
Chile plant exhibiting symptoms of Tomato
Spotted Wilt Virus (Photo: NMSU - PDC)
We've highlighted this disease before, but it's such an interesting disease that we decided to do it again! This chile plant was submitted for disease analysis exhibiting classic symptoms of Tomato Spotted Wilt Virus (TSWV). Symptoms of TSWV can be quite variable, but some of the more common symptoms include leaf deformity (especially of the new growth), terminal necrosis, leaf drooping (wilt-like appearance), stunting, mottling, and spotting (necrotic and chlorotic). In this particular case, several leaves on the plant displayed characteristic chlorotic spots with concentric rings (see images below).  Fruit symptoms can also be quite striking. Fruit symptoms include: chlorotic concentric ringspots, raised bumps, uneven ripening and deformity. Plants infected early in the season may produce little or no fruit. In this case, the plant produced only one small, deformed fruit which ripened prematurely. Most viral diseases are confirmed using some form of DNA analysis. In this case, there is an antibody-antigen test that can detect the virus in just a few minutes. A small amount of plant tissue is ground in buffer and then a strip embedded with antibodies to the virus is placed in the sap solution. If the virus is present, two purple lines appear on the strip (if only one line appears, it indicates that the test worked, but the virus was not present). Below is a photo of the antibody-antigen test for this plant.

The terminal branches of a chile plant with leaf curling and deformity caused by a virus
Terminal leaf deformity caused by Tomato Spotted
Wilt Virus (Photo: NMSU-PDC)

A composite photo of chile pepper leaves with chlorotic concentric ringspots caused by a virus
Chile pepper leaves exhibiting chlorotic concentric ringspots characteristic
of TSWV (Photo: NMSU-PDC)

A plastic sample bag with a crushed up chile peper leaf and an indicator stick showing the leaf sample is infected with a virus
Antibody-antigen test results for Tomato Spotted Wilt Virus -
two purple lines indicated that the tissue is infected
with Tomato Spotted Wilt Virus (Photo: NMSU-PDC)

Tuesday, September 13, 2016

Tubakia Leaf Spot

Tubakia Leaf Spot

Oak leaves with irregular brown spots caused by a fungus
Tubakia leaf lesion on Oak.
Tubakia sp. a fungal pathogen that causes a late season leaf spot and twig canker. All species of oak appear to be susceptible to the disease but those in the red oak group ( black, red & pin oak) appear to be most susceptible.

Tubakia leaf spot tends to develop in the late summer to early fall and begins as small reddish-brown foliar spots. As the disease progresses the leaf spots enlarge and may coalesce to form large blighted areas. Spotting may occurs on or near leaf veins causing death of the vein and collapse of leaf tissue beyond that point. Premature leaf drop and small twig cankers can form in severe cases.

Tubakia sp. overwinter in twig cankers and in infected leaf tissue on or around the tree. Spores are easily spread by wind and rain splash to uninfected tissue in the spring and early summer months. In most cases, Tubakia leaf spot is a cosmetic disease. In general, when leaf spots are sporadic, the ultimate damage to the plant is relatively insignificant. When leaf spots occur year after year on the same tree, cumulative damage may result in dieback and decline.

Management of Tubakia begins with good sanitation practices such as raking and destroying fallen leaves. This reduces the initial inoculum for the following year. Selectively pruning branches to increase air circulation within the tree canopy will further reduce the incidence and severity of the disease. Branches exhibiting cankers or dieback should also be removed from the tree. This will improve the appearance of the tree as well as reduce overwintering sites for the pathogen. Lastly ensuring that the tree is properly cared for (irrigation and fertility programs) will reduce plant stress and decrease susceptibility to the disease.

Close up of irregular brown spots on an oak leaf caused by a fungus
Tubakia sp. causing leaf vein death and
collapse of leaf tissue beyond that point.
Oaks leave dying and turning light brown due to fungus infection
 Advanced stages of Tubakia leaf spot note coalescing
lesions, twig cankers and premature defoliation.

Microscopic photograph of fungus spores
The conidia (dark, ovoid structures pictured here) are produced
by the fungus and serve as inoculum for new infections.