Gaura Rust

Gaura lindheimeri is a plant native to southwestern Louisiana and Texas that is becoming increasingly popular as a landscape plant because it is a sun-loving, summer-flowering perennial that once established is fairly drought tolerant. For more on the horticultural merits of this plant, please see Dan Gill’s “Get It Growing” video at the LSUAgCenter.com  (http://tinyurl.com/cl235nv).  The one drawback I see to this plant from personal experience is that it does get a rust, caused by Uromyces plumbarius, that can get quite severe on the foliage. Planting in a well-drained, sunny location with good air circulation should help keep this problem to a minimum.

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Cercospora Leaf Spot of Hydrangea

Received these photos of Cercospora leaf spot on hydrangea from Gerald Roberts, county agent in Lafayette Parish.  This disease is caused by the fungus Cercospora hydrangeae, which reproduces on infected leaves that have fallen to the ground. The spores produced on these fallen leaves are either splashed onto the lower foliage or are spread by air movement.  The pathogen requires moisture on the leaves to grow and cause disease, so avoiding overhead watering is one means of reducing the chance of disease. Many of the standard fungicides available to home gardeners are also effective in controlling this disease, but they must be applied to prevent the disease from becoming established.

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Fig Rust Causing Early Defoliation

Fig rust, caused by the fungus Cerotelium fici, is already causing defoliation of fig trees in parts of southern Louisiana. This is considerably earlier than normal, but this spring’s abundance of rainfall made conditions ideal for disease development. For more information on this disease, please see the fact sheet at http://tinyurl.com/72td69s

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Powdery Mildew Blighting Crape Myrtles Now

Powdery mildew, caused by the fungus Erysiphe lagerstroemeriae, is once again wreaking havoc on the blooms of susceptible crape myrtle varieties. The warm, humid weather during late spring and early summer has been ideal for disease development on plants in partially shaded or areas protected from afternoon sun. Fungicides applied on a regular basis can help prevent most of this, but spraying large crape myrtles in the landscape is not really practical.

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Cucurbit Downy Mildew 2012

We have yet to see downy mildew on cucurbits here in Louisiana this year, but it was recently reported on cucumbers in Baldwin County, Alabama, which is east of Mobile Bay. Growers, especially those growing watermelons and pumpkins, and county agents in the Southeast Region of Louisiana should be on the alert and prepared to spray fungicides should it arrive here. Although it does not attack the fruit, it can defoliate these crops exposing the fruit to sunburn. See page 185 of the 2012 Southeastern U.S. Vegetable Crop Handbook (http://www.thegrower.com/south-east-vegetable-guide/) for information on fungicides used to control this disease.

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Impatiens Downy Mildew Update

Downy mildew was observed on Impatiens walleriana in a residential landscape in New Orleans today. The affected plants had been purchased at a “big box” store in the area, but it was not the same chain as the store in the Baton Rouge area where the disease was found previously.

Defoliation of impatiens due to downy mildew.

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Gummy Stem Blight of Watermelon

Visited a couple of commercial watermelon producers in Washington and Tangipahoa Parishes with Henry Harrison last week. Poor pollination early in the season has been the major problem in the area due to lack of bee activity early in the season.  However, we did see quite a bit of gummy stem blight on one farm. The relatively cool, wet spring weather was ideal for the development of this disease. Spores of the causal fungus, Didymella bryoniae, are released during or shortly after periods of rainfall or dew and are wind-borne. Infection of the crown leaves results in  the development of greasy-looking lesions in which pycnidia of the anamorph, Phoma cucurbitacearum, are formed. Spores produced in the pycnidia are then rain-splashed and often cause lesions on the runner vines and the crown of the plant. These lesions may eventually girdle the stems causing individual vines or the whole plant to collapse.  The pathogen may be seed-borne, and an early season fungicide spray program is essential to manage it in years with consistently wet spring weather.

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