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Alternaria Leaf Spots

October 14, 2011

Many ornamentals (annuals, perennials, and shrubs) and greenhouse crops are susceptible to leaf spots caused by the fungal pathogen Alternaria. Although these leaf spots usually do not kill the plants they infect, the presence of leaf spots may greatly reduce the appearance and marketability of the crop.

This disease is most prevalent under moist growing conditions and is favored by conditions that stress the host plant such as temperature extremes or closed boxes during shipping. Alternaria spreads from plant to plant by splashing water or in air currents.

Alternaria symptoms vary by the plant species of the infected plant and takes on numerous appearances. On numerous plants, the symptoms appear as small (< 1/8-inch wide) spots which initially appear water-soaked and often enlarge up into 1/4-inch diameter circular patterns with reddish brown centers and yellow borders (halos or concentric rings).  Other plants exhibit leaf spots with tan to white centers or the spotting takes on a reddish-brown or black appearance. In most cases, Alternaria symptoms first appear on the older, lower leaves.  These spots frequently coalesce or merge together and may infect the entire leaf surface or other parts of the plant.

In some plants, the symptoms are not as typical as those described above.  Alternaria symptoms on Dianthus (Carnation) appear as gray to brown leaf and petal spots with purple margins. Symptoms on Cathranthus (Vinca) are expressed as small black leaf spots on the leaves, petioles, stems, and/or flower petals. Symptoms appear as white spots with purple margins on Tagetes (Marigold). In severe cases, Alternaria leaf spots may lead to leaf drop or blight.  

Needless to say, Alternaria leaf spot symptoms take on numerous forms and appearances. To confuse matters, the leaf spots caused by Alternaria appear similar to several other fungal pathogens.  It is often necessary to submit samples to a clinic for proper identification.  

Of the fungal leaf spot diseases, Alternaria is slightly more difficult to control.  However, there are many steps growers can take to reduce or prevent infections from occurring.  The best method to reduce the occurrence of Alternaria is to reduce the period of time the leaves have free moisture on them.  The foliage must remain wet for at least 10 hours for infections (sporolation) to occur.  

Avoid overhead irrigation susceptible crops late in the day or during overcast conditions to help reduce the time the leaves remain wet after watering.  Where possible, provide good air circulation by placing crops at wide plant spacing, minimize contact from plant to plant, and water early in the day to allow the foliage to dry quickly.  If these methods are not providing adequate control then the application of fungicides is necessary.

Keep in mind that fungicide applications work best when they are applied on a preventative basis or just as infections are beginning to occur. There are numerous chemical products available that provide sufficient control including chlorothalonil, fludioxonil, imazalil, iprodione, kresoxim methyl, polyoxin, and pyraclostrobin + boscalid.

As with most diseases, good cultural practices along with early detection and implementation of control strategies will greatly alleviate the severity of Alternaria.

Paul Pilon
Perennial Solutions Consulting

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