TechNotes are provided by Syngenta GreenCast.
Alternaria Leaf Spots
Alternaria spp. leaf spots usually become problematic under very hot, wet conditions. This disease infects numerous plant species and takes on numerous symptoms. 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.
Alternaria leaf spots have numerous appearances and can be difficult to identify without sending samples to a diagnostic clinic. Some of the most common symptoms appear as small (< 1/8 inches wide) spots which initially appear water-soaked and often enlarge up into ¼-inch-diameter circular patterns with reddish brown centers and yellow borders. Other plants exhibit leaf spots with tan to white centers or the spotting takes on a reddish-brown or black appearance. These spots frequently merge and may infect the entire surface of the leaf or plant.
In some plants, the symptoms are not typical as those described above and appear as gray brown leaf and petal spots with purple margins on Dianthus (Carnation), as small black leaf spots on the leaves, petioles, stems, and/or flower petals of Cathranthus (Vinca), or as white spots with purple margins on Tagetes (Marigold). Needless to say, Alternaria leaf spots take on numerous forms and appearances. In severe cases Alternaria leaf spots may lead to leaf drop or blight.
To confuse matters, the leaf spots caused by Alternaria appear similar to several other fungal pathogens. It may be worth while to submit samples to a clinic for proper identification. The good news is the methods used to control these pathogens are similar to those used to control Alternaria.
Although not always possible, the best method to reduce the occurrence of Alternaria is to reduce the period of leaf wetness. The leaves must remain wet for at least 10 hours for infections to occur. 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.
Although fungicides containing thiophanate-methyl are effective at controlling numerous pathogens which cause leaf spots, growers should note that thiophanate-methyl is not effective at controlling Alternaria and should not be applied to control it. There are numerous chemical products available that provide sufficient control including: azoxystrobin, chlorothalonil, copper hydroxide, copper hydroxide plus mancozeb, copper sulfate pentahydrate, fludioxonil, iprodione, kresoxim methyl, mancozeb, and triflumizole.
As with most diseases, early detection and implementation of control strategies will greatly alleviate the severity of this pathogen.
Paul Pilon, Perennial Solutions Consulting - paul@perennial-solutions.com
The mention of specific active ingredients does not constitute an endorsement or recommendation of, or discrimination against similar products not mentioned. ALWAYS READ PRODUCT LABELS AND USE THEM AS DIRECTED ON THE LABEL.
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