Leaf blight diseases.
A couple of new fungicides (Cabrio and Pristine) have been recently approved for the management of fungal leaf blight diseases of carrots in New York, therefore the following is a brief update and review of current management practices and available chemicals. Aside from planting high quality and pathogen-free seeds, crop rotation is a very important management tool for fungal leaf blight diseases as well as root diseases, including root-knot nematode. The current recommended minimum 2 year rotation out of carrots facilitates the decomposition of crop debris that can serve as a source of inoculum for subsequent carrot plantings. Once the crop has emerged, it is important to scout weekly for both Alternaria and Cercospora leaf blight disease symptoms (Cercospora leaf blight has been predominant since 2000). Alternaria leaf blight lesions are irregular in shape and size, dark brown to black in color and are typically more evident initially on the lower leaves compared to Cercospora leaf blight lesions. Lesions from Cercospora carotae infections are typically tan and circular in shape with dark definitive margins. The first fungicide spray should be applied once 25% disease incidence is observed in the field (e.g. 12 of 50 leaves scouted show symptoms of leaf blight). Even though none of the cultivars are highly resistant to leaf blight diseases, it is important to scout cultivars individually because of the difference in the level of susceptibility which can affect the development of disease, thus the number of fungicide applications necessary. In 2004 for example, Carson and Eagle planted in one field required 2 and 6 fungicide applications, respectively. Subsequent fungicide applications should be based on increasing disease severity as well as the weather forecasts that favor disease development. Both Alternaria dauci and Cercospora carotae require prolonged leaf wetness with temperatures between 61-77ºF and 68-86ºF, respectively for infection to occur (night temperatures >59ºF also favor disease development). Alternatively, subsequent fungicide applications can be based on the accumulation of >15 disease severity values (DSV) as predicted by the Tom-Cast (tomato’s early blight disease) model.

Due to the explosive nature of bacterial blight under hot, wet, and windy conditions; only the trace presence of symptoms in the field will require management. Currently, only copper-based products are registered for use to manage bacterial blight, but additional materials are currently being evaluated in research trials. A hot water seed treatment (125ºF for 25 min.) may reduce bacterial infestation levels, but exposure to excessive temperatures may reduce seed germination rates. For additional information on fungal and bacterial leaf blight diseases of carrot and scouting methods, an updated fact sheet is available at http://www.nysipm.cornell.edu/factsheets/vegetables/misc/clb.pdf or check the Veg MD website.

Other foliar and root diseases.
Although foliar blight and crown rot caused by Rhizoctonia solani have been more frequently observed in recent years, they are still a sporadic problem in New York carrot fields. Cultural practices that minimize injury to the root, minimize the amount of soil thrown over the crown and increase soil drainage are recommended. Seed treatments with Maxim are also effective for managing seed and seedling diseases, while foliar applications of azoxystrobin fungicides are highly effective against Rhizoctonia crown and foliar blights. Symptoms of Rhizoctonia foliar blight appear first on the petioles or near the crown. 

Cavity spot caused by Pythium spp. was observed in wet, low lying areas of some fields in 2004. The sunken elliptical lesions that develop laterally on the roots are often infected by secondary organisms like bacteria that cause the carrots to rot. Crop rotation (3 to 4+ years) with grain crops to reduce the build-up of inoculum along with the fungicide seed treatment of Apron are effective management tools as well as practices that promote good soil drainage will reduce the incidence and symptom severity. Metalaxyl (Ridomil) applications have been reported to reduce cavity spot in other carrot producing regions.

Sclerotinia white mold caused by Sclerotinia sclerotiorum is also favored by poor soil drainage, cool temperatures and injury. Lesions will develop a characteristic cottony white mycelium on the surface and the production of the characteristic black sclerotia (survival structures of the fungus).

Table 1.  Fungicides for managing leaf blight diseases of carrot in New York.  Chemical efficacy is designed on a scale from most effective (++++) to poor (+) or effect unknown/chemical registration for that disease is not available in New York (--).

^w Alternate with another chemical class for resistance management except for Quadris Opti which already is mixed with chlorothalonil.

^x Strobilurin fungicides have been more effective the past two years due to their ability to quickly penetrate and dry on the leaf surface and therefore resist being washed off due to rain compared to other products.

^y Cabrio EG:  8 to 12 oz./A, a maximum of 3 applications per season, total of 48 oz./A/season.

^z  Pristine:  8 to 10.5 oz./A, a maximum of 6 applications per season, total of 63 oz./A/season.

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