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Agronomy Day 2010

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New Vegetable Diseases in Illinois

Mohammad Babadoost
Mohammad Babadoost
Associate Professor
Department of Crop Sciences
(217) 333-1523;
babadoos@illinois.edu

Three new vegetable diseases occurred in Illinois in 2009, which likely will occur again in future. These diseases were: 1) downy mildew of basil, caused by the oomycete Peronospora belbahrii, which occurred in Illinois for the first time; 2) late blight of tomato, caused by the oomycete Phytophthora infestans, which has been reported from Illinois in the past but was not observed in the past 10 year; and 3) leaf mold, caused by the fungus Fulvia fulva, which had not been observed since 1999.

Downy Mildew of Basil
Symptoms and signs of disease include yellow spots on upper leaf surface (Fig. 1A, B), gray mold on the lower leaf surface (Fig. 1C, D), and foliar blight. The pathogen is spread by wind, seed, and plant material. The cool, wet growing conditions favor development of this disease. Use of fungicide is necessary to protect plants against downy mildew. Azoxystrobin (Quadris) and two phosphorous acid fungicides (ProPhyt and K-Phite) have downy mildew under herbs listed on the. Also, a section-18 permit has been issued for use of mandipropamid (Revus) in basil fields in Illinois during in 2010. Weekly applications of Quadris + ProPhyt alternated with Revus + ProPhyt can effectively control of downy mildew of basil.

Late Blight of Tomato
The disease develops on all aboveground parts of the tomato plant. Leaf lesions first appear as indefinite, water-soaked spots, which may enlarge rapidly into pale green to brown lesions (Fig. 2A). The undersides of the leaves often show a downy white growth in moist weather. Petioles and stems are affected in a similar manner (Fig. 2B). Infection of fruit produces large, irregularly shaped brown blotches (Fig. 2C). Infected fruits rapidly deteriorate into foul-smelling masses. The pathogen is most active during cool, moist weather. Late blight of tomato can be managed by the following practices: 1) destroy volunteer potato and tomato plants; 2) keep tomato field as far as possible from potato fields; 3) manage disease by prevention; 4) keep plants as dry as possible; 5) remove and destroy infected plants; 6) clean up the site after harvest; and 7) use current recommendations for chemical control (http://www.btny.purdue.edu/Pubs/ID/id-56).

Leaf Mold of Tomato
Leaf mold is primarily a problem on high tunnel and greenhouse tomatoes. Symptoms usually develop on foliage, with fruit infections being rare. The first leaf symptom is the appearance of small, pale-green, or yellowish spots with indefinite margins on the upper leaf surface (Fig. 3A, B). On the corresponding areas of the lower leaf surface the fungus begins to sporulate. The fungus appears as an olive green to grayish purple velvety growth, composed mostly of spores (Fig. 3 C). Leaf infection occurs most rapidly when humidity levels at the leaf surface fluctuate between 85% (day) and 100% (night). The most effective method for managing the leaf mold in high tunnels and greenhouses is to keep the relative humidity below 85%, and prevent free moisture from forming. Providing good ventilation helps to lower the relative humidity. Keeping night temperatures in the tunnels or greenhouses warmer than outside air temperatures prevents moisture formation on leaves. Reducing primary inoculum levels through sanitation (removing plant debris after harvest) and hot-water treatment of seed is effective in managing the disease. For chemical control of the disease, refer to the Midwest Vegetable Production Guide for Commercial Growers (http://www.btny.purdue.edu/Pubs/ID/id-56).

Downy mildew of basil. (A,B) yellow spots on upper leaf surface; (C,D) gray mold on the lower leaf surface.Fig. 1. Downy mildew of basil. (A,B) yellow spots on upper leaf surface; (C,D) gray mold on the lower leaf surface.
Late blight of tomato. (A) leaf infection; (B) stem infection; (C) fruit infection. Photo courtesy M. T. McGrath.Fig. 2. Late blight of tomato. (A) leaf infection; (B) stem infection; (C) fruit infection. Photo courtesy M. T. McGrath.
Leaf mold of tomato. (A) foliage infection; (B) spots on upper leaf surface; (C) spots on lower leaf surface.Fig. 3. Leaf mold of tomato. (A) foliage infection; (B) spots on upper leaf surface; (C) spots on lower leaf surface.
Agronomy Day 2010