**Control of Major Diseases in Horticulture**

Emma W. Gachomo1,2, Jose C. Jimenez-Lopez3, Adéchola P. P. Kayodé4, Lamine Baba-Moussa5 and Simeon O. Kotchoni1,2 *1Department of Biology, Rutgers University, Camden, 2Center for Computational and Integrative Biology, Rutgers University, Camden, 3Department of Biochemistry, Cell and Molecular Biology of Plant, Estacion Experimental del Zaidin, Consejo Superior de Investigaciones Cientificas, Granada, 4Département de Nutrition et Sciences Alimentaires, Faculté des Sciences Agronomiques, Université d'Abomey-Calavi, Cotonou, 5Laboratoire de Biologie et de Typage Moléculaire en Microbiologie, Faculté des Sciences et Techniques, Université d'Abomey-Calavi, Cotonou, 1,2USA 3Spain 4,5Benin* 

#### **1. Introduction**

166 Fungicides for Plant and Animal Diseases

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OH 192.9 after air drying. *Biocontrol Science and Technology* 16: 281-293 Zhang, Z. & Pierson III, L.S. (2001). A second quorum-sensing system regulates cell surface

*Applied and Environmental Microbiology* 67: 4305-4315

carbon loading of production media influence freeze-drying survival and biocontrol efficacy of *Cryptococcus nodaenisis* OH 182.9. *Phytopathology* 95: 626-632 Zhang, S., Schisler, D.A., Jackson, M.A., Boehm, M.J., Slininger, P.J. & Liu, Z. (2006). Cold

shock during liquid production increases storage shelf-life of *Cryptococcus nodaensis*

properties but not phenazine antibiotic production in *Pseudomonas aureofaciens*.

Cultivation of fruits, vegetables, flowers, and/or ornamental plants is one of the economically important sectors of crop production. However cultivation of these plants plagued with wide range of most destructive, reoccurring and widespread stress factors both biotic (diseases) and abiotic (environmental) (Gachomo et al., 2009). Plant diseases are caused by many different types of organism that include fungi, bacteria, viruses etc. While plant disease may be caused by many factors, majority of them are caused by fungi and fungi like organisms (FLO). Over 8,000 species of fungi and FLOs have been reported to cause disease (Ellis et al., 2008). Disease caused by fungi and FLOs have been recorded to cause damage that has had far reaching effects on human population as a result of the suffering that follow crop loss (Gachomo, 2005) e.g. the great famine of 1845-1852 that occurred in Ireland as a result of potato crop failure due to potato blight infection. During this famine about a million people died of hunger and another million emigrated out of Ireland. Control of plant disease is both expensive and time consuming and represents a large segment of the pesticide market (Gachomo et al., 2009; Killian & Steiner, 2003), despite continuous efforts to control their detrimental effect on crop production. For example cost of control and damage by *Phytopthora infestans* (that causes late blight of potatoes) in Europe is estimated at more than €1,000,000,000 (\$1,400,000,000) per year (Haverkort et al., 2008). The typical symptoms of these fungal induced diseases cover all color spectra (from white to black) and form different types of symptoms both internal and external, microscopic and macroscopic, local and systemic which appear (but are not limited to) as leaf spots, galls, lesions, blights and cankers incase of local symptoms or as wilting, yellowing, and dwarfing incase of systemic symptoms because they involve most or all of the plant (Gachomo et al., 2006; Gachomo & Kotchoni 2007; Gachomo et al., 2010). Common fungal diseases include leaf spots, cankers, powdery mildews, downey mildews, smuts, blights, damping off,

Control of Major Diseases in Horticulture 169

The powdery mildews are caused by a group of more than 300 related fungal species which are host specific, meaning that a fungal species that attacks one type of plant species might not infect another plant species. Examples **of fungi causing powdery mildew**include *Sphaerotheca pannosa* that infects roses; *Erysiphe cichoracearm* that infects Chrysanthemums. The primary inoculum comes from dormant infections that survive the winter. The overwintering structures called cleistothecia produce sexual spores, which infect young leaves and blossoms when the weather is favorable. Mycelia formed in the infected tissues form conidia, which infected other leaves, shoots and buds. Several generations of conidia are formed in the growing season as long as the weather remains favorable. Towards the end of the season when the conditions become unfavorable for disease development, cleistothecia are formed in the

infected tissue and will serve as a source of inoculum during the next growing season.

Fig. 1. Disease symptom of powdery mildew on the upper leaf surface of Euonymous plant (A), grass blades: Photo, courtesy of P. Flynn (B) and pumpkin leaves: Photo, courtesy of J.

Spores of the fungal causing disease are spread by wind and water splash; it is therefore crucial to avoid wetting the leaves during watering as one of the most fundamental strategies of powdery mildew control. In addition, avoiding the use of sprinkling irrigation but rather watering at the base of the plants or use of drip irrigation are the general rules in controlling this disease. In order for the disease to develop, the spores require plants wetness for several hours to be able to infect the plants, therefore water the plants preferentially in the morning rather than in the evening so that they have time to dry during the day, therefore reducing time of leaf wetness. Varieties that are susceptible should not be planted under the shade but in areas exposed to the sun because shading does not allow dew to dry quickly. The plants should be well spaced and kept free of weed to increase aeration around the plants and reduce moisture retention around the base of the plant. Plants should be pruned to reduce dense foliage and increase aeration. The fungus overwinters in fallen plant debris, therefore it is very important to clean up all fallen plant materials and prune infected tissues at the end of the season, which will ultimately reduce the source of inoculum for the next season. Up to date, the efficient control of powdery mildew disease relies on fungicide application. Several fungicides have been developed for the management of this disease. In Table 1, we show some

**2.3 Control and prevention of powdery mildew disease** 

of the widely used fungicides in powdery mildew disease control.

**2.2 Life cycle of powdery mildew causal agents** 

Kunbino (C).

blasts, anthracnose, wilts, fruit rots, root rots, stem rots, rusts and molds among any others (Bowen & Roark, 2001; Walker et al., 1995).

Once the fungal diseases are established, they are generally difficult to control despite a combination of practices including sanitation measures and fungicide applications (Behe et al., 1993). Management of the diseases relies often on intensive use of fungicides (Bowen & Roark, 2001; Reddy et al., 1992; Walker et al., 1995). Fungicides used in fungal disease management include dormant sprays, protectant and systemic fungicides (Bowen & Roark, 2001; Killian & Steiner, 2003). Protectants, also known as contact fungicides, remain on the outside of the plant and kill parts of fungal structures growing on the surface of the plant (Gachomo et al., 2009). However, the activity of protectant fungicides decreases with time because of exposure to environmental factors such as ultraviolet (UV) radiation and wash-off caused by the rain, and the expansion of foliage surface in growing plants leading to a breaking effect on protective layer. Consequently, protectant fungicides need frequent applications to be effective. On the other hand, systemic fungicides are absorbed into the plant tissue and can minimize disease symptoms after infection has occurred (Gachomo, 2005). One group of systemic fungicides, the demethylation inhibitors (DMI), has been widely used to control fungal diseases in horticulture (Killian & Steiner, 2003). DMI common mode of action is centered on the fungal sterol biosynthesis, i.e. the inhibition of demethylation at position 14 of lanosterol or 24 methylene dihydrolanosterol, the precursors of sterols in fungi (Kuck et al., 1996). However, strobilurins constitute the novel promising group of fungicides to control a wide range of pathogens infecting cereals, potatoes, fruit, grapevine, vegetables, turf grass and ornamentals (Ebeling et al., 2003; Stark-Urnau et al., 1997; Wojdyla & Orlikowski, 1999). The strobilurins inhibit the mitochondrial respiratory chain by blocking the electron transport at the Qo site of cytochrome b. Forthermore, the strobilurins used either alone or in combination with other fungicides control a wide array of fungal diseases, including diseases caused by water moulds, downy mildews, powdery mildews, fruit rotters, rusts, leaf spotting and blight fungi (Ammermann et al., 1992, Margot et al., 1998; Reuveni, 2001; Ypema & Gold 1999).

In this chapter, we give a detail overview of disease symptoms, their causal agents and fungicide dependent control of selected major fungal diseases in horticulture such as powdery mildew, gray mold, blackspot, late and early blight diseases, and Fusarium wilt disease. Optimum number and timing of fungicide sprays for an efficient disease control in horticulture are here discussed. In addition, we report several aspects of fungicide activity, including the effects on fungal establishment within the host.
