**2. Diseases of flowers**

#### **2.1 Powdery mildew**

Powdery mildew as the name suggests initially appears as a white grayish coating on leaves (Figure 1 A). When the disease is severe it produces a white gray coating on stems, petals and buds. This white coating spreads over time to cover the whole leaf (Figure 1 B, C). The affected leaves or petals get distorted, shrivel and may fall to the ground. Several disease cycles occur within the plants' growth season. The disease makes the plants unattractive and repeated disease cycles cause reduced vigor of the plants (Gachomo, 2005). Among the most often affected flowers and shrubs are columbine, dahlia, delphiniums, honeysuckle, ivy, lilac, phlox, privet, snapdragons, lungwort, bee balm, garden phlox, verbena, rose, and zinnias.

blasts, anthracnose, wilts, fruit rots, root rots, stem rots, rusts and molds among any others

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).

including the effects on fungal establishment within the host.

**2. Diseases of flowers** 

verbena, rose, and zinnias.

**2.1 Powdery mildew** 

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,

Powdery mildew as the name suggests initially appears as a white grayish coating on leaves (Figure 1 A). When the disease is severe it produces a white gray coating on stems, petals and buds. This white coating spreads over time to cover the whole leaf (Figure 1 B, C). The affected leaves or petals get distorted, shrivel and may fall to the ground. Several disease cycles occur within the plants' growth season. The disease makes the plants unattractive and repeated disease cycles cause reduced vigor of the plants (Gachomo, 2005). Among the most often affected flowers and shrubs are columbine, dahlia, delphiniums, honeysuckle, ivy, lilac, phlox, privet, snapdragons, lungwort, bee balm, garden phlox,

(Bowen & Roark, 2001; Walker et al., 1995).

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

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. Kunbino (C).

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

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 of the widely used fungicides in powdery mildew disease control.

Control of Major Diseases in Horticulture 171

in combination with the good sanitation measures will ultimately help to contain the disease. It is imperative to apply preventive fungicides as soon as disease is detected. Fungicides with the active ingredients such as chlorothalonil, dichloran, fludioxonil, trifloxystrobin, iprodione, mancozeb, copper sulfate pentahydrate, fenhexamid, azoxystrobin, and thiophanate methyl are registered for *Botrytis* control and therefore recommended to use in case of gray mold disease. Be sure to rotate applications among

Blackspot is a disease of roses. It appears as small black spots on the upper surface of the leaves (Figure 3 A), which first appear on the lowest leaves and may first appear as purple spots on stems that eventually turn black. The area around the spots turns yellow and the spot may coalesce to form black blotches (Figure 3 A, B, C). The yellow leaves easily fall off the plants. The disease spreads from lower leaves to younger upper leaves leading to further defoliation. Severe defoliation reduces vigor of the plants and decrease flower production

chemical classes as fungicide resistant strains of *Botrytis* have been reported.

Fig. 3. Photographs of rose leaves infected with *Diplocarpon rosae* showing,

acervuli (Adopted from Gachomo, et al., 2010)

**2.6.1 Life cycle of blackspot causal agents** 

season. Once established the disease is difficult to control.

(A) the symptoms on leaves followed by the yellowing of the leaves (B) a close up of a sporulating spot showing the dome shaped-unopened acervuli that have pushed the cuticle upwards, (C) a close up of a sporulating spot where a mass of white conidia oozes out of the

Blackspot disease is caused by a fungal pathogen, *Diplocarpon rosae*. The fungus overwinters on infected canes and fallen debris (Gachomo, 2005). During the favorable weather conditions the spores are splashed from infected plant parts to young leaves by rain splash and irrigation water. The fungus produces conidia within 10 to 14 days (Figure 4 A, B) which are splashed to other young leaves. Several disease cycles can occur within a growing

**2.6 Blackspot disease** 

(Gachomo et al., 2010).

#### **2.4 Gray mold disease**

Generally, the gray mold disease appears as a gray to brown mold on dying plant tissue such as spent flowers, leaves and stems (Figure 2 A, B). The symptoms are observed as light spot surrounded by maroon halos on petals. These spots enlarge and turn brown over time. Infected petals may fall off. Cankers and dieback may be observed on stems. A wide range of plant varieties are affected by this disease. Susceptible plant species to gray mold include house plants such as African violets, amaryllis, Amazon-lily, azaleas, begonias, cacti, caladium, calla lily, camellias, castorbeans, chrysanthemums, cinerarias, coleus, corn flowers, dalias, dracaenas, dusty millers, ferns, fig, fuchsias, ardenias, gloxinias, heliotrope, orchids, passion flower, and poinsettias. In addition, members of herbaceous annual plants susceptible to gray mold are ageratum, begonia, caldium, carnation, celosia, chicory, geranium, gerbera, gladiola, impatiens, marigold, nasturtium, pansy, petunia, snapdragon, statice, stock, sunflower, sweet pea, verbena, and zinnia. Furthermore herbaceous perennials including anemone, aster, baby's-breath, bellflower, bleeding heart, bloodroot, bluebell, buttercup, calendula, candytuft, carnation, chrysanthemum, chicory, coralberry, cranesbill, dandelion, daylily, delphinium, Dutchman's- pipe, foxglove, globe-amaranth, hyacinth, iris, Jack-in-the-pulpit, liy, lily-of-the-valley, lupine, Maltese cross, narcissus, peony, phlox, pinks, plantain lily, poppy, primrose, purple coneflower, rose, snowdrop, tulip, vinca, and violet are susceptible to gray mold disease.

Fig. 2. Disease symptom of Gray mold disease on red raspberry fruit (A) and on tomato fruit (B). Photo, courtesy of P. R. Bristout.

#### **2.5 Life cycle of gray mold causal agents**

Gray mold disease is caused by many strains of the fungus *Botrytis cinerea*. This fungus is not host specific and therefore infects many different types of plant species. Symptoms vary with the plant species attacked. The fungus needs moisture to cause infection and a favorable temperature ranging from 18° C to 25° C.

#### **2.5.1 Control of gray mold disease**

To contain the disease all infected tissue should be remove as soon as possible; infected debris should be collected. The plants should be well spaced and weeded to allow good air circulation; sprinkler irrigation and wetting of the foliage should be avoided because this provides the humidity needed for the proliferation of the disease. Application of fungicides in combination with the good sanitation measures will ultimately help to contain the disease. It is imperative to apply preventive fungicides as soon as disease is detected. Fungicides with the active ingredients such as chlorothalonil, dichloran, fludioxonil, trifloxystrobin, iprodione, mancozeb, copper sulfate pentahydrate, fenhexamid, azoxystrobin, and thiophanate methyl are registered for *Botrytis* control and therefore recommended to use in case of gray mold disease. Be sure to rotate applications among chemical classes as fungicide resistant strains of *Botrytis* have been reported.
