**2.6 Blackspot disease**

170 Fungicides for Plant and Animal Diseases

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,

Fig. 2. Disease symptom of Gray mold disease on red raspberry fruit (A) and on tomato fruit

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

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

tulip, vinca, and violet are susceptible to gray mold disease.

(B). Photo, courtesy of P. R. Bristout.

**2.5.1 Control of gray mold disease** 

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

favorable temperature ranging from 18° C to 25° C.

**2.4 Gray mold disease** 

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 (Gachomo et al., 2010).

Fig. 3. Photographs of rose leaves infected with *Diplocarpon rosae* showing, (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 acervuli (Adopted from Gachomo, et al., 2010)

#### **2.6.1 Life cycle of blackspot causal agents**

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 season. Once established the disease is difficult to control.

Control of Major Diseases in Horticulture 173

Bottom rot disease of lettuce can be recognized by brown spots that initially appear on the midribs of the lower leaves that are in contact with the soil (Figure 5 A). The rot spreads rapidly under favorable conditions to affect larger sections of the midrib and leaf blades, and may affect the inner leaves of the head. Symptoms are more severe during

Fig. 5. Disease symptom of bottom rot (A) and Fusarium wilt of lettuce (B). Photos courtesy

Bottom rot is caused by a soilborne fungal pathogen *Rhizoctonia solani*. The fungus overwinters in the soil or in crop debris as sclerotia or mycelia. It may survive in alternate hosts and serve as a source of inoculum, sexual spores. It is disseminated by wind or rain splash in the next growing season. *R. Solani* has a wide host range e.g. eggplant, soybean,

Cultural measures includes three year rotations with non-host plants; collecting plant debris and burying it or plowing it deep in the soil; planting varieties that have an upright architecture to reduce contact with the soil; keeping the fields weed free and removing volunteer crops to reduce possible alternate hosts. Since *R. Solani* is able to survive on non decomposed organic matter, it is important to avoid planting lettuce in a field that has high amounts of organic matter that is not decomposed; avoid overhead irrigation during heading of the plants; plant lettuce on ridges which increases aeration and helps avoid plants contact with the soil. Fungicides (Table 2) are the most effective means to control bottom rot disease. However, fungicide control is only satisfactory when used in combination with cultural control strategies. Proper placement and timing of fungicide

**3. Diseases of vegetables** 

heading.

**3.1 Bottom rot disease of lettuce** 

of A. F. Sherf (A) and T. A. Zitter (B).

**3.1.2 Control of bottom rot disease** 

**3.1.1 Fungal agent of bottom rot disease** 

potato, cotton, alfafa, maize, wheat and several weed species.

applications are key elements for effective disease management.

Fig. 4. Light microscope photographs of *Diplocarpon rosae* growing on artificial malt agar medium: (A) *D. rosae* two-celled conidial structures before and during germination. (B) Three day-old conidium germination. (Courtesy of E. W. Gachomo).

#### **2.6.2 Control of blackspot disease**

Control practices start with planting resistant rose varieties where available. Good sanitation is key to keeping the rose disease free. Recommended cultural practices are: All infected debris should be collected and burnt or buried; all infected canes must be pruned; overhead irrigation must be avoided because it tends to splash conidia from infected to non-infected parts of the plants. It recommended to water plants at the base; the plants should be preferably watered in the morning as opposed to the evening, because the conidia require several hours of wetness to cause infection, therefore watering in the morning reduces the hours of leaf wetness. In addition, plants should be well spaced and kept weed free to allow for aeration. Furthermore, one must avoid planting susceptible plants under the shade. When blackspot disease is establish, its control relies heavily on fungicides. In Table 1, the fungicides recommended in blacksopt disease management are listed.


\*FRAC (Fungicides Resistance Action Committee)

Table 1. Fungicides labeled for the control of powdery mildew on roses.
