1、APPLE SCAB Apple scab exists worldwide but is more severe in areas with cool, moist springs and summers. Similar scab diseases affect pears (Venturia pyrina) and hawthorns (V. inaequalis sp.f. pyracanthae).Scab is the most important disease of apples. Its primary effect is reduction of the quality o
2、f infected fruit. Scab also reduces fruit size or results in premature fruit drop, defoliation, and poor fruit bud development for the next year;and it reduces the length of time infected fruit can be kept in storage. Losses from apple scab may be 70% or more of the total fruit value. In most apple-
3、producing areas, no marketable fruit can be harvested if scab control measures are not taken. Symptoms At first, light, olive-colored, irregular spots appear on the lower surface of sepals（萼片） or young leaves of the flower buds. Soon after, the lesions become olive green gray with a velvety surface.
4、 Later, the lesion; appear metallic（金属的） black in color and may be slightly raised. Lesion on older leaves generally form on the upper surface of the leaves. Lesions may remain distinct or they may coalesce（联合） . Leaves infected young remain small and curled and may later fall off. Occasionally,smal
5、l scab spots are produced on twigs and blossoms. Infected fruit develop circular scab lesions, velvety and olive green at first but later becoming darker, scabby, and sometimes cracked. The cuticle of the fruit is ruptured（破裂、裂开） at the margin of the lesions. Fruit Infected early become misshapen（畸形
6、） and cracked, and frequently drop prematurely . Fruit infected when approaching maturity form only small lesions, which, however, may develop into dark scab spots during storage. The Pathogen: Venturia inaequalis The mycelium in living tissues is located only between the cuticle and the epidermal c
7、ells. There, it produces short, erect, brownish conidiophores that give rise to several, one- or two-celled, and spilocaea-type(环痕式 ) conidia of rather characteristic shape. In dead leaves the mycelium grows through the leaf tissues and produces ascogonia and antheridia; following fertilization, pse
8、udothecia form. The latter are dark brown to black with a slight beak and an opening. Each pseudothecium（假囊壳）contains 50 to 100 asci, each with eight ascospores consisting of two cells of unequal size.Development of Disease The pathogen overwinters in dead leaves on the ground as immature pseudothec
9、ia. Pseudothecia complete their growth in late winter and spring, and ascospores mature as the weather becomes favorable for growth and development of the host. Most pseudothecia and asci mature in the period during which the fruit buds open. When pseudothecia become thoroughly wet in the spring, th
10、e asci forcibly discharge the ascospores into the air which carry them to susceptible green apple tissues. Ascospore discharge may continue for 3 to 5 weeks after petal fall. Ascospores germinate and cause infection when kept wet at temperatures ranging from 6 to 26C. For infection to occur, the spo
11、res must be continuously wet for 28 hours at 6C, for 14 hours at 10C, for 9 hours at 18-24C, or for 12 hours at 26C. Infections, however, are more abundant during cool, wet periods of spring, early summer, and fall, while they are infrequent or absent in the dry, hot summer weather. After infected l
12、eaves fall to the ground, the mycelium invades the interior of the leaf and forms pseudothecia, which carry the fungus through the winter.On germination on an apple leaf or fruit, the ascospore germ tube pierces(穿透) the cuticle and grows between the cuticle and the outer cell wall of the epidermal c
13、ells. At first the epidermal cells and later the palisade（栅栏）and the mesophyll（叶肉）cells show a gradual depletion of their contents, eventually collapsing and dying. The fungus, however, continues to remain largely in the subcuticular position. The mycelium soon produces enormous numbers of conidia,
14、which push outward, rupture the cuticle, and, within 8 to 15 days of inoculation, form the olive-green, velvety scab lesions. Conidia germinate and cause infection in the same way ascospores do. Control Several apple varieties resistant to scab are available, but many popular ones are moderately to
15、highly susceptible. It appears that all apple cultivars are susceptible to some apple scab fungus isolates and resistant to others. So far, no effective practical biological control of apple scab has been developed. Introducing endochitinase genes from fungi into apple increased the resistance of ap
16、ple to scab, but it also reduced vigor of the plant. Shredding of apple leaf litter or treating them with urea in the fall reduced the risk of scab by about 65%. Apple scab, however, can be controlled thoroughly by timely sprays with the proper fungicides. Since the early 1990s, considerable progres
17、s has been made in developing simple or computerized apple scab prediction systems of spore release and infection for scheduling fungicide applications for scab control. The tobacco mosaic virus can attack a wide range of plants, including tomato, pepper, eggplant, tobacco, spinach, petunia, and mar
18、igold.On tomato, virus infection causes light and dark green mottled areas on the leaves. Stunting of young plants is common, and often is accompanied by a distortion and fern(蕨类植物)-like appearance of the leaves. Older leaves curl downward and may be slightly distorted. Certain strains of the virus
19、can cause a mottling, streaking, and necrosis of the fruits. Infected plants are not killed, but they produce poor quality fruit and low yields. The cucumber mosaic virus has one of the broadest host ranges of any of the viruses. The disease affects a number of important vegetables and ornamentals i
20、ncluding tomato, pepper, cucumber, melons, squash, spinach, celery, beets, and petunia.Tomatoes infected with the cucumber mosaic virus develop a slight yellowing and mottling of the older leaves. Expanding leaves typically become twisted, curl downward, and develop a shoestring appearance as a resu
21、lt of a restriction of the leaf surface to a narrow band around the midrib of the leaf. Diseased plants are stunted and produce small quantities of fruit.The tomato spotted wilt virus also has a wide host range and can affect a number of ornamental plants as well as tomato. Early symptoms of spotted
22、 wilt on tomato are difficult to diagnose. Young, infected plants may show an inward cupping of leaves, and the foliage may appear off-color or have a slight bronze cast. In some cases, leaves with exhibit dark purple flecks or small necrotic concentric rings. Development of DiseaseThe tobacco mosai
23、c virus is very stable and can persist in dry contaminated soil, in infected tomato debris, on or in the seed coat. The virus is transmitted readily from plant to plant by mechanical means. This may simply involve picking up the virus while working with infected plant material, then inoculating heal
24、thy plants by rubbing or brushing against them with contaminated tools, clothing, or hands. Aphids are not vectors of the tomato mosaic virus, although certain chewing insects may transmit the pathogen. The cucumber mosaic virus overwinters in perennial weeds and may be transmitted to healthy plants
25、 by aphid vectors (although tomatoes are not the preferred host of aphids) or by mechanical means.The cucumber mosaic virus cannot withstand drying, or persist in the soil. It also is more difficult than tobacco mosaic to transmit mechanically. Thus, cucumber mosaic tends to progress more slowly tha
26、n tobacco mosaic in a field or garden.The spotted wilt virus is transmitted from plant to plant by several species of small insects called thrips. Thrips are less than one-quarter inch in length, light green to brown, and are extremely difficult to find on the plants. Several weedy hosts and ornamen
27、tal plants may serve as alternate hosts for the virus.ControlVirus diseases cannot be controlled once the plant is infected. Therefore, every effort should be made to prevent introduction of virus diseases into the garden. Many varieties of tomato are available with resistance to tobacco mosaic viru
28、s. Sanitation is the primary means of controlling the other two virus diseases. Infected plants should be removed immediately to prevent spread of the pathogens. Perennial weeds, which may serve as alternate hosts, should be controlled in and adjacent to the garden. Avoid planting tomatoes next to c
29、ucurbits, spinach, or other vegetables and flowers susceptible to these diseases. Control of insects, especially aphids and thrips, will help reduce the likelihood of cucumber mosaic and spotted wilt.Tomato Leaf Mould Leaf mould is a disease of tomatoes only. Leaf mold is primarily a problem on gree
30、nhouse tomatoes. The disease is most destructive in the greenhouse during the fall, early winter, and spring when the relative humidity is most likely to be high, and air temperatures are such that heating is not continuous.When humidity is high, the fungus develops rapidly on the foliage, usually s
31、tarting on the lower leaves and progressing upward. If the disease is not controlled, large portions of the foliage can be killed, resulting in significant yield reductions. Early infections are most threatening.SymptomsMajor symptoms occur on the foliage but they may also occur on petioles（叶柄）, ste
32、ms, and fruit peduncles（果梗） . All stages of growth are attacked.The first leaf symptom is the appearance of small, white, pale green, or yellowish spots with indefinite margins on the upper leaf surface. On the corresponding areas of the lower leaf surface the fungus begins to sporulate. The fungus
33、appears as an olive green to grayish purple velvety growth composed mostly of spores (conidia) of the leaf mold fungus. Disease development is slow and severe symptoms are usually present only late in the season; however, major yield losses are possible if infection occurs early.Seed-borne infection
34、 has not been reported.PathogenFulvia fulva，synonym Cladosporium fulvumSpores germinate in water films or when humidity levels are above 85 percent, at temperatures between 4 and 34. The optimum temperature for germination is between 24 and 26 .Disease Cycle The fungus survives between seasons as co
35、nidia (spores), as sclerotia on plant debris, in seed, and as a soil saprophyte. Conidia are resistant to drying, and may survive in the greenhouse at least 1 year in the absence of a susceptible host, and new conidia can be produced from surviving sclerotia. The conidia act as primary inoculum to i
36、nfect plants when conditions become favorable.Leaves are infected through stoma when humidity levels are 85 percent or higher. Infection occurs most rapidly when humidity levels at the leaf surface fluctuate between 85 percent (day) and 100 percent (night). Symptoms usually begin to appear approxima
37、tely 10 days after inoculation, with spore formation beginning a few days later. Spore production is most abundant at relative humidity between 78 and 92 percent, but can occur at humidity as low as 58 percent.Large numbers of spores are produced on the undersurface of infected leaves , and these sp
38、ores are easily spread from plant toplant by air currents, splashing water, on tools, and clothing of workers, and possibly by insects.Conditions for Disease DevelopmentMany cycles of disease development are possible during the growing season.Disease development is influenced by temperature, relativ
39、e humidity, and long periods of leaf wetness. The presence of moisture on the foliage from dew, rainfall and fog provides good conditions for disease development. Increasing periods of leaf wetness are associated with increasing disease severity; consequently, the disease may become more serious dur
40、ing the rainy season when warm temperatures prevail. Its spores will not germinate if the relative humidity is less than 85%.CONTROL1. Whenever possible, keep the relative humidity in the greenhouse below 85 percent. This will inhibit the development and spread of the leaf mold fungus.a. Provide goo
41、d ventilation and as much light as possible. b. Attempt to avoid wetting the leaves when watering. Water early in the day to allow leaves to dry by mid-afternoon.d. Maintain a temperature of at least 16 to 18 throughout the season.e. Provide adequate plant and row spacing to avoid excessive shading.
42、2. Leaf mold resistant varieties are available, but because the fungus mutates readily (there are at least 12 races of the pathogen) resistant varieties are of limited use. Because new virulent races can develop in only a few years, a tomato variety which is resistant one year may be very susceptibl
43、e the next. If available, grow varieties with more than one leaf mold resistance gene. 3. A fungicide spray program may help control the disease, but should be considered secondary to environmental control measures.A weekly spray program may be necessary. For current recommendations refer to the abo
44、ve mentioned circular. Be sure to thoroughly cover all aboveground parts of every plant, especially the lower surface of the foliage, with each spray.4. Reduce primary inoculum levels through sanitation, steam treatment of greenhouses, and seed treatment.a. After harvest, carefully remove and destro
45、y (burn) all plant debris.b. Where possible, steam entire greenhouse sections between crops, preferably on a bright, hot day when little steam will be needed. Close all ventilators, and maintain the temperature at 57 for at least six hours.c. Where necessary, use hot water treated seed. Treat seed f
46、or 25 minutes at exactly 50. Tomato Gray MoldBotrytis blight, or gray mold, as it is commonly known, has an exceptionally wide host range with well over 200 reported hosts. The fungus can occur as both a parasite and a saprophyte on the same wide range of hosts. This fungus disease is intriguing in
47、that it can cause a variety of plant diseases including damping-off and blights of flowers, fruits, stems, and foliage of many vegetables and ornamentals. It is a major cause of postharvest rot of perishable plant produce, including tomatoes at harvest and in storage. The disease can occur both in t
48、he greenhouse and in the field. Besides tomato, gray mold is of concern to other vegetables including snap and lima beans(菜豆), cabbage, lettuce and endive, muskmelon（香瓜）, pea, pepper, and potato. Symptoms“Ghost spots” appear on fruits after periods of prolonged high humidity. These superficial spots
49、 have a pale halo with a brown to black pinpoint spot in the center. On unripe fruit, the halo is pale green or silvery, and generally the tissue inside the halo is paler green. On ripe fruit, the halo is pale yellow. If warm and sunny weather occurs, then ghost spot symptoms usually do not develop further, although the marketability of the fruit may be affected. Botrytis may develop on dying flowers and subsequently infect the fruit calyx. The lesions on fruit appear as light brown to gray spots, up to 3 cm in dia
