39 in these fields. Lettuce fields should not be placed
adjacent to plantings of perennial hosts, such as
Gazania species, of Lettuce mosaic virus. Crucifer veg-
etables may experience increased pressure from several diseases if planted near oilseed rape fields.
Consider other pertinent environmental factors that are related to sites. Crops planted close to an ocean may be more at risk from downy mildew diseases due to the consistently high humidity and cool temperatures. However, moving a few miles inland from the ocean can change these conditions and reduce downy mildew severity. Choosing a site that has lighter textured soils that drain well reduces the risk of damping-off and root rot for sensitive crops such as spinach.
EXCLUSION
Exclusion is preventing any contaminated, infested, or infected materials from entering the propagation, pro- duction, and harvest systems. Because seedborne pathogens are a primary means of pathogen introduc- tion for a number of vegetable diseases, do not allow infested or infected seed to be used in the propagation system or production field. Growers should purchase seed that has been tested and certified to be below a certain infestation threshold level, or seed that has been treated to reduce pathogen infestation levels. Some seedborne diseases have well defined seed infestation levels, such as black rot of crucifers (caused by Xantho-
monas campestris pv. campestris) and Lettuce mosaic virus of lettuce. For many others, however, seedborne
thresholds have not been established. Note that the des- ignations ‘pathogen-free seed’ and ‘disease-free seed’ are convenient marketing terms only, as it is not possible to scientifically prove that a seed lot is actually void of all pathogens; pathogen-free seed usually means that the pathogen incidence in a seed lot is below that which can be detected with standard methods.
The field of seed pathology and seed treatments is a highly developed one. The nature of seedborne pathogens and how to manage them has been exten- sively researched and studied. Many refinements have been made in producing seed so that pathogens on seed plants are minimized, pathogen detection is improved, and seed treatments are more effective. Some key steps in growing seed having minimal pathogen populations are the following: selecting and placing seed production sites in areas where the pathogen is not present and con- ditions do not favor disease development (dry, arid
conditions with no rain during the crop cycle); careful, regular monitoring of seed fields for disease symptoms; roguing (removing) symptomatic and off-type seed plants; applying preventative spray treatments; employ- ing appropriate harvest and processing methods so as to avoid contaminating seed; using seed health testing to evaluate seed for pathogens and viability. These advances in seed pathology are important to the vegetable industry because a number of damaging diseases are seedborne in these crops (see Table 8, page 40).
Seed treatments are an important means of excluding pathogens from the seed used to initiate transplants and crops. Effective seed treatments that do not significant- ly reduce germination of the seed depend on the following factors: the species of vegetable seed being treated; the target pathogen(s); the treatment itself (hot water, steam, chlorine, other chemicals); dose of the treatment substance; length of treatment time; volume of seed being treated at any one time; post treatment handling of the seed (cooling, rinsing, drying, coating, storing, etc.). Examples of seed treatments commonly used to deal with seedborne pathogens include the following: hot water soaks (carrot: 50° C for 30 minutes; celery 48° C for 30 minutes; crucifers: 50° C for 20–30 minutes); sodium hypochlorite (tomato: 1.05% for 40 minutes; pea: 10% for 1–5 minutes); antibiotics such as agrimycin; fermentation and acid treatments (various cucurbits); other chemical treat- ments such as trisodium phosphate (tomato: 10% for 15 minutes for treating for Tomato mosaic virus); dry heat treatments (tomato: 70° C for 2–4 days for treating for Tomato mosaic virus).
Other seed treatments are intended to deposit pro- tecting fungicides onto the seed coat. Such deposits will protect the seed during the first few days after planting and will protect seed and newly emerged seedling from soilborne damping-off pathogens. These seed treat- ments usually consist of fungicides applied to seeds such as beans, sweetcorn, and cole crops.
Because of the intense international marketing and transporting of vegetable seed, the importance of seed health needs to be continually examined. International standards for seedborne pathogen detection, testing methodology, seed treatment, seed viability levels, and other pertinent parameters will require continual research and subsequent discussion and acceptance by nations producing and selling vegetable seed.
40 INTRODUCTION TOVEGETABLECROPS ANDDISEASES
CROP PATHOGEN
Alliums Aspergillus niger Botrytis allii
Pseudomonas syringae pv. porri
Asparagus Asparagus virus 2
Fusarium oxysporum f. sp. asparagi
Basil Fusarium oxysporum f. sp. basilicum
Bean Bean common mosaic virus
Colletotrichum lindemuthianum Pseudomonas syringae pv. phaseolicola Xanthomonas campestris pv. phaseoli
Beet Beet mild yellowing virus
Beet western yellows virus Cercospora beticola
Peronospora farinosa f. sp. betae Phoma betae
Brassicas Alternaria brassicae Alternaria brassicicola Phoma lingam
Mycosphaerella brassicicola
Pseudomonas syringae pv. alisalensis Pseudomonas syringae pv. maculicola Xanthomonas campestris pv. campestris
Broad bean Ascochyta fabae
Broad bean stain virus Broad bean true mosaic virus
Broccoli raab Alternaria brassicae
Pseudomonas syringae pv. alisalensis
Carrot Alternaria dauci Alternaria radicina Cercospora carotae
Xanthomonas campestris pv. carotae
Celery Cercospora apii Phoma apiicola
Pseudomonas syringae pv. apii Septoria apiicola
Cilantro Cilantro yellow blotch virus
Pseudomonas syringae pv. coriandricola
Cucurbit Acidovorax avenae subsp. citrulli Cladosporium cucumerinum Colletotrichum orbiculare Cucumber mosaic virus Didymella bryoniae Fusarium oxysporum
Fusarium solani f. sp. cucurbitae
TABLE 8 Important seedborne pathogens of vegetables*
CROP PATHOGEN
Cucurbit Pseudomonas lachrymans
continued... Squash mosaic virus
Tobacco ringspot virus Zucchini yellow mosaic virus
Lettuce Lettuce mosaic virus Septoria lactucae Verticillium dahliae
Xanthomonas campestris pv. vitians
Parsley Septoria petroselini
Parsnip Phoma complanata
Pea Ascochyta pinodes
Ascochyta pisi
Fusarium oxysporum f. sp. pisi Pea early browning virus Pea seedborne mosaic virus Pseudomonas syringae pv. pisi
Pepper Colletotrichum species Cucumber mosaic virus
Xanthomonas campestris pv. vesicatoria Pepper mild mottle virus
Tobacco mosaic virus Tomato mosaic virus
Spinach Cladosporum variabile Cucumber mosaic virus Stemphylium botryosum Verticillium dahliae
Sweetcorn Ustilago maydis
Swiss chard Cercospora beticola Tomato Alfalfa mosaic virus
Alternaria solani
Clavibacter michiganensis
subsp. michiganensis
Colletotrichum coccodes Cucumber mosaic virus
Fusarium oxysporum f. sp. lycopersici Pseudomonas syringae pv. tomato Tomato mosaic virus
Xanthomonas campestris pv. vesicatoria
Watercress Septoria sisymbrii
* This table lists those pathogens that have been documented to be seedborne, and in which the seedborne aspect plays a role in vegetable disease epidemiology
.
41 Diseased or contaminated transplants likewise
should not be purchased or used to plant production fields. The growing of high-quality, healthy transplants entails the use of many disease management steps, many of which are discussed in the sanitation section of this chapter. For a few vegetable crops such as asparagus and artichoke, vegetative crown tissue is divided and used as propagation material to start new fields. The disease control principle of exclusion demands that only healthy, uninfected crown divisions be used to propagate such crops.
Exclusion also means preventing contaminated equipment, water, soil, and other objects from entering the vegetable production area. While not always practical or easy to achieve, tractors and vehicles should be washed or cleaned of contaminated soil prior to entering a clean field. For at least two diseases, varnish spot (caused by Pseudomonas cichorii) and lettuce dieback (Lettuce necrotic stunt virus) of lettuce, the pathogen is found in infested water; such water should be excluded and not be used to irrigate the crop. If livestock are fed crucifer residues containing the clubroot organism (Plasmodiophora brassicae), the pathogen’s resting spores survive passage through animal digestive systems and can infest manure. Such manure should be excluded from the field. Aspects of contaminated objects are also discussed under sanita- tion and cultural practices.
RESISTANTPLANTS ANDCULTIVARS
Resistant plants are an obvious and effective control measure and are one the most important components in an integrated disease control program. Cultivars should be selected that are resistant to the main pathogens of concern. The most valuable cultivars will also have resistance to other pathogens, desirable horticultural characteristics, and be suitable for the par- ticular region and climate where it is placed. Likewise, growers can select cultivars that can tolerate the pathogen even if such plants are not technically resistant. For Verticillium wilt of cauliflower in Cali- fornia, some vigorous hybrids showed excellent tolerance to the pathogen in the field and produced high yields, even though under experimental conditions the cultivar was susceptible to the pathogen (V. dahliae).
Resistance in a plant can be expressed through the action of a single gene that confers immunity (resistance to certain races of the Fusarium wilt pathogen, for
example) or through multiple genes that result in a broad resistance to many pathogens. Single gene resist- ance is called vertical resistance, and it limits the initial level of infection and subsequent production of inoculum. However, single gene resistance can be overcome by new strains of the pathogen. The breakdown of resistance due to such changes in the pathogen poses a constant concern for growers. For example, during the past 50 years in California, a new race of spinach downy mildew (caused by Peronospora
farinosa f. sp. spinaciae) would periodically occur and
cause significant damage to the previously resistant cultivars. Plant breeders would counter with new cultivars having resistance genes to the new race. Growers would then enjoy several years of mildew-free spinach until the development of yet another race. This back-and-forth dynamic has taken place for each of the races that so far has been found in California. Similar dynamics exist for lettuce downy mildew (caused by
Bremia lactucae) in both Europe and the USA. In
contrast, multiple gene resistance is called horizontal resistance, and it limits the rate of disease development, meaning that some disease may develop but at a low, generally tolerable level.
Perhaps the greatest limitation of resistant plants as a disease control option is that resistance is not available for all crops. For several of the most damaging plant diseases, such as late blight of tomato (caused by
Phytophthora infestans) and white rot of onion and
garlic (caused by Sclerotium cepivorum), growers do not yet have cultivars with high degrees of acceptable resistance. There are no known disease resistant cultivars for most of the smaller acreage, specialty veg- etables such as the following: arugula, broccoli raab, cilantro, fennel, jicama (Pachyrhizus erosus), leafy mustards, radicchio, Swiss chard, tomatillo, and many Asian vegetables and herbs. Another major limitation is that resistance may be present in cultivars that lack adequate horticultural characteristics. There are celery cultivars with acceptable resistance to the Fusarium yellows pathogen (Fusarium oxysporum f. sp. apii); however, some of these selections lack the color, yield, and appearance qualities that the celery market currently requires. Finding a cultivar with multiple resistances can also be difficult for growers. Lettuce that is resistant to Lettuce mosaic virus may be quite sus- ceptible to corky root disease (caused by Rhizomonas CONTROLLINGDISEASE
42
suberifaciens); a lettuce selection that resists corky root
may be very susceptible to downy mildew.
Modern molecular technology and the production of genetically modified plants will provide novel sources of disease resistance and other benefits. One example is the development of transgenic summer squash (Cucurbita pepo) cultivars that are resistant to several important virus pathogens. This resistance is an example of pathogen-derived resistance in which genes from the virus pathogen itself (in this case genes that code for the virus coat protein) are introduced and inte- grated into the genome of the squash host. In 1994 a yellow crookneck summer squash was the first virus- resistant transgenic plant to be marketed in the USA. However, this technology has not yet gained general public support, so most vegetable breeding efforts apparently will rely, in the short term, on conventional breeding methods.
CULTURALPRACTICES
The disease control category of cultural practices is a broad and diverse collection of production practices and choices used to reduce the effects of diseases. Such practices are designed to help plants avoid contact with pathogens, reduce inoculum in the environment of the host plant, and create environmental conditions unfa- vorable for disease development.
Crop rotations
A significant factor in disease problems is the growing of the same crop or closely related crops in consecutive plantings, or growing the same crop too frequently over a period of a few seasons. Growers need to rotate crops so that the pathogens of one crop do not continue to increase and survive on that particular crop. Implementing crop rotations that have diverse species will also encourage diversity in the soil microbe popu- lation. Crop rotations are useful, advisable strategies for all crops and for all diseases; however, rotating crops is particularly important for combating soilborne pathogens. Crop rotations should also include, when possible, the use of cover crops that encourage soil microbe diversity and add organic matter to soil. Accurate records must be maintained so that crop rotation schemes are documented for future planning. The subject of crop rotations also encompasses the strategy of host-free periods. Researchers have found that some virus diseases are controlled much more
readily if the crop rotation strategy includes a period of time in which no fields of the host plant are present in the region. For example, in coastal California’s Salinas Valley, government regulations enforce a host-free period in which no celery can be planted in the field for the month of January and no lettuce can be present in fields from December 6 to 20. These mandatory host- free periods greatly assist in the management of Celery
mosaic virus and Lettuce mosaic virus, respectively,
because of the elimination of the primary virus hosts during a time in the winter when the aphid vectors are inactive and reduced in populations. This step prevents the virus pathogens from bridging from the fall pro- duction season into the following spring plantings. In another example, researchers found that overwintered carrot fields in the Salinas Valley were the primary source of the virus complex that causes carrot motley dwarf of carrot. When carrot growers stopped the practice of keeping carrot fields through the winter, the virus disease almost disappeared from the region.
Research indicates that certain plants, in addition to being revenue-generating crops, also have partial sup- pressive effects on various pathogens. For example, after broccoli crops are harvested and the plant residue is plowed into the soil, the decomposition of the broccoli stems and leaves releases chemicals that either directly inhibit soilborne pathogens or perhaps alter soil microflora populations that subsequently compete with pathogens. Broccoli as a rotation crop and even as a cover crop is now being used by California growers to take advantage of this suppressive effect. Cabbage crop residues and mustard cover crops show similar effects on soilborne pathogens.
When devising crop rotation strategies, growers must consider which crops and cover crops might increase disease problems. Vetch cover crops, if planted in fields having populations of Sclerotinia minor, can greatly increase the number of infective sclerotia of this pathogen. Oilseed radish cover crops can be used as trap crops to reduce cyst nematode (Heterodera species) populations in the soil; however, oilseed radish could cause increases in clubroot disease.
Fertilizers, soil amendments, and composts
Adding amendments and composts to the soil is benefi- cial for a number of fertility and soil conditioning reasons. However, with few exceptions, there is a lack of empirical data that clearly document a commercial INTRODUCTION TOVEGETABLECROPS ANDDISEASES
43 level disease control benefit from such additions to soil.
One exception is the application of lime that success- fully reduces clubroot disease of crucifers. Amendments and composts should continue to be used, however, for plant nutrition and growth considerations. Implement balanced, appropriate fertilizer programs to encourage vigorous growth. Do not over apply fertilizers such as nitrogen, as too much nitrogen can result in excessive, succulent foliage that can be more susceptible to foliar pathogens.
Planting
Time of planting can offer an opportunity for minimiz- ing diseases. In California, susceptible cauliflower that is planted in Verticillium-infested fields in the spring or summer will likely experience significant disease; however, cauliflower planted in the same fields in the late fall or winter will exhibit no Verticillium wilt symptoms. This difference in disease severity is attrib- uted to soil temperatures; winter soil temperatures are too cool for the fungus to develop and cause significant problems. In the UK, delayed planting can reduce the impact of Aphanomyces on beet because warmer soils encourage rapid germination of seedlings. Early planting can reduce rhizomania on beet and clubroot on crucifers because of reduced pathogen activity in cold soils. Therefore, choose planting dates that might reduce disease pressure for the particular crop under consideration.
Proper soil preparation prior to planting can reduce seed decay and seedling damping-off diseases by tilling to reduce plant residues left from previous crops and by making raised beds with good soil tilth and drainage. Proper bed preparation will also assist in the establish- ment of transplants. At planting, place seed and trans- plants at proper depths. Placing plants too deeply can delay plant emergence or establishment, and thus increase disease problems.
Irrigation
For most foliar diseases, overhead sprinkler irrigation enhances pathogen survival and dispersal, and subse- quent disease development. Bacterial diseases are espe- cially dependent on rain and sprinkler irrigation. Therefore, eliminate or reduce the use of sprinkler irri- gation if possible. The use of surface or buried drip tape for vegetable production has increased greatly in California and other areas in recent years and helps
reduce the severity of many diseases. Drip irrigation usually allows for a more precise delivery of water, resulting in better water management, reduced soil sat- uration, and a lowered risk of soilborne diseases such as root rots. Where overhead irrigation is required, appli- cation should be made early in the day so that foliage can dry during the remainder of the day. For all irriga- tion schedules, carefully monitor irrigations so that excess water is not applied to the crop. For a few diseases, the pathogen can be present in irrigation water. Examples of such pathogens are Pseudomonas cichorii and Lettuce necrotic stunt virus in lettuce and Phyto-
phthora capsici in several vegetable crops. In such cases,
exercise caution when using such infested water.
SANITATION
Sanitation is the general practice of cleaning up or removing diseased or contaminated materials. During the process of producing vegetable transplants, for example, sanitation involves the use of clean or sanitized transplant trays, bench tops, and mowing equipment (used to mow the tops of transplants and encourage thicker stem development). Workers should