PLP 6404 Epidemiology of Plant Diseases Spring 2015

Transcription

1 PLP 6404 Epidemiology of Plant Diseases Spring 2015 Ariena van Bruggen, modified from Katherine Stevenson Lecture 25b: Epidemiology and disease management strategies. Reducing the rate of epidemic development (cont d) Host resistance (horizontal) The genes in the host plant that condition horizontal resistance result in the slowing of the epidemic rate, r. This slowing of the epidemic rate is the overall average of one or more effects of the individual components of resistance. When van der Plank wrote his first book (1963), he proposed that horizontal resistance (the slowing of the epidemic rate) would occur from these components: 1. longer latent period, p; 2. shorter infectious period, i; 3. lower spore numbers per lesion over time; and 4. lower infection efficiency (fewer infections). Since the publishing of this book, numerous examples of rate-reducing components of resistance have been described. Using this type of resistance is one of the most widely practiced and effective means of disease control. Van der Plank concluded that all rate-reducing resistances were race-nonspecific and horizontal. This assumption was widely accepted but Parlevliet (Wageningen) states that it is certainly NOT true. Parlevliet stated that all resistances (race-specific and race-nonspecific) all reduce the epidemic rate, r. For example, a high level of resistance to Pyricularia blast in rice was found and assumed to be horizontal. However, several years later, races of Pyricularia were found that attacked this resistance in rice, hence, it was a rate-reducing vertical resistance. The resistance components in cereals to rust have been studied in the most detail. The general, over-all effect for the slowing of epidemic rate for rusts is commonly termed slow rusting. Alternatively, for some powdery mildew diseases, the descriptive term is slow mildewing. We also have slow blasting in rice, slow leaf-spotting in peanuts, and slow blighting in potatoes. When making a selection for rate-reducing resistance, field tests need to be designed carefully. Interplot interference can greatly reduce the difference in apparent resistance level. For example, when a susceptible and a resistant cultivar are each planted in isolated plots, the difference in disease may be to 2000-fold. However, when these two cvs. are planted in adjacent plots, the difference may be only 10- to 30-fold. The high spore load overrides several resistance components. 1

2 The lengthening of the latent period exerts the strongest, most definite slowing of the epidemic rate of all resistance components. This component is not commonly identified in natural epidemics because the time(s) of inoculation is (are) not usually known. The identification of a lengthened latent period requires specific tests in a growth chamber or greenhouse. The rate at which lesions enlarge also has a great influence on the epidemic rate. This rate of lesion expansion is determined by the horizontal resistance of the host. If a lesion increases in radius by 0.1 mm/day or more, then 90% of the diseased area can come from lesion expansion! Example: Partial resistance of soybean cultivars to infection by Phytophthora megasperma 2

3 Example: Late blight epidemics on 11 partially resistant potato cultivars Example: Partial resistance to glume blotch in cultivars of soft red winter wheat (Cunfer, B.M., et al., Euphytica 37:19-140) 3

4 Use of host mixtures to reduce the rate of disease increase Race-specific (vertical) resistance delays the epidemic until a new race emerges that can overcome the resistance. The epidemic rate is not changed if the new race has the same level of aggressiveness and fitness as the old race. However, we can manage the use of race-specific resistance so that the overall epidemic rate by one or more races of the pathogen can be reduced. There are several approaches that may have this rate-reduing effect: species mixtures (intercropping, etc.) cultivar mixtures multilines Reference: Garrett, K.A., and Mundt, C.C Phytopathology 89: Multilines (R2 is genotype compatible with race 2, but not with race 1) Simple mixtures one susceptible and one immune plant genotype the 2 genotypes are completely mixed in space 4

5 inoculum is randomly distributed through field Leonard (1969) predicted that the reduction in disease due to decreased susceptible host tissue would be where y = proportion of infected host tissue in a pure stand of the susceptible genotype y = proportion of infected host tissue in the mixture y 0 = proportion of host tissue initially infected m = proportion of susceptible plants in the host mixture n = number of generations of disease increase This equation can be simplified to: How realistic is this simple model? 5

6 Effects of autoinfection: The proportion of pathogen inoculum retained on the same host plant on which it was produced. The degree of autoinfection is determined by interaction between plant size and dispersal gradient. Steeper dispersal gradients predicted to result in smaller host-diversity effects. Effects of genotype unit area (GUA) GUA = the area occupied by an independent unit of host tissue of the same genotype In general, the smaller the GUA, the greater the effect of host diversity. However, when disease occurs in discrete foci, large GUAs produced a larger decrease in disease. The number of GUAs may actually be more important than GUA itself, although this is very difficult to test experimentally. Example: Effect of GUA on crown rust of oats (Mundt, C.C. and Browning, J.A., Phytopathology 75: ) 6

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8 Effects of lesion expansion More auto-infection as there is a greater lesion expansion. Mixtures of 2 hosts with different disease resistance: A - one susceptible, one immune (simple mixture) B - both susceptible, but to different degrees (race non-specific) C - one susceptible to one race, one susceptible to another race (race-specific) For case B: Host diversity effect will depend on the difference in susceptibility between the two genotypes - the greater the difference in susceptibility, the greater the effect. where z = ratio of the susceptibility of the more resistant genotype to that of the more susceptible genotype For case C: Differential (race-specific) resistance results in dramatic host diversity effects Why? - reduced proportion of tissue that is susceptible to races that can infect it - physical barrier effect - competition/compensation effects - increased potential for induced host resistance - interactions among pathogen races? 8

9 Predicting host diversity effects 9

10 Reducing the duration of the epidemic Control based on decreasing the duration of the epidemic by decreasing the time that the crop is actually exposed to the pathogen. 1. Use of transplants to quickly establish crop Example: Effects of direct seeding vs. transplants on severity of corky root on lettuce (Van Bruggen and Rubatzky, Plant Dis 76: ) 2. Use of short season or early-maturing varieties 10

11 3. Adequate fertilizer and soil moisture to promote rapid plant growth and development 4. Manipulation of planting and harvesting dates Diseases reduced by early planting downy mildew of corn leaf blight of corn common smut of wheat fungal root rots of wheat powdery mildew of peas blackleg of canola Diseases reduced by late planting black rot of peanut barley yellow dwarf in rice downy mildew of sunflower Fusarium root rots of peas and beans Fusarium wilt of tomato (winter-fl) Fusarium wilt of lettuce (winter-az) Example: Fusarium wilt of lettuce (Matheron, M. E., McCreight, J. D., Tickes, B. R., and Porchas, M Effect of planting date, cultivar, and stage of plant development on incidence of Fusarium wilt of lettuce in desert production fields. Plant Disease 89: ) 11

12 Summary of epidemiological effects of various methods of disease control (Zadoks and Schein, Epidemiology and Plant Disease Management) 12