Characterization of Resistance to Early Blight in Three Potato Cultivars: Incubation Period, Lesion Expansion Rate, and Spore Production

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1 Ecology and Epidemiology Characterization of Resistance to Early Blight in Three Potato Cultivars: ncubation Period, Lesion Expansion Rate, and Spore Production J. R. Pelletier and W. E. Fry First author: Agriculture Canada, Research Station, P.O. Box 457, St.-Jean-sur-Richelieu, Quebec, Canada J3B 6Z8; second author: Department of Plant Pathology, Cornell University, thaca, New York We thank Jim Sweigard for helping field work. Supported in part by Agriculture Canada and USDA PM Grants 85-CRSR and 87-CRSR The first author was a recipient of an FCAC postgraduate fellowship. Contribution 335/89.05.R, Research Station, Research Branch, Agriculture Canada, St-Jean-sur-Richelieu, Quebec, Canada. Accepted for publication 6 September ABSTRACT Pelletier, J. R., and Fry, W. E Characterization of resistance to early blight in three potato cultivars: ncubation period, lesion expansion rate, and spore production. Phytopathology 79: Early blight incubation periods (degree-hours from inoculation until 18-day-old Norchip shorter than on Kennebec and Rosa; 45-day-old lesion radius reaches 0.35 mm) and lesion expansion rates (micrometers of plants no longer markedly different. Lesion expansion rates lesion radius per degree-hour during leaf wetness) measured on intact lowest on Rosa. Cultivar differences became less apparent as plants plants of field-grown potato cultivars Kennebec, Norchip, and Rosa at grew older. Spore production per square millimeter of lesion was measured about 18, 30, and 45 days after emergence. ncubation periods shorter from individual lesions on leaves harvested from field-grown plants at 74 and lesion expansion rates greater on lower leaves and on older plants. and 92 days after planting. Spore production increased as a linear function During leaf wetness, lesion expansion increased in a linear manner over a of lesion area. Plant age (Pdays after emergence) and leaf position did not range of 6 to 24 hr and in response to temperatures ranging from 9 to 27 C. have a significant effect on spore production. Spore production was lowest ncubation periods and lesion expansion rates described by a linear on Kennebec; Norchip and Rosa not significantly different. Spore function of leaf position, plant physiological age (Pdays after emergence), production was not correlated maturity class of cultivar. and interaction of leaf position and plant age. ncubation periods on Additional keywords: Alternaria solani, heat summation, resistance components, Solanum tuberosum. Early blight, caused by Alternaria solani (Ell. & Mart.) Jones & NY. Norchip (early-maturing) is reported as highly susceptible, Grout, is one of major causes of defoliation of potatoes in Kennebec (mid-season) as having an intermediate level of norastern United States. t has been repeatedly observed that resistance, and Rosa (late-maturing) as being highly resistant to early-maturing cultivars are more susceptible than later-maturing early blight (1). Potato seed consisted of small whole tubers or seed cultivars (1,4,11). Furrmore, older leaves are more susceptible pieces (Kennebec, 1984) weighing about 50 g each. Seed pieces to early blight, and susceptibility increases as plants grow older treated mancozeb dust (Manzate 8D, E.. dupont de (9,11-13,16). We observed that infection efficiency (number of Nemours and Co., Wilmington, DE). Four-row plots (16 lesions per number of conidia applied) was greater on older leaves plants/row), 3.7 m long, 0.9 m between rows, planted and increased markedly as plants grew older (8). However, a 23-cm spacing between plants. The plots planted in differences in infection efficiency of A. solani among cultivars did areas that not planted potatoes for two previous not fully explain early blight resistance ranking of three potato years. Four plots of each cultivar planted in a completely cultivars. Thus, or components of resistance, such as randomized design. Fertilizer (167 kg each of N, P, and K per incubation period (P), lesion expansion rate (LER), and spore hectare) and insecticide carbofuran (Furadan 15G, 3.4 kg production (SP), may contribute to field-observed differences in a.i./ha, FMC Corp., Middleport, NY) applied at planting. cultivar resistance (4,9,11,12,16). For example, Harrison et al observed large lesions on lower leaves, while upper leaves The herbicide linuron (Lorox 50 WP, 1.7 kg a.i./ha, E.. du Pont de Nemours and Co., Wilmington, DE) was applied after planting asymptomatic even though y harbored pathogen (4). but prior to plant emergence. During growing season, Most studies, however, do not mention wher larger lesions insecticide azinphos-methyl (Guthion 25EC, 0.88 kg a.i./ha, result from a shorter P, a greater LER, or a combination of both. Chemagro Corp., Kansas City, MO) or endosulfan (Thiodan Likewise, cultivar differences in SP have been documented for A. 5OWP, 1.1 kg a.i./ha, FMC Corp., Middleport, NY) was applied solani on tomato (7). However, in order to construct an early blight when necessary. Metalaxyl (Ridomil 2EC, kg a.i. /ha, Cibasimulation model which is useful in pest management, re Geigy Corp., Greensboro, NC) was applied to prevent late blight, remains need to determine wher leaf position and plant age caused by Phytophthora infestans (Mont.) de Bary, on a 2-wk affect SP in potato, schedule from 12 July to 9 August 1984 and from 1 August to 15 n this report, plant-age-related changes in P, LER, and SP in August Percent plant emergence was determined for 256 three potato cultivars different degrees of resistance are plants per cultivar on 13 June, 15 June, 19 June, and 22 June 1984 described. These detailed data are essential in construction of a and for 384 plants per cultivar on 15 June, 17 June, 19 June, 22 simulation model for management of potato early blight. June, and 26 June Plants hilled on 3 July 1984 and July MATERALS AND METHODS Production of inoculum. Conidia of a field isolate of A. solani used for P and LER experiments. noculum was Cultural conditions. Certified seed tubers of Kennebec, prepared by a modified technique for conidium production of Norchip, and Rosa potatoes planted on 25 May 1984 and 24 Cochliobolus heterostrophus (0. C. Yoder and J. Leach, personal May 1985 at Homer C. Thompson Research Farm at Freeville, communication). Mycelium-permeated mesh circles of Handi- Wipe (Colgate-Palmolive Co., New York, NY) produced by 1989 The American Phytopathological Society placing mesh on inoculated V-8 agar petri dishes for 7 days at Vol. 79, No. 5,

2 21 C in a dark incubator. The mesh was n removed, scraped potato growth, based on a minimum temperature of 7 C, an a rubber policeman, and placed on lid of an inverted water optimum of 21 C, and a maximum of 30 C (14). The Pdays foreach agar petri dish. The dishes placed for 5 days at 18 C under day determined by calculating physiological time Cool White fluorescent tubes (Sylvania, F40WW) a 12-hr accumulated during each hour and summing for each 24-hr period. photoperiod. The conidium-covered mesh was n stored in a Physiological time was calculated as follows: desiccator at 4 C until use 2 to 8 wk later. Quantification of effects of cultivar, plant age, and leaf position Pi = 0 Ti < Ta on P and LER. Because of difficulty in maintaining vigorous detached leaves for periods greater than 7 days, se studies PT=O.417[1-((Ti- Tb) 2! (Tb-- Ta) 2 )] Ta Ti Tb carried out in field plots on intact potato plants of cultivars Kennebec, Norchip, and Rosa. The P and LE R experiments carried out as follows: n 1985, four stems from two plants of each Pi=0.417[1-((Ti- Tb) 2 / (T,.- Tb) 2 )] T5 < Ti < T,. cultivar inoculated on 28 June, 10 July, and 25 July a Pi = 0 T" T, (1) single field isolate of A. solani. The plants sprayed a conidial suspension at 9:00 P.M. and covered plastic bags until 9:00 following morning. The position of leaves present at where Pi is physiological time accumulated during ith hour time of inoculation was noted as node number from soil of day, Ti is mean temperature (C) during ith hour, Ta is line. Leaves photographed at 4- to 12-day intervals until 7 C, T. is 21 C, and T, is 30 C (14). abscission occurred. The photographs enlarged and lesions Effects of duration of leaf wetness and temperature on lesion darkened a fine-tipped felt pen. The prints placed on a expansion. Because of space limitations, effects of temperature and light table, and area of individual lesions was determined a leaf wetness duration on radial growth of lesions video image analysis system (Delta-T Devices Ltd., 128 Low Road, determined in separate greenhouse and incubator experiments. Burwell, Cambridge, England). Norchip plants grown in greenhouse in clay pots, 12.7 cm The Ps and LERs corrected for variable environmental in diameter, containing a peat-vermiculite mixture (1:1, v/v) conditions in field, using temperature data and duration of amended 2.3 kg of (N-P-K) per cubic yard of periods of leaf wetness recorded during experiments. Leaf mixture. The plants inoculated 50 days after emergence wetness duration was measured a leaf wetness sensor (3) an aqueous conidial suspension (3,000 conidia per milliliter) of fastened to adaxial surface of an intact potato leaflet. The field isolate of A. solani. The plants placed in a mist chamber sensor was placed in a plot of Norchip at 30 cm from soil and at 20 C for 24 hr a 12-hr photoperiod. They returned to tilted at 300 from horizontal. Both sensor and leaf secured greenhouse bench until appearance of necrotic spots on wooden stakes to prevent movement and damage to leaves (about 7 days after inoculation). The plants n leaflet's surface by sensor. The sensor was connected to an AC returned to mist chamber (20 C) and exposed to a 12-hr channel of a CR21 data logger (Campbell Scientific, nc., Logan, light-dry and 12-hr dark-mist regime for 7 days. Under se UT), and leaf wetness readings recorded every 20 min. The conditions, lesions expanded and assumed characteristic response of sensor ranged from 0 mv (dry leaf surface) to a "target board" appearance of lesions in field. maximum of 150 mv (continuous water film). From observations The effect of temperature during leaf wetness on lesion of leaves in a mist chamber, readings of 10 mv and over expansion was quantified early-blighted leaves, which considered to be from wet leaves. Because canopy densities of detached, tagged, and photographed. The leaves n misted three cultivars similar for duration of experiment distilled water, placed in sealed plastic bags, and placed for 12 (Table 1), leaf wetness data from Norchip canopies hr in dark incubators set at 9, 15, 18, 21, 24, or 27 C. After used for Kennebec and Rosa. A preliminary study in 1984 revealed exposure to different temperatures, leaves that when plants still small ( a leaf area of about 0.6 photographed again. The areas of lesions before and after m2 per plant), leaf wetness duration was not markedly different for treatments measured Delta-T image analysis system. sensors placed at heights of 15, 30, and 45 cm in canopy. Later The lesion radius was estimated by formula for a circle. The in growing season, leaf wetness durations at 15 cm experiment was performed three times, and areas of 141 lesions markedly shorter than those at 30 or 45 cm. However, most of measured. leaves at 15 cm had died by mid-season and no longer a factor. The effect of duration of leaf wetness on lesion expansion Temperature was recorded a hygrormograph in a louvered was determined in a similar manner. Leaves lesions wear shelter placed between plots at 20 cm above soil treated as above and placed in an incubator at 21 C for 6, 9, 12, 15, surface. 21, or 24 hr. Leaf surfaces kept wet while in incubator, and From temperature data, physiological age of plants leaves photographed immediately after leaf wetness was expressed as Pdays accumulated from median emergence exposure. The experiment was done twice, and areas of 101 date of each cultivar. Pdays are a measure of rmal time for lesions measured. The radial growth of lesions was determined as for temperature experiments. Calculation of P and LER. To incorporate effects of time and temperature on fungal growth, Ps and LERs expressed TABLE 1. Leaf area per plant at three inoculation times for potato in degree-hours. The degree-hours for development ofa. solani cultivars Kennebec, Norchip, and Rosa calculated from relationships between relative growth rate of lesions and temperature and duration of leaf wetness Day of year (see Results). Firstly, equation 5 (see Results) was simplified by subtracting minimum temperature, to give Cultivar ( 18)" (30) (45) Kennebec 0.20h RLRG = X (TEMP ) (2) (0.02) (0.06) (0.06) where RLRG is lesion radial growth relative to that at 27 C, and Norchip TEMP is temperature (C). Equation 2 was n multiplied by (0.03) (0.14) (0.15) 0.041, from equation 5. An intercept of 0 was used for effect of Rosa leaf wetness duration on relative lesion radial growth, because lesions on greenhouse-grown plants did not expand in absence (0.05) (0.09) (0.10) of leaf wetness, and y intercept of equation 5 was not 'Days after median emergence. significantly different from 0. Degree-hours n calculated bleaf area (in 2 ) per plant. Numbers in parenses are standard errors. from mean hourly temperatures for each hour after inoculation: 512 PHYTOPATHOLOGY

3 n degree-hours Y X (T ) (3) i=1 12- A. KENNEBEC where T; is average temperature during ith hour Because preliminary analysis revealed that lesion radius increased in a linear manner cumulative degree-hours, a / simple linear equation of lesion radius as a function of cumulative 0" 8- / degree-hours was calculated for each leaf on each stem. From se F / equations, P for each leaf was calculated as degree-hours U. / from inoculation to occurrence of a mean lesion radius of 0.35 z1: 6-1 / mm. This end point was selected because of limits of resolution W of image analysis system and because lesions smaller P / 2 diameters are still turgid from internal leaf moisture. Fungal CO D 4- /, 2 Z) W 3 growth in lesions a radius of 0.35 mm or less appeared to be o 0 independent of leaf surface moisture. Microscopic examination of z ' 2-, lesions on Kennebec, Norchip, and Rosa revealed that A. solani did not sporulate in lesions less than 0.35 mm in radius. The LER for each leaf was estimated by slope of curve plotting 0-A lesion radius as a function of cumulative degree-hours Measurement of SP. Experiments done in 1984 on leaves L POSTO harvested from field-grown Kennebec, Norchip, and Rosa potato LEAF POSTON 12 B A. NORCHP O,' 8- o.6-6- OW z Qo. m (5 M_ ua W/ w C) Z.-----Y Q z 2- > 0.2- / LU Ž0.2 0r LEAF POSTON TEMPERATURE (0C) 12- C. ROSA 2 o czt 6-1 O LU OU Z - 2U2.._z LU 0.4 / 0_,.n B. 10- Ho.2- LU rr 5 9 LEAF POSTON Fig. 2. Effect of plant age (Pdays) and leaf position on incubation TME: (HOURS) period of Kennebec (A), Norchip (B), and Rosa (C). The smallest numbers on x axis designate leaves occupyinig lowest postions in canopy. Fig. 1. A, Effect of temperature during a 12-hr period of leaf wetness on The numerals 1, 2, and 3 are means (n =4) of observed incubation lesion radial growth, relative to mean radial growth at 27 C. B, Effect of periods for plants inoculated at first, second, and third inoculation duration of leaf wetness at 21 C on lesion radial growth, relative to times (35, 47, and 62 days after planting, respectively). The lines are mean radial growth for 24 hr. model predictions. Vol. 79, No. 5,

4 plants to determine influence of plant physiological age and ml of 1% CuSO 4 (w/w) and two drops leaf position of Tween on 20 SP. per Leaves 450 ml. harvested at 3:00p.m. at 74and 92 The CuSO 4 was added to prevent days spore after germination. planting. Plant physiological age in Pdays was The spores from calculated each lesion at each harvest. n transferred Leaf position to a grid in for canopy was noted as counting by filtering spore number suspension of nodes through from a piece soil of graph line. Conidia that had formed in paper a 2.5-mm grid. The conidia on field filter prior paper to SP experiments blown off both sides of counted under a lesions dissecting microscope. a compressed air jet. The leaves n placed mmediately after harvest, upright in leaves trays of distilled photographed, water. Selected prints leaflets misted made, and area distilled of necrotic water and tissue enclosed in each in lesion a moisture was chamber to induce measured from photographs sporulation. a Delta-T A moisture image chamber analysis system. consisted of a plastic 9-cm petri For remainder of this paper, term lesion dish refers lined only to moistened filter paper, a notch cut in side of necrotic portion dish to of admit lesions. petiolule. The leaves placed in a dark Statistical analysis. incubator Median at emergence 18 C for 12 dates hr. Conidia calculated n by collected from both probit analysis of emergence as sides a function of each of lesion days after planting, a vacuum by collector (ER Machine Shop, means of PROC PROBT of owa Statistical State University, Analysis 124 System ER Bldg., (15). Ames) into a vial containing To determine effect of temperature on LER, radial TABLE 2. Regression results for effect of leaf position and plant age on incubation periods for potato cultivars Kennebec, Norchip, and Rosa Cultivar Sourcea dfb SSC R 2 adjd Variable Coefficient SS le P Kennebec Model Error Total Norchip Model Error Total Rosa Model Error Total a Only significant variables are listed. 'Degrees of freedom. 'Sequential sums of squares. dcoefficient of determination (adjusted for degrees of freedom). 'Partial sums of squares. 'Leaf position from soil line. g Plant age in Pdays accumulated from emergence. ntercept Leaf' Age' X leaf ntercept Leaf Age ntercept Leaf Age X leaf TABLE 3. Regression results for effect of leaf position and plant age on lesion expansion rate for potato cultivars Kennebec, Norchip, and Rosa Cultivar Source" dfh SSc R 2 adjd Variable Coefficient SS le P Kennebec Model Error Total Norchip Model Error Total Rosa Model Error Total a Only significant variables are listed. bdegrees of freedom. C Sequential sums of squares. dcoefficient of determination (adjusted for degrees of freedom). C Partial sums of squares. fleaf position from soil line. g Plant age in Pdays accumulated from emergence. ntercept Leaf Ageg X leaf X l ntercept Leaf Age Age X leaf X ntercept Leaf Age X leaf X PHYTOPATHOLOGY

5 growth of lesions for each temperature was expressed as a by lack of interaction of plant age and leaf position, while proportion of mean radial growth at 27 C (at which lesion effect decreased significantly in Kennebec and Rosa, as seen by expansion was most rapid) and regressed as a function of lower slopes of regression lines on older plants than on younger temperature. To determine effect of duration of leaf wetness plants (Fig. 2). This was supported by significant interaction of on lesion growth, radial growth of lesions was expressed as a plant age and leaf position (Table 2). The P became shorter on proportion of mean radial growth for 24 hr and regressed as a older plants, which is indicated by downward shift in function of duration of wetness period, regression lines for plants of increasing age (Fig. 2). Cultivar Linear regressions of lesion radius as a function of cumulative degree-hours since inoculations done PROC GLM of Statistical Analysis System (15) Preliminary analyses suggested linear effects of plant A. physiological age (Pdays) and leaf position on P, LER, and SP. KENNEBEC Therefore, only linear variables included in models. The r-- effects of leaf position, plant physiological age, and product of < leaf position and plant age on P, LER, and SP per square z 0 millimeter identified for each cultivar by stepwise regression 0, by means of PROC STEPWSE of Statistical Analysis System Z L1.>Q (15). Variables added to models if F statistic was < 500- significant at 0.15 level. After a variable was added, variables X. already in model removed if y did not produce an F z E statistic that was significant at 0.15 level. z E 3 ý uj RESULTS Plant emergence. Probit analysis revealed that in 1984, Norchip 0- emerged first, at 18.7 days after planting, followed by Kennebec at days and Rosa at 23.0 days after planting. The median LEAF POSTON emergence dates in , 17.2, and 17.6 days after planting for Norchip, Kennebec, and Rosa, respectively Therefore, for P and LER experiments in 1985, Norchip was B inoculated at 18.7, 30.7, and 45.7 days after emergence (137.9,. - NORCHP 238.6, and Pdays, respectively); Kennebec was inoculated at,-, , 29.8, and 44.8 days after emergence (131.6, 232.3, and < 750- Pdays, respectively); and Rosa was inoculated at 17.4, 29.4, and z days after emergence (128.7, 229.4, and Pdays, 0 - \q:, respectively). For SP experiments in 1984, Norchip was W j inoculated at 51.3 and 69.3 days after emergence (413.7 and < tr Pdays, respectively); Kennebec was inoculated at 49.5 and 67.5 W -U - 3 days after emergence (400.2 and Pdays, respectively); and z 1 \ Rosa was inoculated at 47.0 and 65.0 days after emergence ( E", and Pdays, respectively). co- 2 1, Effects of temperature and duration of leaf wetness on radial -j growth of lesions. The effect of temperature on radial growth was as follows: 0-1 l RLRGtemp) = X TEMP R 2 = (4) LEAF POSTON in which RLRG(tmp) is lesion radial growth relative to that at 27 C, C. and TEMP is temperature (C) (Fig. A). The minimum ROSA temperature for lesion expansion (3.52 C) was estimated by " calculating x intercept of equation 4. r--, The effect of duration of leaf wetness on radial growth was as m 70 follows: ZO 1: RLRG(time) = XTME R 2 =80.31 (5). zwl0 ( in which RLRG(time)is lesion radial growth relative to that for 24 hr, w3 rn.x"'- and TME is leaf wetness duration (hr) (Fig. 1B). Z E 1 Effect of leaf position, plant age, and cultivar on P. For each qo 25 - "."- cultivar, P was regressed as a function of leaf position, plant age, 1 2 :3 and product of leaf position and plant age. Pairwise comparisons revealed that multiple regression lines of three cultivars significantly different (P = 0.001). Therefore, 0-,, model containing only significant independent variables was identified for each cultivar by stepwise regression (15). Stepwise LEAF POSTON regression revealed that for three cultivars, P was shortest on lower, and refore oldest leaves (Fig. 2). This was Fig. 3. Effect of plant age (Pdays) and leaf position on lesion expansion rate of Kennebec (A), Norchip (B), and Rosa (C). The smallest numbers on supported by positive coefficients for leaf position effect x axis designate leaves occupying lowest positions in canopy. (Table 2). Leaf position effects greater for Kennebec and The numerals 1, 2, and 3 are means (n = 4) of observed lesion expansion Rosa than for Norchip, based on coefficients from stepwise rates for plants inoculated at first, second, and third inoculation times regression (Table 2). The effect of leaf position on P did not (35, 47, and 62 days after planting, respectively). The lines are model change markedly in Norchip as plants grew older, as indicated predictions. Vol. 79, No. 5,

6 differences most apparent early in growing season. Visual leaf disks (6). The shortest P observed in comparison field was about of 56 regression hr lines for plants inoculated at three (about 0.04 degree-hours per hour), which different was ages consistent revealed that Ps for Norchip plants for first previously observed values of hr inoculation (6), hr (11), markedly and 48 hr shorter than Ps for Kennebec and (13). The longest P we observed was about 12 days on leaves Rosa. of However, Ps for three cultivars not markedly young Rosa plants. different by third inoculation. Ultimately, Ps and leaf Differences among three cultivars more pronounced for position effects became similar on three cultivars. These trends reflected by negative coefficients for plant age and for interaction of leaf position and plant age (Table 2) Effect of leaf position and plant age on LER. For each cultivar, A. LERs regressed as a function of leaf position, plant age, and KENNEBEC product of leaf position and plant age. Pairwise comparisons revealed that multiple regression lines of three cultivars significantly different (P = 0.001). Therefore, model containing only significant independent variables was identified for each cultivar by stepwise regression (15). Stepwise regression - - results revealed that LERs for three cultivars greater on lower, older leaves than on younger leaves (Fig. 3). This was Z - + supported by negative coefficients for leaf position effect 0 (Table 3). For Norchip and Rosa, difference in LER for leaves in different positions was greatest early in growing season. For Kennebec, leaf position effect did not change increasing plant age. This is indicated by parallel regression lines of LER + A + against leaf position at different plant ages for Kennebec in Figure A and lack of a significant term for interaction of leaf _+ position and plant age (Table 3) The 60 LER in 80 three 100 cultivars increased as plants grew older, LESON AREA C mm 2 ) which was reflected by upward shift in regression lines (Fig. 3) and positive coefficients for plant age and for interaction of leaf position and plant age (Table 3). Early in season, LERs for early-blight-susceptible Norchip greater than B. those for Rosa, while LERs for Kennebec tended to be greater NORCHP than those for Norchip and Rosa. By third inoculation, LERs for three cultivars similar. Effect of cultivar, lesion area, plant age, and leaf position on SP For each cultivar, stepwise regression revealed that neir plant < age nor leaf position had a significant effect on number of conidia + produced per square millimeter of lesion area (Table 4). 10 Spore production was a simple linear function of lesion area (Fig. 4). 0 Pairwise comparison of regression lines indicated per that square SP millimeter 0 of lesion area was significantly lower on Kennebec than on Norchip or Rosa. The regression lines for + + Norchip and Rosa not significantly different at 0.05 level From y intercept of regression equations listed in Table 4, + + minimum lesion area for SP was estimated to be mm 2 for h Kennebec, mm 2 for Norchip, and mm 2 for Rosa. 0 " DSCUSSON LESON AREA ( mm 2 ) The P of A. solani was shorter and expansion rate of early blight lesions was greater younger, on upper older, leaves lower and leaves on older than potato on plants than on 3000 younger C. ROSA ones. These findings for plot-grown potatoes confirm previous observations on potted potatoes (6,10,12,13,16) and on floating TABLE 4. Regression results for effect of potato cultivar and lesion < 2000 area on spore production Cultivar Source dfa SSb P Coefficient O Kennebec ntercept 1 324, ( Area 1 31,867, Error 66 9,960,664 Norchip ntercept 1 890, Area 1 31, 197, Error 56 11,872,2230 Rosa ntercept 1 2,403, Area 1 32,303, LESON AREA C mm 2 ) Error 48 3,706,781 "Degrees of freedom. 5 Fig. 4. Number of conidia produced per Sums square of squares. millimeter of lesions on Kennebec (A), Norchip (B), and Rosa (C). 516 PHYTOPATHOLOGY

7 P than for LER, P being shortest on early-blight- studies (unpublished), we believe that we can now explain susceptible and early-maturing Norchip. However, re was no relative resistance rankings of three cultivars. Norchip discernible trend between P for three cultivars and tuber (susceptible) has a short P, a rapid LER, and high sporulation, dry weight or ratio of tuber to shoot dry weight. Likewise, no and it supported highest infection efficiency. Kennebec discernible relationship was found between LER and se two (moderately resistant) has a moderate P (long on young leaves or growth variables (unpublished results). young plants), a moderate LER, and moderately high sporulation, We used same temperature response function for both P and it supported a high infection efficiency. Rosa (most resistant) and LER. These two components both reflect process of host has a moderate P, a low LER, and high sporulation, but it colonization. Because A. solani is present only in necrotic host supported lowest infection efficiency (unpublished results). tissue (11), we assumed that lesion expansion was dependent on leaf surface moisture and temperature. Our findings that LER LTERATURE CTED increased temperature are contrary to those of Pound (9) for potted tomato plants. However, since he kept his plants on a 1. Douglas, D. R., and Pavek, J. J Screening potatoes for field greenhouse bench and leaf surfaces presumably kept dry, 2. Habgood, R. M Resistance of barley cultivars he may have been measuring effect of temperature on Rhynchosporium secalis. Trans. Br. Mycol. Soc. 69: collapse and desiccation of chlorotic tissue around necrotic 3. Hackel, H Eine elektrische Methode zur Messung der tissue, and not colonization of leaf tissue per se. Bldittbenetzungs-dauer unmittelber am Blditt. Agric. Meteorol. The number of conidia produced per unit of lesion area was 13: found to depend only on cultivar and not on plant age or leaf 4. Harrison, M. D., Livingston, C. H., and Oshima, N position. There was no correlation between maturity class of Epidemiology of potato early blight in Colorado.. nitial infection, cultivars and SP, although our results based on only three disease development and influence of environmental factors. Am. cultivars. For example, re was no significant difference between Potato J. 42: elater-maturing Rosa. Our 5. Hooker, A. L., Hilu, H. M., Wilkinson, D. R., and Van Dyke, C. G. early-maturing Norchip and because Rosa. our Additional sources of chlorotic-lesion resistance to results are consistent those of ors because we have found Helminthosporum turcicum in corn. Plant Dis. Rep. 48: no examples in literature of plant age and leaf position effects 6. McCallan, S. E. A., and Wellman, R. H A greenhouse method of on SP by necrotrophic pathogens. However, lack of effect in evaluating fungicides by means of tomato foliage diseases. Contrib. our study may also be due to advanced age of plants when Boyce Thompson nst. 13: leaves harvested (about 400 and 530 Pdays). f effect of 7. O'Leary, D. J., and Shoemaker, P. B Components of resistance leaf position on SP decreased as plants grew older, such for tomato early blight. (Abstr.) Phytopathology 73:803. differences may have disappeared by time we harvested 8. Pelletier, J. R Computer simulation of cultivar resistance and lesions. fungicide effects on epidemics of potato early blight. Ph.D. sis, lower SP millimeter of lesion area on Kennebec Cornell University, thaca, NY. 127 pp. Thelativer to per andrsquare millmeterplesioned ba lower rtene 9. Pound, G. S Effect of air temperature on incidence and relative to Norchip and Rosa may be explained by a lower rate of development of early blight disease of tomato. Phytopathology sporulation. Lower rates on resistant cultivars have been observed 41: for A. solani on tomato (7) and or necrotrophic pathogens, such 10. Pscheidt, J. W Epidemiology and control of potato early blight, as Helminthosporium turcicum (5) and Rhynchosporium secalis caused by Alternaria solani. Ph.D. sis, University of Wisconsin, (2). However, although sporulation rate of R. secalis was lower Madison. 242 pp. on resistant barley cultivars, re was no marked difference in 11. Rands, R. D Early blight of potatoes and related plants. Wis. total number of spores produced (2). An alternative explanation is Agric. Exp. Stn. Res. Bull pp. that lesion expansion outpaced SP in Kennebec, resulting in a 12. Rotem, J., and Feldman, S The relation between ratio of thar lower nyield number of spores per unit of lesion area.s.jagi.r to foliage and incidence of early blight in potato and tomato. minimum lesion areas for SP smaller than those found sr. J. Agric. Res. 15: The Ranimum (11), son obsrvedta for Sw t ere tanrths found o 13. Rowell, J. B Leaf blight of tomato and potato plants: Factors by Rands (11), who observed that conidia rarely found on affecting degree of injury incited by Alternaria dauci f. sp. solani. lesions areas less than about 12.6 mm 2. Univ. R.. Agric. Res. Stn. Bull The linear relationship between SP and lesion area was 14. Sands, P. J., Hackett, C., and Nix, H. A A model of somewhat difficult to understand. A curvilinear function might development and bulking of potatoes (Solanum tuberosum L.).. have been expected, arising from relationship between Derivation from well-managed field crops. Field Crops Res. 2: amount of sporulating and nonsporulating tissues on lesions of 15. SAS nstitute SAS User's Guide: Statistics. SAS nstitute, varying sizes. The linear relationship between SP and lesion area Cary, NC. 584 pp. 16. Sinden, S. L., Goth, R. W., and O'Brien, M. J Effect of potato alkaloids on growth of Alternaria solani and ir possible role as center of lesion. resistance factors in potatoes. Phytopathology 63: From information produced in this study and in companion Vol. 79, No. 5,