Nov 7, 2013 - Abstract Black leaf mold (BLM), caused by. Pseudocercospora fuligena is a serious threat to tomato production in the humid tropics. Accurate ...
Eur J Plant Pathol (2014) 138:39–49 DOI 10.1007/s10658-013-0295-3
Effects of temperature, wetness duration and leaf age on incubation and latent periods of black leaf mold (Pseudocercospora fuligena) on fresh market tomatoes Zelalem Mersha & Shouan Zhang & Bernhard Hau
Accepted: 12 September 2013 / Published online: 7 November 2013 # KNPV 2013
Abstract Black leaf mold (BLM), caused by Pseudocercospora fuligena is a serious threat to tomato production in the humid tropics. Accurate information about the incubation (IP) and latent period (LP) under various host susceptibility and weather favourability circumstances will help to formulate holistic approaches to manage this disease. In this study, effects of temperature, wetness duration, and leaf age on the monocyclic components (IP and LP) of BLM were studied from growth chamber (GC) and greenhouse (GH) experiments as well as detached leaf assays in growth cabins. Linear interpolation and inflection point (of logistic regression model) methods were used to determine IP and LP. These two methods were highly correlated in GC (r2=0.89; P 85 % were delineated as favourable based on earlier reports (Hartman et al. 1991; Wang et al. 1996). A value of 1 (favourable) was nominally assigned to a given day or night if mean temperature or RH falls within the above range. Similarly, a value of 0 (non-favourable) was assigned if the mean value falls outside the given range. For each experiment, favourability index (FI) of temperature (FIT) and relative humidity (FIRH) were calculated from the mean value of day (FIDT, FIDRH) and night (FINT, FINRH) favourability indices, respectively. Thus, FIT = 0.5 · (FIDT + FINT) and FIRH = 0.5 · (FIDRH + FINRH). In addition, frequency of uninterrupted weather favourability, i.e., prevalence of concurrent optimal T and RH (FITRH) for at least 18 h in any given day, was counted for each experiment. The interval from 6:00 am to 5:59 pm and from 6:00 pm to 5:59 am was considered as day and night times, respectively. Disease assessment Growth chamber Disease incidence in growth chambers was monitored at 2-day intervals for the first 18 DAI and at 4-day intervals afterwards until 34 DAI. Disease incidence of symptomatic units (DISY) was recorded based on the percentage of leaflets with BLM symptoms relative to the total number of inoculated leaflets. Disease incidence of sporulating (those leaflets which showed dark grey to fuliginous lesions) units (DISP) was monitored visually or aided by a 10× magnifying glass. Proportions of DISY and DISP were used to determine IP and LP, respectively, using the two methods stated below. In addition, disease severity on the terminal leaflets of each plant was monitored at the end of an experiment. Detached leaf assay For the detached leaf assay, symptom appearance was monitored and colonies were counted every day for the first 10 DAI and then at 2 to 5-day intervals until 30 DAI. The incubation period was determined as described below (methods 1 and 2). Visual observation of sporulation on detached leaflets, even with a stereo microscope aided at 63× magnification was ambiguous, and hence no latent period was determined in this case.
Eur J Plant Pathol (2014) 138:39–49
Greenhouse Disease incidence of symptomatic (DISY) and sporulating (DISP) leaflets was monitored at 2-day intervals. Inception of sporulation was decided visually or using a hand held magnifying lens. Lesion expansion (in mm) was measured on the marked leaflets at 10, 16, 20 and 24 DAI. Statistical analyses For growth chamber and greenhouse experiments, areas under the curves (AUC) of incidence of symptomatic (DISY) and sporulating leaflets (DISP) were calculated from the progress curves using the trapezoidal method. For the detached leaf assay too, AUC of symptomatic colony density was calculated using the same method. In addition, maximum density of symptomatic colony was compared amongst the treatments for the detached leaf assay in petri dishes. For all experiments, statistical significance of the three main factors (T, A and WD) as well as their interactions was tested using PROC GLIMMIX procedure in SAS (SAS Institute Inc., Cary, NC). All means were separated according to Tukey’s test at P=0.05. Determination of incubation (IP) and latent periods (LP) Method 1 — linear interpolation In growth chamber and greenhouse experiments, IP was calculated by counting the number of days from inoculation to the time when 50 % of the maximum observed DISY was achieved. For detached leaf assays, the time at which 50 % of the maximum observed colony number attained was considered IP. Since sampling intervals were spaced unequally, a linear interpolation (Eq. 1) method was used to estimate IP. h i . . IP ¼ t 1 þ maxDI SY 2−DI SY ðt 1 Þ ⋅ðt 2 −t 1 Þ ð1Þ ½DI SY ðt 2 Þ−DI SY ðt 1 Þ In Eq. 1 for the incubation time IP, t1 is the time of the last observation of DISY just before 50 % of the maximum observed DISY (max DISY) was reached, t2 is the observation time just after surpassing the 50 % of the maximum value. To determine the latent period LP,
Eur J Plant Pathol (2014) 138:39–49
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in the GC and GH experiments, all DISY notations in Eq. 1 were replaced with DISP.
significant or there is a big difference in IP and LP determined using the two methods.
Method 2 — inflection point
Effect of wetness duration on monocyclic components
At times when asymptotic behavior was observed, three-parametric logistic model fitted well to the actual disease progress of DISY, DISP or colony density over the assessment time (t). Based on the principle that a logistic growth function has an inflection point at a half disease level compared to the maximum, the incubation time IP was estimated as time when this inflection point was reached by fitting the modified logistic growth function (Eq. 2) to DISY.
In growth chamber experiments, no disease appeared on leaves of 7-week old tomato plants or the disease was distinctively lower when inoculation with P. fuligena was not followed by extended wetness duration, compared to those leaves which were maintained humid for durations of 1–3 days (Fig. 2). Area under the curve (AUC) comparisons revealed an average of 84–100 % and 97–100 % more incidence of symptomatic leaflet (DISY) (Fig. 3, left) and sporulating leaflets (DISP)
DI SY ðt Þ ¼ DI SY max
.h i 1 þ e−rL ðt−IPÞ
ð2Þ
a
Growth Chamber Experiments y = -0.38 + 1.05x (r2 = 0.89, P < 0.0001)
The parameter DISYmax is the estimated maximum DISY, and rL is the estimated logistic disease rate. To determine LP, Eq. 2 was fitted to data of DISP.
Results Comparison of linear interpolation and inflection point methods
IP and LP (in days) according to inflection point method
35
30
25
20
15
10
5 5
15
b
20
25
30
35
Greenhouse Experiments 16
IP and LP (in days) according to inflection point method
There was a very strong linear correlation between the two methods that were used to determine the incubation and latent period of black leaf mold (Fig. 1). In growth chamber experiments, the correlation was high (r2=0.89, P
