357 For. Snow Landsc. Res. 76, 3: 357–360 (2001)
Small format aerial photography to assess chestnut ink disease
Luis M. Martins, Maria I. Lufinha, Carlos P. Marques and Carlos G. Abreu
Universidade de Trás-os-Montes e Alto Douro, Dep. Prot. Plantas, Apt. 202, 5000-911 Vila Real, Portugal. lmartins@utad.pt
Abstract
Chestnut ink disease is a major threat to the orchards and areas with newly planted chestnut trees in Portugal. In this study, 460 ha of chestnut stands in north-east Portugal were selected as a test area for aerial photographic monitoring. A Cessna 150 aircraft, as the aerial platform, and a small format aerial colour photographic system, were used to survey the trees.
Results indicate that vegetative condition can be detected on 35 mm normal colour photo- graphs. According to the aerial observations, 5.3% (9.6% for ground truth) of the trees were dead. Moreover, 21.2% (21.9%) of the canopies had sparse and yellow foliage. Overall, the gener- al level of damage was 1.90 (1.93) on a 0–4 scale (0 = healthy).
No significant differences were found between the results of the ground surveys and aerial photographic observations. Therefore, small format aerial photography may be an effective and inexpensive tool to assess stress conditions associated with dieback diseases in chestnut stands.
Keywords: small format aerial photography, Castanea sativa
1 Introduction
Chestnut (Castanea sativa Mill.) is an important tree for forestry and agriculture in north- east Portugal. Interest in chestnut orchard cultivation is increasing due to the increased market demand for chestnut fruit. The result has been an increase in chestnut orchard plantation, mainly in the north of the country. However, poor phyto-sanitary conditions, in particular the chestnut ink disease, is still a major constraint for the recovery of chestnut planted areas.
The ink infection, induced by the soil-borne fungus Phytophthora cinnamomi Rands, causes the rot of chestnut root and collars. It produces yellowish, sparse foliage, dieback of branches and a gradual decline on infected trees. This condition, known as dieback-decline disease, makes the management of chestnut groves difficult because of the different biotic or abiotic factors which influence the fungus (ABREUet al. 1993a). The ink disease in north- east Portugal has a dispersed distribution. Affected trees can be found located near healthy trees and also grouped in clusters without healthy trees. This distribution means that system- atic sampling is required to quantify the incidence of the disease.
Early detection of the visible effects on the foliage and canopies of affected trees is possible using large-scale aerial photography (ABREUet al. 1993b, 2000, MARTINS1997, MARTINSet al.1997). Small format aerial photography (SFAP) techniques are useful in monitoring the diseases of chestnut stands due to their relatively low cost and the way in which they can individualize chestnut trees (ABREUet al. 2000, AMBROSINIet al.1997).
SFAP, using colour and colour infrared films, can be used to detect forest decline, insect attacks and tree root diseases (HOFFERand MYHRE1989, LILLESANDand KIEFER1999, WARNERet al. 1996). The infrared film can also predict these problems due the lower near infrared reflectance from infected trees (LILLESAND and KIEFER 1999). The goal of this study, however, was to detect the trees affected by ink disease with visible symptoms.
In this paper, a rapid, low cost yet effective system is presented, based on a 35 mm door- mounted camera designed for the Cessna 150 aircraft. The colour photos obtained compare well with ground truth observations.
358 Luis M. Martins et al.
2 Materials and methods
A study site from “Soutos da Padrela” (north-east Portugal) was selected as a test area for aerial photographic monitoring. It has 460 ha of orchards with chestnuts of different ages as well as newly planted areas. Black-and-white aerial photography (August, 1994; scale 1: 17 000) from the Instituto Português de Cartografia e Cadastro was used to identify the area and select the chest- nut stands.
A camera door-mount was designed for the Cessna 150 aircraft and adapted to a 35 mm reflex camera (Nikon F70). It consists of three sections that include a leading edge fillet, a camera com- partment, and a trailing edge fillet (Fig. 1). It is located in the right cabin window and makes it possible to take horizontal photographs and achieve a good orientation relative to the flight line.
An electronic intervalometer, adapted to the reflex camera was also made. It permits aerial photos with 60% overlapping.
On September 30, 2000, a flight at an average height of 400 m was made over the study area, with the orientation given by a Global Position System (Garmin III Pilot). The reflex camera, equipped with a standard 50 mm lens, was loaded with a Fujifilm Superia film (200 ASA). The intervalometer was programmed to take one photo every 2.3 seconds. A total of 8 x 36 photo- graphs were taken (negative scale = approximately 1: 8500) (Fig. 2). The negatives (24 x 36 mm) were amplified four times up to the standard format (10 x 15 cm) for visual interpretation.
Black and white aerial photographs were used in order to select sample plots in the study area.
Thirty-two plots (n1) were selected and their location transferred to colour SFAP. In order to assess the level of damage, 321 trees present in these chestnut plots (1000 m2, corresponding to 3.2 ha of the study area) were evaluated by visually interpreting the structural and chromatic charac- teristics of the foliage using a mirror stereoscope. Five levels of damage (0–4) were identified as part of the interpretation procedure (0 = healthy; 1 = slightly damaged; 2 = moderately damaged;
3 = heavily damaged; 4 = dying).
In the same study area, thirty (n2) independent plots (1000 m2) were also systematically inter- preted via ground surveys, using a diagrammatic scale for visual analyses (MARTINS1997).
For each level of damage, a t-test for discrete distributions was undertaken, since this test can be applied if the sample size (n) exceeds 30 (SNEDECORand COCHRAN1982). The formula for t is modified by substituting means and standard errors with the corresponding expression for the binomial distribution:
t = , Where p1= a1 = the number of items having a given
attribute in sample 1 and a2= the num-
p = ber in sample 2
q = 1 – p.
p1, p2= proportion of chestnut trees with attribute 0–4 on sample 1, sample 2;
n = sample size
Sample 1 and sample 2 = colour SFAP and ground truth.
Fig. 1. Cessna 150 aircraft with a camera door-mount on right cabin window.
(p1- p2) pq(n1+ n2)
n1n2
a1 n1 (a1+ a2) (n1+ n2)
359 For. Snow Landsc. Res. 76, 3 (2001)
3 Results
The number of chestnut trees separated by level of damage is shown in Table 1. The five levels of damage found in the colour photos are not significantly different from ground truth analyses according to the one-sided t-test (with 95% of significance). Overall, the mean level of damage was 1.90 (1.93 on ground truth) i.e. rougly corresponding to “moderately dam- aged”. This constitutes evidence that most chestnut trees in this area are declining.
Table 1. Average levels of damage assessed using colour photographs and ground truth, showing sig - nificant differences with t-test for Discrete Distributions. p1proportion of chestnut trees with levels of damage 0–4 in the colour photographs; p2proportion of chestnut trees with levels of damage 0–4 in the ground truth survey; n.s. no significant difference.
35 mm normal colour photographs Ground truth Average level Number of Number of
of damage chestnut trees % p1 chestnut trees % p2 t-test Significance
0 17 5.3 0.053 28 9.6 0.096 2.04 n.s.
1 102 31.8 0.318 84 28.8 0.288 0.81 n.s.
2 116 36.1 0.361 88 30.1 0.301 1.57 n.s.
3 68 21.2 0.212 64 21.9 0.219 0.22 n.s.
4 18 5.6 0.056 28 9.6 0.096 1.87 n.s.
Total 321 100 1 292 100 1
4 Discussion
The colour aerial photographic survey using a Cessna aircraft and standard photographic equipment was a more cost effective method of investigation than the conventional aerial photographic reconnaissance for monitoring chestnut ink disease. This tested technology is suitable for non-professional operators and its good results were obtained with commercial equipment.
Geographic Coordinates
7 6 5 4 3 2 1
Flight line Chestnut plots 20”
13”
6”
58”
51”
44”
37”
31”
41°35’0”
Chestnut area Scale
41°34’00”N
7°30’0”W 7°26’0”
214.2 m 8
Padrela e Tazém
S. João de Corveira
1000 m N
Fig. 2. Flight lines with geographic co-ordinates, covering the chestnut area.
360 Luis M. Martins et al.
The camera door-mount combined with the intervalometer design offers an inexpensive and efficient approach to SFAP. The Cessna aircraft reconnaissance has shown its effective- ness in surveying and analysing limited areas up to 1000 ha in size. Internal access to exter- nally mounted cameras overcomes some of the difficulties imposed by the small amount of cabin space in the aircraft. This makes flying and imaging operations easier. The idea can easily be adapted to suit various Cessna aircraft wing heights and a variety of small format cameras (LONGet al. 1986). With this methodology accurate aerial photographic exposures can be obtained in a short time at limited cost and with high resolution. It is easy to repeat flights within a short time period. Moreover, the cost of aerial photo interpretation is lower and the information from the study area can also be made available for future research. The research flight described here cost about EUR 300 (without the cost of the reflex camera) which is approximately 15% of the cost of a conventional photographic survey (EUR 2200).
Recently the high quality imagery provided by digital CCD cameras, attributable to advances and improvements in electronics, has revolutionised imaging technology. The air- borne image acquisition provided by this system is encouraged by EVERITTet al. (1997), and LEE(1989) who suggest the use of computer image analysis to identify and quantify disease damage. The research described in this paper confirms the usefulness of this technique in mapping diseased areas of chestnuts.
5 References
ABREU, C.G.; COUTINHO, J.F.; CARDOSO, A.O.; CAMPOS, J., 1993a: Suppressive soils and chestnut ink disease. In: ANTOGNOZZI(ed) International Congress on Chestnut, Spoleto. 533–537.
ABREU, C.G.; MARQUES, C.P.; VAREJÃO, E.S.; CARDOSO, A.O., 1993b: Aplicação da fotografia policromada de infravermelho na detecção da doença da tinta do castanheiro. Cienc. Biol.
Ecol. System 23, 1/2: 20.
ABREU, C.G.; MARTINS, L.M.; SILVA, C.A.; CARDOSOA.O.; JÓIA, H.A., 2000: Chestnut ink disease: remote sensing and resistance. Cost Meeting, Lithocoro, Greece. 90–91.
AMBROSINI, I.; GHERARDI, L.; VITI, M.; MARESI, G.; TURCHETTI, T., 1997: Monitoring diseases of chestnut stands by small format aerial photography. Geocarto Internacional 12, 3: 41–46.
EVERITT, J.H.; ESCOBAR, D.E.; NORIEGA, J.R.; DAVIS, M.R.; CAVAZOS, I., 1997: A digital imaging system and its application to natural resource management. In: The First North American Symposium on Small Format Aerial Photography, Minnesota. 123–136.
HOFFER, R.M.; MYHRE, R.J., 1989: Color and color infrared photography for vegetation assess- ment. In: Color Aerial Photography and Videography, Nevada. 1–5.
LEE, Y.J., 1989: Aerial photography for the detection of soil-borne disease. Can. J. Plant. Pathol.
11: 174–176.
LILLESAND, T.M.; KIEFER, R.W., 1999: Remote sensing and image interpretation. Fourth Edition.
New York, John Wiley & Sons, Inc. 725 pp.
LONG, D.S.; TAYLOR, J.E.; MCCARTHY, J., 1986: Cessna aircraft cabin door mount for photographic and videographic cameras. Photogramm. Eng. Remote Sens. 11: 1753–1755.
MARTINS, L.M., 1997; Influência dos solos supressivos sobre a doença da tinta do castanheiro.
Mestrado em Engenharia dos Recursos Florestais, Vila Real, UTAD. 129 pp.
MARTINS, L.M.; ABREU, C.G.; MARQUES, C.P., 1997: Evolução do estado sanitário do castanheiro numa área limítrofe à serra da Nogueira (Trás-os-Montes) In: I Congreso Forestal Hispano Luso, II Congreso Forestal Español, Pamplona. 265–270.
SNEDECOR, J.P.; COCHRAN, W., 1982: Statistical methods. 7ª ed. Aims, USA, The Iowa State University Press. 507 pp.
WARNER, W.S.; GRAHAM, R.W.; READ, R.E, 1996: Small format aerial photography. Maryland, American Society for Photogrammetry and Remote Sensing. 348 pp.
Accepted 28.1.02
