## 3 Sensing Per Crop
### Sensing Potato Diseases
#### 3.5.1 Sensors and Algorithms  
Tuber physiological disorders such as brown center, hollow heart, and translucent end, as well as secondary growth, growth cracks, bruise susceptibility, and heat necrosis have been associated with water stress and/or wide variations in soil moisture content [209-212]. A vision system graded potatoes by size, and detected external defects such as greening, mechanical damages, rhizoctonia, silver scab, common scab, cracks and growth cracks [213]. Mirrors were used to obtain a 360\({}^{\circ}\) view of the potato with a 3-CCD line-scan camera to inspect the potato in flight as they pass under the camera (Table 2). The color segmentation procedure used Linear Discriminant Analysis (LDA) and a Fourier Transform based shape classification technique based on the boundary distances and the centroid of the potato. A color machine vision system was trained to distinguish between good and green potatoes [214]. The vision system achieved over 90% accuracy in inspection of potatoes with hue histograms and multivariate discriminant techniques (Table 3). NIR detected defective potato tubers [215] which are _Phytophthora_ infected using discriminant analyses of reflectance rates (Table 4). As a result, for the ranges of specific wavelengths, there were clear differences between untreated and defective potato tubers. The light source was applied not only to the defective side, but to the entire surface of the potato tubers. Magnetic resonance imaging was applied to detect non-visible internal bruise and spraying symptoms caused by a virus and to get insight on the chemical and anatomical causes of such defects [53]. _P. infestans_, _Phoma foveata_, _Fusarium sulphureum_ y _F. coeruleum_ fungus were detected using magnetic resonance [216]. Previous X-ray methods were modified to enhance the contrast of the hollow heart potatoes [217]. A potato was placed in an X-ray field and a scanning detector traversed the length of the potatoes. The output of detector was amplified, digitized, and transferred to a computer for analyzing and recording. Ultrasonic transmission through potatoes is possible for the detection of hollow heart at a frequency of 50 kHz with a power level of 0.22 W [36]. At this frequency, ultrasound penetrated the whole potato tuber in the transverse direction along the longitudinal axis of the tuber, and yields information about the inside of the potato tuber. The analysis of the waveform of transmitted signals in time domain is sufficient to indicate the presence of hollow heart in potatoes. Average transmission losses of hollow heart potatoes were found to be greater than 0.28 dB/mm, and when this value was used as threshold, a predictive accuracy of 98 percent was achieved. Forecasting of late blight uses data from in-field automatic weather stations for limiting agrochemical use during the potato growing stage. Ideal conditions for _Phytophthora infestans_ spore production are relative humidity greater than 95% and temperature over 10\({}^{\circ}\)C at nighttime [218]. Serious infection occurs when free water is available on the crop surface, being rainfall and prolonged high relative humidity required after spore production. Critical periods can be predicted when minimum temperature is over 10\({}^{\circ}\)C, relative humidity greater than 90% for a period from 12 to 48 hours and rainfall is present in the period following up to 10 days. Prediction should be based on hourly observation of temperature and relative humidity. Consistently rainy summer or fall weather promotes late blight. However, in the 1990's, epidemics of late blight developed in potato crops in arid production areas of the Pacific Northwest where late blight had not been a problem [219]. Irrigation that tends to keep the foliage wet may contribute to thisdeveloping risk. Potatoes cultivated under center pivot irrigation can receive a relatively low volume of irrigation water for a long time near the pivot, favoring late blight occurrence.