## 1 Introduction  
Sensors and algorithms have been developed to detect fruit and vegetable diseases. Diseases can be encountered during the growing stage, the postharvest storage, and at the point of retail where the consumer comes in contact with the product. Crop residue-borne fungi and other soil-surfaceinhabiting microbes, including numerous plant pathogens, are encountered during the production stage. These microbes rely on water at or near the soil surface for survival, growth, and reproduction [1,2] The presence of wetness (moisture) at the soil-air interface or within the top few millimeters of soil is thought to be a very important factor in the development of certain plant pathogens and plant diseases [3,4]. Soil surface moisture, often associated with precipitation events or dew formation in the canopy [5], is presumed to be one of the critical environmental factors affecting the development of these and other residue-borne pathogens [2]. Leaf wetness influences the initiation and development of many fungal and bacterial plant diseases. Bacterial diseases increase in severity in direct relationship to the length of time the leaves are wet [6]. Most foliar fungal disease pathogens require specific leaf surface conditions for their spores to germinate; favorable temperature and a film of water on the leaf surface are required [7]. The longer the leaf surface is wet, the greater the risk of infection and the greater the number of infections per leaf. Even patch diseases have been reported to be more severe with prolonged periods of leaf wetness [8,9]. Thus, disease management prediction requires leaf wetness assessment. Trees and orchards suffer damages by fungi that decay live or dead trees. It is estimated that for every 100 million board feet of timber harvested every year in the United States, heart decay fungi destroys about 30 million board feet of timber volume [10]. Bacterial wet-wood infections are present in hardwood and some conifers causing annual losses due to drying defects in oak lumber of about 500 million board feet [11]. The battle against postharvest decay of fruits and vegetables has been fought for decades and consumers who shop for quality fresh fruits and vegetables must often discard spoiled produce [12]. Harvested fruits have to be sorted in order to eliminate those with disease. If one rotten fruit is present inside a box, a high probability of infection in healthy fruits exists. Although the development of modern fungicides and storage technologies have greatly extended the shelf life of fruit after harvest, postharvest losses vary from an estimated 5 percent to more than 20 percent in the United States, and can be as high as 50 percent in developing countries [13]. Postharvest losses were measured in Costa Rica at rates of 44%, 35%, 32% and 30% for mango, avocado, melon and papaya, respectively [14]. Losses between 40 and 50% occurred in retail shops. Fungicide efficiency has decreased with the development of pathogen resistance and to public perception that pesticides are harmful to human health and the environment [12]. Thus, alternative methods to control postharvest diseases are urgently needed. The structure and physiological status of a plant is represented by reflectance patterns. The amount of reflected light depends on an amount of leaf-related factors, such as external morphology, internal structure, concentration, and internal distribution of biochemical components. Much remote sensing research has been done at plant leaf-level to ascertain the amount of stress in plants. Alteration of the interaction of light with the foliar medium and its effect on physiological changes has been studied [15]. The most common and widespread change occurs in the proportion of light-absorbing pigments [16], most notably chlorophylls \(a\) and \(b\) which absorb light in the 430-660 nm region. Investigators have observed differences in reflectance due to stress induced changes in pigment concentration in the green peak (525-605 nm) and along the red edge (\(\sim\)750 nm) [17,18].