Summary: Plant discases caused by viruses, bacteria, fungi, and other pathogens severely impact global crop yields. Rapid and accurate identification of these causal agents directly in the field is essential for implementing effective control measures to minimize crop losses. However, current molecular diagnostic methods are inefficient to monitor the pathogen attacks and spread over time, as the diagnostic tests are either performed in a laboratory setting (e.g., nucleic acid amplification assays) or lack sensitivity and specificity (e.g., lateral flow test strips). This dissertation describes a rapid solution for in-field diagnosis of plant diseases by combining microneedle (MN) extraction technology with a smartphone-enabled nucleic acid diagnostic device for quick biomarker (e.g. DNA and RNA) extraction, amplification, amplicon detection, and result reporting all in a single device. A polymeric MN patch, made of polyvinyl alcohol (PVA), extracted both DNA and RNA molecules from plant leaf tissues by two simple steps: 1) press and 2) rinse with Tris-EDTA (TE) buffer or deionized water. The MN rinsing solution was direcetly applicable to downstream nucleie acid amplification assays (e.g., polymerase chain reaction (PCR) and loop-mediated isothermal amplification (LAMP) assay) without additional purification, which reduced sample preparation time from several hours in a conventional extraction method to less than a minute. The MN method was successfully applied to isolate host DNA from various plant species such as potato, tomato, pepper, and soybean. Moreover, MN patches successfully extracted pathogen DNA (e.g, Phytophthora infestans, an oomyeete infeceting tomato and potato) and RNA (e.g., tomato spotted wilt virus) from laboratory-inoculated and field-collected tomato leaves. To amplify the MN- extracted DNA and RNA samples directly in the field, a smartphone-based LAMP amplification platform was developed in this dissertation. The integrated MN-smartphone platfom detected both P. infestans and TSWV simultaneously from co-infected tomato leaves down to I pg sensitivity within 30 minutes.In addition to pathogen detection, MN extraction technology can be utilized to isolate plant DNA for genotyping or sequencing assays. These assays are usually designed to detect unique genes which are present in very low copy numbers. Thus, the genotyping and sequencing assays require more input DNA to initiate the reaction than the PCR and LAMP assays used for pathogen detection. To increase the DNA yield, the operational steps of the PVA MN extraction method were optimized. Increasing the MN patch pressing time as well as pressing the patch for a second time on a new leaf area yielded more DNA. To further increase the DNA extraction capacity of a PVA MN patch, polyethylenimine (PEI), a positively charged polymer, was mixed with PVA in a weight ratio of 1:9 to introduce positive charges on the MN surfaces. The PVA+PEI MN patch yielded -2 times more DNA compared to the PVA MN patch. However, DNA samples extracted by PVA+PEI MN patches did not amplify in PCR and LAMP reactions. PEI caused inhibition in both PCR and LAMP reactions. Later, chitosan and carboxymethyl chitosan were tested as an altenative to PVA for MN patch fabrication. The extraction efficiency of the carboxymethyl chitosan MN patch was similar to the PVA patch. However, chitosan MN patches extracted -3.5 times less DNA than the PVA patches. The extracted DNA samples by chitosan and carboxymethyl chitosan MN patches could be directly utilized in PCR and LAMP reactions.Finally, the MN extraction technology has been expanded for plant stem and seed DNA extraction. A PVA MN patch with 1.5 mm long MNs successfully extracted DNA from the geranium stem tissue. For seed DNA extraction, PVA MN patch, sewing needle, and hollow (syringe) needle were tested, and all three needles successfully isolated DNA from water-soaked soybean seeds.