Contact Reflectance Spectroscopy for Rapid, Accurate, and Nondestructive Phytophthora infestans Clonal Lineage Discrimination

Populations of Phytophthora infestans, the oomycete causal agent of potato late blight in the United States, are predominantly asexual, and isolates are characterized by clonal lineage or asexual descendants of a single genotype. Current tools for clonal lineage identification are time consuming and require laboratory equipment. We previously found that foliar spectroscopy can be used for high-accuracy pre- and postsymptomatic detection of P. infestans infections caused by clonal lineages US-08 and US-23. In this work, we found subtle but distinct differences in spectral responses of potato foliage infected by these clonal lineages in both growth-chamber time-course experiments (12- to 24-h intervals over 5 days) and naturally infected samples from commercial production fields. In both settings, we measured continuous visible to shortwave infrared reflectance (400 to 2,500 nm) on leaves using a portable spectrometer with contact probe. We consistently discriminated between infections caused by the two clonal lineages across all stages of disease progression using partial least squares (PLS) discriminant analysis, with total accuracies ranging from 88 to 98%. Three-class random forest differentiation between control, US-08, and US-23 yielded total discrimination accuracy ranging from 68 to 76%. Differences were greatest during presymptomatic infection stages and progressed toward uniformity as symptoms advanced. Using PLS-regression trait models, we found that total phenolics, sugar, and leaf mass per area were different between lineages. Shortwave infrared wavelengths (>1,100 nm) were important for clonal lineage differentiation. This work provides a foundation for future use of hyperspectral sensing as a nondestructive tool for pathovar differentiation.

Effects of a Potato Spindle Tuber Viroid Tomato Strain on the Symptoms, Biomass, and Yields of Classical Indicator and Currently Grown Potato and Tomato Cultivars

The Chittering strain of potato spindle tuber viroid (PSTVd) infects solanaceous crops and wild plants in the subtropical Gascoyne Horticultural District of Western Australia. Classical PSTVd indicator hosts tomato cultivar Rutgers (R) and potato cultivar Russet Burbank (RB) and currently widely grown tomato cultivars Petula (P) and Swanson (S) and potato cultivars Nadine (N) and Atlantic (A) were inoculated with this strain to study its pathogenicity, quantify fruit or tuber yield losses, and establish whether tomato strains might threaten potato production. In potato foliage, infection caused spindly stems, an upright growth habit, leaves with ruffled margins and reduced size, and upward rolling and twisting of terminal leaflets (RB, A, and N); axillary shoot proliferation (A); severe plant stunting (N and RB); and necrotic spotting of petioles and stems (RB). Tubers from infected plants were tiny (N) or small and “spindle shaped” with (A) or without (RB) cracking. Potato foliage dry weight biomass was decreased by 30 to 44% in A and RB and 37% in N, whereas tuber yield was diminished by 50 to 89% in A, 69 to 71% in RB, and 90% in N. In tomato foliage, infection caused epinasty and rugosity in apical leaves, leaf chlorosis, and plant stunting (S, P, and N); cupped leaves (S and P); and reduced leaf size, flower abortion, and necrosis of midribs, petioles, and stems (R). Mean tomato fruit size was greatly decreased in all three cultivars. Tomato foliage dry weight biomass was diminished by 40 to 53% (P), 42% (S), and 37 to 51% (R). Tomato fruit yield was decreased by 60 to 76% (P), 52% (S), and 64 to 89% (R), respectively. Thus, the tomato strain studied was highly pathogenic to classical indicator and representative current tomato and potato cultivars, causing major losses in fruit and tuber yields. Tomato PSTVd strains, therefore, pose a threat to tomato and potato industries worldwide.

Contribution to the study of potato aphids in Greece

Four aphid species were identified in the Metsovo potato center, Greece. Rhopalosiphoninus latysiphon, infesting subterranean parts of potato, is reported for the first time in Greece, while Aphis frangulae, infesting the potato foliage, is reported for the first time on potato plants in Greece. High resistance to methamidophos was observed with the latter aphid species. Potato virus Y (PVY) was transmitted by all four species of aphids

Pathogenicity and Aggressiveness of Three Alternaria spp. on Potato Foliage in the U.S. Northwest

Alternaria spp. were collected from potato foliage showing symptoms of early blight and brown spot in the Columbia Basin, WA and Bonners Ferry and Rupert, ID between 2009 and 2011. The aggressiveness of three Alternaria spp. on potato was quantified on nonwounded and wounded detached leaves of ‘Russet Norkotah’ potato; wounded detached leaves of ‘Alturas’, ‘Ranger Russet’, ‘Russet Burbank’, and ‘Umatilla Russet’; and whole plants of Russet Norkotah. Mean infection frequencies (MIF) and area under the lesion expansion curve (AULEC) were significantly greater for Alternaria solani (P = 0.0072 and 0.0002, respectively) than for A. arborescens or A. arbusti on nonwounded leaves. Wounding of tissue significantly increased MIF and AULEC for A. arbusti (P = 0.008 and 0.0047, respectively) and AULEC for A. arborescens (P = 0.01) relative to nonwounded tissue. AULEC did not differ significantly among the three Alternaria spp. when inoculated onto wounded foliage of whole plants (P = 0.34); the AULEC of whole plants was positively and significantly correlated with AULEC on detached leaves (P = 0.03). Umatilla Russet was the most susceptible and Russet Burbank was the least susceptible based on MIF and AULEC for all three pathogen species. Results indicate that A. solani was the more aggressive pathogen of potato in the Columbia Basin, because both A. arborescens and A. arbusti require wounds and A. arbusti lesions do not expand significantly in comparison with A. solani or A. arborescens following inoculation.

Etiology of a Tuber Rot and Foliar Blight of Potato Caused by Phytophthora nicotianae

Although Phytophthora nicotianae is not normally considered to be an important pathogen of potato (Solanum tuberosum), intermittent outbreaks of a foliar blight and tuber rot have been reported in the United States over the past 75 years. Due to the sporadic nature of these reports, little is known about the etiology of the disease in potato. However, foliar disease and tuber rots caused by this pathogen are usually centered near areas of standing water in the field and along pivot tracks. Moreover, soil particles adhering to the foliage of infected potato plants suggest that water splash is involved in P. nicotianae dissemination and infection. Soil infestation and water splash dissemination studies were conducted under greenhouse conditions to examine the role that zoospores of P. nicotianae may play in disease on potato. In the soil infestation study, inoculum of P. nicotianae was added to soil at four rates (0.0, 1.0 × 103, 5.0 × 103, and 4.0 × 104 zoospores/ml) and three timings (at planting and 7 and 14 days after planting). Direct infection of aboveground plant tissues was achieved via splash dissemination of inoculum onto potato foliage. All soil infestations significantly reduced emergence, with the exception of the 1.0 × 103 zoospores/ml treatment, and no plants emerged from soil infested with 4.0 ×104 zoospores/ml. Significant reductions in stem number were observed with infestations of 1.0 × 103 and 5.0 × 103 zoospores/ml at planting and 5.0 × 103 zoospores/ml at 7 days after planting. Progeny tuber infections were confirmed with infestations at 1.0 × 103 zoospores/ml at planting and 7 days after planting. Lesions developed on leaflets, petioles, leaf axils, and stems in all water splash dissemination treatments within 3 days of inoculation, significant differences in the lesion number were observed, and disease severity generally was proportional to inoculum concentration. Relative area under the disease progress curve of the 5.0 × 103 and 4.0 × 104 zoospores/ml splash dissemination treatments was significantly greater than the 0.0 zoospore and 1.0 × 103 zoospores/ml treatments. Progeny tuber infections were observed with all water splash dissemination treatments but infection rates did not differ significantly among treatments. These studies confirm the hypothesis that water splash dissemination of P. nicotianae inoculum is a likely means by which infections of this pathogen are initiated in potato.