Jeffrey M. Stein

Screening for Late Blight Susceptibility in Potato Tubers by Digital Analysis of Cut Tuber Surfaces

A method for quantification of late blight (Phytophthora infestans) in potato tuber tissue using a digital scanner and image analysis software is presented. The average reflective intensity of light reflected from the cut surface of sample tubers measures the darkened, diseased potato tuber tissue amid lighter, late blight-free tissue. In the absence of disease, potato variety, tuber shape, and tuber size do not influence the scan results. While digital quantification of late blight in tubers under controlled inoculation conditions is consistent, the digital assessments of late blight did not correspond exactly with those from a conventional subjective visual method. Used together, the methods can provide complementary information regarding varietal susceptibility to P. infestans development on the tuber surface and internal tuber tissue. The method of image analysis presented may be used to determine susceptibility of potato tubers to late blight in varietal development programs, storage research programs, or other tuber research programs.

Field optimization of dimethomorph for the control of potato late blight Phytophthora infestans: application rate, interval, and mixtures

Aspects concerning the field use of dimethomorph for the control of foliar and tuber potato late blight were examined. When application rate and interval manipulation for a season-long dimethomorph and mancozeb mixture was performed,the rate increasing through the season and 80% of full rate programs had equal final foliar blight control as the full rate program,regardless of interval. The minimum application rate for control equivalent to the full rate program was 1.34 kg ha � 1 week � 1 . When dimethomorph was tank-mixed with one of three protectant fungicides and integrated into a chlorothalonil-based late blight control program,all programs were as effective as the season-long chlorothalonil program. None of the mixtures were more effective than the others. When tank-mixed with pyraclostrobin and alternated with chlorothalonil applications,rate reduction to 50% of full rate gave foliar blight control equivalent to a full rate,for a dimethomorph and pyraclostrobin mixture and pyraclostrobin alone. r 2003 Elsevier Science Ltd. All rights reserved.

Influence of storage temperature on rate of potato tuber tissue infection caused byPhytophthora infestans (mont.) de bary estimated by digital image analysis

SummaryPotato tubers were inoculated with two biotypes ofPhytophthora infestans then stored at 3,7, 10 and 15°C. Image analysis quantified average reflective intensity (ARI) of diseased tissue from cut surfaces of sample tubers. Tuber tissue infection and infection rate were measured by calculating Mean ARI of samples. Average tuber tissue infection and infection rate was minimal at 3°C (P.i.-US8 orP.i.-US1). Tuber tissue infection increased at temperatures >3°C, from 220 Mean ARI seven days after inoculation (dai) to 190–150 Mean ARI 50 dai (depending on cultivar and biotype ofP. infestans). Rate of tuber tissue infection caused byP.i.-US1 at 7°C was about zero in cv. Snowden but greater than −0.2 ARI day−1 (cvs Russet Burbank and Superior). Rate of late blight infection in tuber tissue generally increased with temperature from −0.2 ARI day−1 (at 7°C) in all cultivars to a maximum of −0.8 ARI day−1 (10°C).

Containment of existing potato late blight (Phytophthora infestans) foliar epidemics with fungicides

Critical timing of application of foliar fungicides to limit further infection of potato foliage by Phytophthora infestans and the critical threshold of foliar infection level at which individual or combinations of specific fungicides limited further spread of infection with P. infestans were determined. In most seasons, key application timings at which the foliar late blight epidemic was contained to o50% of that in untreated control plots, were programs initiated 72 h before and 72 h after inoculation with P. infestans, and when foliar area with late blight lesions was estimated to be 1% or less. Delaying initiation of application of any fungicide until 5% and 10% estimated average foliar area with late blight lesions resulted in late blight development similar to the untreated control. Most fungicides and fungicide mixtures examined resulted in 30–50% reduction of late blight development in comparison to the untreated control as long as they were applied not later than 1% estimated average foliar area with late blight lesions. r 2002 Published by Elsevier Science Ltd.

Late blight (Phytophthora infestans (Mont) De Bary) development from potato seed-pieces treated with fungicides

Fungicides were applied as seed-piece treatments to control potato late blight, caused by Phytophthora infestans, US8, A2 biotype in controlled environment and field experiments. Efficacy of seed treatments for controlling late blight was examined under three disease development regimes simulated by artifical inoculation; (a) seed-borne infection, (b) transmission of infection resulting from spread during the seed-cutting operation, and (c) infection of foliage by aerial inoculation. Emergence of plants from the seed-borne infection was uniformly low (<40%) in controlled environment and field experiments. In controlled environment experiments some of the plants that emerged from fungicide-treated seed-pieces were infected with late blight. Following exposure of tuber surfaces to P infestans, emergence rates from seed-pieces treated with formulated products that included mancozeb in the formulation were comparable to the untreated and non-inoculated control in controlled environment and field experiments. Plants that emerged from non-inoculated seed-pieces treated with fungicides that contained active ingredients known to be effective against foliar late blight had lower percentage foliar infection after inoculation than the untreated control. Leaves close to the base of the stem had fewer infections than leaves attached at the mid region of the main stem, 14 days after inoculation, in some of the controlled environment studies. In contrast, field experiments conducted under conditions conducive to late blight development showed that none of the seed treatments applied to late blight-free seed-pieces delayed the onset and severity of late blight infection. In potato production areas at risk of early season late blight, seed treatments applied to healthy seed may confer limited protection against late blight between planting and the first scheduled applications of prophylactic foliar fungicides. © 1999 Society of Chemical Industry