Effects of Propamocarb Hydrochloride on Mycelial Growth, Sporulation, and Infection by Phytophthora nicotianae Isolates from Virginia Nurseries

Abstract

Propamocarb hydrochloride is a systemic fungicide commonly used for control of Phytophthora diseases of nursery crops. Here we report on the effect of this compound on different growth stages of Phytophthora nicotianae, a major pathogen of numerous herbaceous and some woody ornamental plants. A total of 71 isolates were assayed for sensitivity to propamocarb at two concentrations of 1.8 mg/ml (label rate) and 10 mg/ml using clarified V8 agar as a base medium. All isolates grew at 10 mg/ml with the most sensitive isolate having 34.8% relative growth compared with growth on nonamended medium. Nine isolates were selected and further tested for mycelial growth at 0, 1, 10, 25, 50, and 100 mg/ml, and for sporangium production, zoospore motility, and germination at 0, 5, 50, 500, 5,000, and 50,000 μg/ml. EC50 values ranged from 2.2 to 90.1 mg/ml for mycelial growth, 133.8 to 481.3 μg/ml for sporangium production, 88.1 to 249.8 μg/ml for zoospore motility, and 1.9 to 184.6 μg/ml for zoospore germination, respectively. In addition, 17 selected isolates were evaluated for propamocarb sensitivity on Pelargonium × hortorum cv. White Orbit. Two days after seedlings were treated with propamocarb at 3.6 mg/ml, they were inoculated by either inverting one 5-mm-diameter plug of a 3-day-old culture or applying a 10-μl drop containing 20 zoospores onto each cotyledon. Propamocarb hydrochloride provided good protection of geranium seedlings from zoospore infections but not from mycelial infections. These results suggest that this fungicide must be used preventively for Phytophthora disease management and that mycelial growth may not be the most reliable measurement to determine the development of fungicide resistance to this compound in Phytophthora species at production facilities and in the landscape.

Sensitivity of Phytophthora infestans (Mont.) de Bary isolates to fluazinam, fosetyl-Al and propamocarb-hydrochloride

“…The results were similar to those of Hu et al (2007) for Phytophthora nicotianae (EC 50 = 2.2-90.1 mg l -1 ). FRAC(b) (2011) states that propamocarb-hydrochloride, in terms of resistance, is a low to medium risk fungicide.…”

Mefenoxam sensitivity and fitness analysis of Phytophthora nicotianae isolates from nurseries in Virginia, USA

“…In vitro screening of isolates using a single concentration of mefenoxam has been the major approach for sensitivity evaluation in Phytophthora species (Ferrin & Kabashima, 1991; Timmer et al ., 1998; Parra & Ristaino, 2001; Taylor et al ., 2002; Jeffers et al ., 2004; Hwang & Benson, 2005). However, it has been known that in vitro screening of oomycete isolates may not accurately predict the sensitivity to chemicals under in vivo conditions (Bruck et al ., 1980; Joseph & Coffey, 1984; Moorman & Kim, 2004), including a previous study with propamocarb (Hu et al ., 2007). The inconsistency between in vitro and in vivo tests may be caused by various factors such as plant‐fungicide interactions, aggressiveness of isolates, cultivar susceptibility and methods of fungicide application.…”

“…White Orbit). Seedlings were treated with mefenoxam then challenged with P. nicotianae as described previously (Hu et al ., 2007). A plastic container (9 cm in diameter and 4·5 cm deep) was used to hold a smaller Petri dish (6 cm in diameter) with a filter paper (Whatman #4) on its top.…”

“…To determine the percentage of resistant subpopulations in resultant zoospore suspensions, 100 µ L of zoospore suspension containing ~36 zoospores was spread on PARP medium in 9‐cm diameter dishes to allow zoospore germination (Hu et al ., 2007). After 36 h of incubation at 25°C in darkness, sixty colonies were selected arbitrarily from each treatment and subcultured on CV8 agar amended with mefenoxam at 100 µ g mL −1 .…”

Fungicides Reduce the Abundance of Yeast-like Symbionts and Survival of White-Backed Planthopper Sogatella furcifera (Homoptera: Delphacidae)

“…To test whether this control strategy could also be applied to WBPHs, we used denaturing gradient gel electrophoresis (DGGE) to investigate the composition of WBPH YLS. We then investigated the abundance of two YLS species and WBPH survival in response to five fungicides that are often used to control plant pathogens [24][25][26][27][28]. These findings may guide the development of an effective management strategy against WBPH.…”