First Report of Fusarium poae Causing Fusarium Head Blight of Wheat in Georgia, USA

Fusarium head blight (FHB) is one of the most troublesome fungal diseases challenging US wheat (Triticum aestivum L.) production (Savary et al. 2019). Harmful mycotoxin contamination, primarily due to deoxynivalenol (DON) in the Fusarium-damaged kernels (FDK), can negatively impact human and livestock health (McMullen et al. 1997). Although Fusarium graminearum is the primary causal agent of FHB, several other species including F. poae could pose a risk by producing dangerous mycotoxins such as nivalenol, DON, HT-2, and T-2 (Stenglein 2009). Severe FHB epidemics on wheat have occurred in recent years along with increased corn acreage across the southeast US specifically in Georgia (Ghimire et al. 2020). Five symptomatic wheat heads displaying bleaching symptoms were randomly collected from 19 different fields across 13 counties of Georgia in late spring of 2018. Infected kernels were dipped in 6% sodium hypochlorite for 10 min and rinsed three times with sterilized water. Blot dried kernels were placed on potato dextrose agar (PDA) and incubated for 7 days at 25°C under 12-h photoperiod. Three isolates (GA18W-2.1.6, GA18W-6.1.4, and GA18W-10.2.3) from Terrell, Peach, and Sumter counties exhibited dense, whitish mycelium colony typical of F. poae (Leslie and Summerell 2006). When grown in carboxymethylcellulose broth, isolates produced globose to piriform microconidia (5.1 to 12.4 µm by 4.4 to 11.2 µm) that were aseptate or had a single septation. The morphological identification was further confirmed by DNA sequencing. Single hyphal tip isolates were grown on cellophane overlain on PDA for 10 days. Fungal DNA was extracted using a Qiagen DNeasy Plant Mini Kit. Genomic DNA was sequenced using TEF1 and TEF2 primer pairs that target the translation elongation factor 1-α (EF1-α) locus (O'Donnell et al. 1998). BLASTn query of the obtained sequences of GA18W-2.1.6 (accession no. MT856907) and GA18W-10.2.3 (accession no. MT856909) were identified as F. poae with a 99% sequence homology with GenBank reference accession MK629641, while GA18W-6.1.4 (accession no. MT856908) displayed 100% similarity with F. poae accession KJ947343. Koch's postulates were performed under greenhouse conditions. Three seeds of the FHB susceptible wheat cultivar 'SS8641' were planted in individual cone-tainers with three replications (two cone-tainers/replicate). Wheat plants were vernalized for six weeks and then moved back to the greenhouse. Each F. poae isolate was spray inoculated (50,000 spores/ml) at the flowering stage onto 18-24 wheat heads. A field isolate of F. graminearum was included as a positive control whereas heads mock-inoculated with water were used as a negative control. Inoculated wheat heads were incubated in black plastic bags for 48 hours. Disease severity and FDK were recorded three weeks post inoculation. Disease severities were 6.7% (GA18W-2.1.6), 8.3% (GA18W-10.2.3), and 15.2% (GA18W-6.1.4) compared to 90.0% in the positive control similar to Arrúa et al (2019). No symptoms were observed in the negative control. FDK was 18% (GA18W-2.1.6), 28% (GA18W-10.2.3) and 44% (GA18W-6.1.4). F. poae was re-isolated from the infected heads and found to be morphologically identical to the isolates used for inoculation. To our knowledge, this is the first report of F. poae associated with FHB of wheat in the state of Georgia, USA. F. poae isolates from Georgia might produce mycotoxins in addition to reducing grain yield which needs further study.

Effect of irradiance upon the population of Pseudomonas coronafaciens in leaves and symptom expression of halo blight of rye

The toxin-induced chlorosis caused by Pseudomonas coronafaciens is influenced by irradiance. Three levels of irradiance caused differences in symptom expression but did not affect the rate of increase or final population of viable cells of P. coronafaciens in rye leaves. Distinct and faint halo blight symptoms appeared in 3--4 days in full light (1425 microW cm-2), and 58% shade (598 microW cm-2) respectively. No symptoms or only faint symptoms appeared after 7 days at 86% shade (202 microW cm-2). When plants kept in 58 and 86% shade were moved to full light 5 days after inoculation, lesion size and chlorosis increased rapidly during the next 2 days. On the 7th day after inoculation, the size of lesions from the 58 and 86% shade treatments exceeded those in full light by 2.5 and 5 times, respectively. A chlorosis index based on lesion size and chlorophyll loss within lesions also reflected this trend although chlorophyll loss was greater in lesions in full light for 7 days. Conditions of low irradiance such as that caused by overcast weather and (or) a dense leaf canopy followed by bright sunshine can cause greater losses from halo blight than a continuous period of high irradiance. Sympton expression may be masked by low irradiance but increase of inoculum is not impaired. Although increased light enhances chlorosis, toxin diffusion or production may be reduced.