Genome Resource of Podosphaera xanthii, the Host-specific Fungal Pathogen that Causes Cucurbit Powdery Mildew

Approximately 33 types of commonly consumed fruits and vegetables are members of the Cucurbitaceae, making it an important crop family worldwide. However, pathogen resistance to pesticides/fungicides has become a growing problem in cultivation practices. The identification of the effector proteins in each unique fungus–host pair would help toward the development of strategies for preventing the infection of important crops. In this study, we characterized the genome of Podosphaera xanthii, the fungal pathogen that causes powdery mildew disease in Cucurbitaceous plants. A first-draft genome of 209.08 MB was assembled and compared with those of 25 other fungal pathogens, particularly for identifying candidate secreted effector proteins. This draft genome can serve as a valuable resource for future genomic/proteomic studies of P. xanthii and its host-specific pathogenesis.

Sensitivities of Cucurbit Powdery Mildew Fungus (Podosphaera xanthii) to Fungicides

This study was conducted to evaluate the sensitivity of cucurbit powdery mildew fungus (Podosphaera xanthii) to major fungicides used for managing this pathogen in the Midwestern United States. Fungicides azoxystrobin from the quinone outside inhibitors group, cyflufenamid from the phenylacetamide group, penthiopyrad from the succinate dehydrogenase inhibitors group, quinoxyfen from the quinolines group, and triflumizole from the demethylation inhibitors group were tested for their effectiveness for preventing infection of cucurbits by P. xanthii. In 2015 and 2016, 37 isolates of P. xanthii were evaluated for their sensitivity to azoxystrobin (Quadris 2.08SC), cyflufenamid (Torino 0.85SC), penthiopyrad (Fontelis 1.67SC), and triflumizole (Procure 480SC) on cucumber ‘Bush Crop’ cotyledon leaves. The number of isolates sensitive to tested concentrations of Quadris 2.08SC, Torino 0.85SC, Fontelis 1.67SC, and Procure 480SC was 8 (22%), 21 (57%), 20 (54%), and 23 (62%), respectively. During 2015 to 2018, Quadris 2.08SC, Torino 0.85SC, Fontelis 1.67SC, quinoxyfen (Quintec 250SC), and Procure 480SC were tested for their effectiveness for managing powdery mildew on pumpkin ‘Howden’ in the field. The results showed that powdery mildew was effectively managed in the plots treated with Procure 480SC and Quintec 250SC. However, management of the disease was less successful in the plots treated with Quadris 2.08SC, Torino 0.85SC, and Fontelis 1.67SC.

Applying Wollastonite to Soil to Adjust pH and Suppress Powdery Mildew on Pumpkin

Although not considered an essential nutrient, silicon (Si) can be beneficial to plants. Si accumulator species such as pumpkin (Cucurbita pepo var. pepo) can absorb Si from soil. Si uptake may reduce plant susceptibility to fungal diseases such as cucurbit powdery mildew (Podosphaera xanthii and Erysiphe cichoracearum). We previously reported that wollastonite, an Organic Materials Reviews Institute–approved natural mineral, can increase soil Si level, increase soil pH, provide pumpkin plants with Si, and increase their resistance to powdery mildew. In this study, we examined the optimum application rate of wollastonite for pumpkins grown in pots and exposed to cucurbit powdery mildew. We confirmed that wollastonite has liming capabilities similar to regular limestone. Regardless of the application rates, wollastonite and limestone showed similar effects on soil chemistry and plant mineral composition. Pumpkin plants grown with the lower doses of wollastonite amendments (3.13 and 6.25 tons/acre) had the greatest tissue Si concentrations and demonstrated the greatest disease resistance. We conclude that wollastonite is a useful material for organic cucurbit (Cucurbitaceae) growers who want to increase soil pH and improve plant resistance to powdery mildew at the same time. Applying wollastonite at rates beyond the amount required to achieve a desirable soil pH for pumpkin production did not further increase Si uptake, nor did it further suppress powdery mildew development.

Monitoring Methyl Benzimidazole Carbamate-Resistant Isolates of the Cucurbit Powdery Mildew Pathogen, Podosphaera xanthii, Using Loop-Mediated Isothermal Amplification

Powdery mildew, caused by the fungus Podosphaera xanthii, is one of the most economically important diseases affecting cucurbit crops in Spain. Currently, chemical control offers the most efficient management of the disease; however, P. xanthii isolates resistant to multiple classes of site-specific fungicides have been reported in the Spanish cucurbit powdery mildew population. In previous studies, resistance to the fungicides known as methyl benzimidazole carbamates (MBCs) was found to be caused by the amino acid substitution E198A on β-tubulin. To detect MBC-resistant isolates in a faster, more efficient, and more specific way than the traditional methods used to date, a loop-mediated isothermal amplification (LAMP) system was developed. In this study, three sets of LAMP primers were designed. One set was designed for the detection of the wild-type allele and two sets were designed for the E198A amino acid change. Positive results were only obtained with both mutant sets; however, LAMP reaction conditions were only optimized with primer set 2, which was selected for optimal detection of the E198A amino acid change in P. xanthii-resistant isolates, along with the optimal temperature and duration parameters of 65°C for 75 min, respectively. The hydroxynaphthol blue (HNB) metal indicator was used for quick visualization of results through the color change from violet to sky blue when the amplification was positive. HNB was added before the amplification to avoid opening the lids, thus decreasing the probability of contamination. To confirm that the amplified product corresponded to the β-tubulin gene, the LAMP product was digested with the enzyme LweI and sequenced. Our results show that the LAMP technique is a specific and reproducible method that could be used for monitoring MBC resistance of P. xanthii directly in the field.

Inheritance of Resistance to Cucurbit Powdery Mildew in Bitter Gourd

Cucurbit powdery mildew (CPM) caused by Podosphaera xanthii (Px) is an economically important disease of bitter gourd (BG; Momordica charantia) in Asia. High-level resistance to CPM is known in various BG accessions that have been used to develop BG breeding lines that originated in different countries. BG breeding lines THMC 113 (Belize), THMC 143 (India), THMC 153 (Thailand), THMC 167 (India), and THMC 170 (Taiwan) possess high-level resistance to BG Px race (BG-CPM), designated Mc-1 from a field at Kamphaeng Saen, Thailand, whereas THMC 144 (India) is susceptible. Our objective was to determine the inheritance of resistance to BG-CPM race Mc-1. To that end, THMC 144 (India) was crossed with the five resistant lines. The parents and their respective F1, F2, backcross progenies were evaluated for BG-CPM disease severity in inoculated field and growth chamber tests. Resistance to BG-CPM race Mc-1 in the five resistant lines was controlled by at least two independent, recessive genes. Intercrosses of the BG-CPM–resistant lines revealed allelic resistances in four of the breeding lines: THMC 113, THMC 153, THMC 167, and THMC 170. Resistance in THMC 143 was clearly non-allelic for resistance to BG-CPM with the other four BG-CPM–resistant lines.

Cucurbit Powdery Mildew-resistant Bitter Gourd Breeding Lines Reveal Four Races of Podosphaera xanthii in Asia

Bitter gourd (Momordica charantia L.) is a commercially and nutritionally important market vegetable in Asia cultivated mainly by smallholder farmers. Cucurbit powdery mildew (CPM) caused by Podosphaera xanthii (Px) is a nearly ubiquitous and serious fungal disease of bitter gourd. Five bitter gourd breeding lines (THMC 113, THMC 143, THMC 153, THMC 167, and THMC 170) were selected at the World Vegetable Center for resistance to a local isolate of Px in Kamphaeng Saen, Thailand. We evaluated the resistance potential of these five inbred lines against local isolates of Px at 12 locations in five Asian countries. Plants were inoculated with the respective local Px isolate 15 and 30 days after transplanting and additional Px-infected plants of the inoculated control were interplanted throughout each test. Plants were rated 60 days after transplanting for CPM reaction using a 0 (no evidence of infection) to 5 (>75% infection evident on individual leaves) disease severity scale. THMC 153 and THMC 167 were resistant to the local race of Px in all locations, whereas THMC 143 was observed resistant in all test locations except one in China. THMC 113 was resistant in each location except one in India. THMC 170 was susceptible in three locations in India. The multilocation tests revealed four unique Px races on bitter gourd in different Asian countries and sources of resistance for breeding CPM-resistant bitter gourd cultivars. Six strains of Px isolated from other cucurbits (Cucumis and Cucurbita) and representing five melon CPM races were unable to infect the susceptible M. charantia accession THMC 144 and the five resistant breeding lines, indicating pathotype differences between them and an isolate of M. charantia origin typed as race 1 on melon. THMC 143 and THMC 167, which originated from India, exhibited good yield potential in trials conducted in Thailand, Myanmar, Vietnam, and Bangladesh.