This research was jointly funded by the Central Public-interest Scientific Institution Basal Research Fund of State Key Laboratory of Tree Genetics and Breeding (grant number CAFYBB2020ZY001-2) and the National Natural Science Foundation of China (grant number 32171776) to Jiaping Zhao.

1. Fungal culture for canker pathogen
<ol>
	<li>Prepare the potato dextrose agar (PDA; potato extraction 6.0 g, dextrose 20.0 g, agar 20.0 g) culture medium for fungal strains; dissolve the above materials in water up to 1000 mL, completely dissolve the medium at 100 &#176;C for 10 min.</li>
	<li>Pour PDA medium into 25 mL tubes (each containing 15.0 mL); sterilize all tubes at 121.1 &#176;C for 30 min. Pour the medium into culture plates (9.0 cm in diameter) and cool the plates at room temperature (RT...

Rapid In Vivo Leaf Inoculation for Screening Poplar Stem Canker Resistance

<ol>
	<li>Du, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Genetic+analysis+and+seedling+selection+of+the+poplar+progenies+of+Aigeiros+section.">Genetic analysis and seedling selection of the poplar progenies of Aigeiros section.</a> Journal of Huazhong Agricultural University. 28 (5), 624-630 (2009).</li><li>Shi, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Identification of canker resistance to selection poplar clones [Master's thesis]. , (2014).</li><li>Zhang, L., Zhang, H., Ou, D., Fan, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Investigation+on+the+resistance+to+canker+of+some+new+leuce+hybrids+inoculated+with+Botryosphaeria+dothidea.">Investigation on the resistance to canker of some new leuce hybrids inoculated with Botryosphaeria dothidea.</a> Journal of Northwest Forestry University. 32 (6), 210-213 (2017).</li><li>Du, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Genomic+selection+of+seedling+growth+traits+in+a+poplar+hybrid+population.">Genomic selection of seedling growth traits in a poplar hybrid population.</a> For Res. 36 (6), 11-19 (2023).</li><li>Li, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+rapid+and+efficient+in+vivo+inoculation+method+for+introducing+tree+stem+canker+pathogens+onto+leaves:+Suitable+for+large-scale+assessment+of+resistance+in+poplar+breeding+progeny.">A rapid and efficient in vivo inoculation method for introducing tree stem canker pathogens onto leaves: Suitable for large-scale assessment of resistance in poplar breeding progeny.</a> BioRxiv. , (2024).</li><li>Li, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fungal+canker+pathogens+trigger+carbon+starvation+by+inhibiting+carbon+metabolism+in+poplar+stems.">Fungal canker pathogens trigger carbon starvation by inhibiting carbon metabolism in poplar stems.</a> Sci Rep. 9 (1), 10111 (2019).</li><li>Xing, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fungal+pathogens+of+canker+disease+trigger+canopy+dieback+in+poplar+saplings+by+inducing+functional+failure+of+the+phloem+and+cambium+and+carbon+starvation+in+the+xylem.">Fungal pathogens of canker disease trigger canopy dieback in poplar saplings by inducing functional failure of the phloem and cambium and carbon starvation in the xylem.</a> Physiol Mol Plant Pathol. 112, 101523 (2020).</li><li>Xing, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparisons+of+photosynthetic+response+and+characteristics+in+leaves+of+Populus+alba+var.+pyramidalis+Infected+by+the+stem+canker+pathogen+Valsa+sodida+and+Botryosphaeria+dothidea+at+early+stage.">Comparisons of photosynthetic response and characteristics in leaves of Populus alba var. pyramidalis Infected by the stem canker pathogen Valsa sodida and Botryosphaeria dothidea at early stage.</a> Scientia Silvae Sinicae. 57 (09), 121-129 (2021).</li><li>Li, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+Valsa+sordida+infection+on+photosynthetic+characteristics+and+carbon-water+metabolism+in+Populus+alba+var.+pyramidalis.">Effects of Valsa sordida infection on photosynthetic characteristics and carbon-water metabolism in Populus alba var. pyramidalis.</a> For Res. 34 (05), 58-68 (2021).</li><li>Balendres, M. A., De Torres, R., Dela Cueva, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Culture+storage+age+and+fungal+re-isolation+from+host-tissue+influence+Colletotrichum+spp.+virulence+to+pepper+fruits.">Culture storage age and fungal re-isolation from host-tissue influence Colletotrichum spp. virulence to pepper fruits.</a> J Phytopathol. 167 (9), 510-515 (2019).</li><li>Ward, K. T., Ostry, M. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Variation+in+Septoria+musiva+and+implications+for+disease+resistance+screening+of+poplars.">Variation in Septoria musiva and implications for disease resistance screening of poplars.</a> Plant Dis. 89 (10), 1077-1082 (2005).</li><li>Asalf, B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ontogenic+resistance+of+leaves+and+fruit,+and+how+leaf+folding+influences+the+distribution+of+powdery+mildew+on+strawberry+plants+colonized+by+Podosphaera+aphanis.">Ontogenic resistance of leaves and fruit, and how leaf folding influences the distribution of powdery mildew on strawberry plants colonized by Podosphaera aphanis.</a> Phytopathology. 104 (9), 954-963 (2014).</li><li>Dunnell, K. L., Le Boldus, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+correlation+between+Septoria+leaf+spot+and+stem+canker+resistance+in+hybrid+poplar.">The correlation between Septoria leaf spot and stem canker resistance in hybrid poplar.</a> Plant Dis. 101 (3), 464-469 (2017).</li><li>Wei, J., Huang, L., Gao, Z., Ke, X., Kang, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Laboratory+evaluation+methods+of+apple+Valsa+canker+disease+caused+by+Valsa+ceratosperma+sensu+Kobayashi.">Laboratory evaluation methods of apple Valsa canker disease caused by Valsa ceratosperma sensu Kobayashi.</a> Acta Phytopathol Sinica. 40 (1), 14-20 (2010).</li><li>Shen, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparative+study+on+the+effectiveness+of+three+inoculation+methods+for+Valsa+sordida+in+Populus+alba+var.+pyramidalis.">Comparative study on the effectiveness of three inoculation methods for Valsa sordida in Populus alba var. pyramidalis.</a> Biology. 13 (4), 251 (2024).</li></ol>

This protocol provides a rapid and efficient inoculation method for poplar canker resistance pathogens, which is suitable for the research fields requiring large-scale disease resistance screening, such as hybrid breeding of poplar canker resistance and pathogenicity screening of stem canker pathogens. 
The first key point of the method is to evaluate disease resistance by newly matured leaf inoculation instead of matured stems/branches. As a result, the selection of stem canker disease-resist...

Poplar stem canker diseases, mainly caused by two necrotrophic pathogens, Cytospora chrysosperma (Pers.) Fr. and Botryosphaeria dothidea (Moug. ex Fr.) Ces. &#38; de Not., severely threaten the development and survival of poplar plantations in the Northeast, North, and Northwest (Three-north) of China. Hybrid breeding is the most direct and efficient method to control and manage tree diseases; however, compared with the progress in breeding for high-yield, fast-growing, or other poplar hybrid clones, research on resistance breeding for poplar canker disease is scarce. Only limited studies of disease-resistance hybrid breeding have been reported1,2,3, and no canker-resistance hybrid clone has been cultivated in poplar afforestation.
The crucial step of regular hybrid breeding is the clone selection based on the phenotype acquisition of hybrid progeny. However, acquiring the acquisition of pathological phenotypes (pathotypes) is time-consuming, laborious, tiring, expensive, and experts-depending&#160;process. For woody plants, it is more challenging due to the time-, labor-, and economy-consuming nature caused by their long lifecycle, slow growth, and massive body. For example, in poplar, the canker resistance screening of hybrid progeny was conducted 5-7 years after hybridization through conventional in vitro stem inoculation methods1,2,3. Moreover, limited by this low-efficiency and high-consuming disease-resistant screening method, researchers re-selected the canker-resistant clones from a small subgroup (for example, the selected high-yield or fast-growing poplar clones), not from all hybrid progeny. Therefore, using the regular stem inoculation methods, it is not sure to identify the authentic resistant clones and can not reveal the disease-resistant diversity of the breeding progeny, thus limiting the exploration of disease-resistance-related genes or gene modules. Compared to the rapid development of poplar fast-growing/high-yield breeding, those breeding programs can obtain hybrids through phenotypic selection or even genomic selection in the first two years of breeding4, the poplar canker-resistance breeding developed slowly. The disease-resistant screening (or detection) of hybrid progeny, or the pathogen inoculation method, has become the crucial speed-limiting step in poplar canker disease-resistance breeding.
In this protocol, we introduce a novel inoculation method for poplar stem canker pathogens, in vivo leaf inoculation method. Using this method, we can quickly and efficiently test the canker disease resistance of dozens of poplar species (cultivars or clones) within 5-7 days. The validation assay illustrated that the in vivo leaf inoculation is consistent with the traditional in vitro stem inoculation on the resistance detection of stem canker disease, suggesting that the leaf inoculation method is suitable for large-scale resistance screening of poplar canker diseases, such as the whole genotype selection of resistance progeny in the breeding of stem canker disease. This method solves the pathological problem of selecting resistant offspring in the hybrid breeding of poplar canker diseases.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Aluminum foil</td>
			<td>Biosharp</td>
			<td>BS-QT-027B</td>
			<td></td>
		</tr>
		<tr>
			<td>C. chrysosperma isolate</td>
			<td>China Forestry Culture Collection Center</td>
			<td>CFCC86775</td>
			<td>Separation and preservation by our laboratory</td>
		</tr>
		<tr>
			<td>Cytospora chrysosperma</td>
			<td>Plant Physiology Laboratory, Institute of Forestry New Technology, Chinese Academy of Forestry</td>
			<td>NCBI accession number: MK994101 for rRNA-ITS and MN025273 for EF1&#945; gene&#160;&#160; CGMCC number:40575</td>
			<td>Separation and preservation by our laboratory</td>
		</tr>
		<tr>
			<td>Epson Perfection V370 Photo</td>
			<td>Epson&#160;</td>
			<td>V370</td>
			<td>Scanner; Scan the leaves into image</td>
		</tr>
		<tr>
			<td>PDA (Potato Dextrose Agar)&#160;</td>
			<td>Solarbio</td>
			<td>P8931</td>
			<td>Provide nutrition for fungal growth</td>
		</tr>
		<tr>
			<td>PE plastic film</td>
			<td></td>
			<td></td>
			<td>To fix fungi on the leaves</td>
		</tr>
		<tr>
			<td>Populus alba var. Pyramidalis</td>
			<td>Plant Physiology Laboratory, Institute of Forestry New Technology, Chinese Academy of Forestry</td>
			<td></td>
			<td>Cultivated by our laboratory</td>
		</tr>
		<tr>
			<td>SPSS</td>
			<td>IBM</td>
			<td></td>
			<td>Data analysis software</td>
		</tr>
		<tr>
			<td>Thermostatic incubator</td>
			<td>Shanghai Kuntian Laboratory Instrument Co., Ltd</td>
			<td>KTD-6000</td>
			<td>Provide an environment for fungal growth</td>
		</tr>
		<tr>
			<td>Tough TG-6 camera</td>
			<td>OLYMPUS</td>
			<td></td>
			<td>To take photos of diseased leaves</td>
		</tr>
	</tbody>
</table>

in vivo leaf inoculation: an alternative method to assess the disease resistance of hybrid clones in poplar breeding of stem canker disease

Stem canker diseases caused by the pathogen Cytospora chrysosperma (Pers.) Fr.) and Botryosphaeria dothidea (Moug. ex Fr.) Ces. &#38; de Not. are the two major forest diseases in the poplar plantations in China, sometimes which can destroy all the poplar seedlings or severely damage mature poplar forests. Hybrid breeding is the most direct and efficient method of controlling and managing tree diseases. However, assessing disease resistance or selecting disease-resistance clones based on In vitro stem inoculation is inefficient, time-consuming, and expensive, limiting the development of hybrid breeding of poplar stem canker disease. In this study, we proposed an alternative method to assess disease resistance to stem canker pathogens through in vivo leaf inoculation. The test materials used in this method can be on 1-year-old poplar saplings or the annual branches of perennial poplars in the greenhouse or the field. The critical step of this alternative method is the selection of inoculating leaves: the 5-7th newly matured leaves might be the most suitable. The second critical step of the leaf inoculation method is to make wounds on plant leaves through needle pierces, providing sufficient lesions to measure disease severity. For the adequate number of leaves produced in the early stage of poplar breeding, this in vivo leaf inoculation contributes to the rapid, accurate, and large-scale screening of the disease-resistance poplar clones to stem canker pathogens. Moreover, this leaf inoculation method will also serve as an efficient method for screening pathotypes of stem canker disease pathogen C. chrysosperma, B.dothidea, or other poplar stem canker pathogens.

Author Spotlight: High-Throughput In Vivo Leaf Inoculation for Accelerating Disease Resistance Screening in Poplar Hybrid Breeding

In Vivo Leaf Inoculation: An Alternative Method to Assess the Disease Resistance of Hybrid Clones in Poplar Breeding of Stem Canker Disease

The scope of this research protocol is to examine the methodology of pathogen inoculation and disease-resistant assessment methods using the poplar hybrid breeding. We are proposing the in vivo leaf inoculation method as an alternative to the traditional method with the aim of advancing the breeding of poplar stem canker disease. Using the leaf inoculation method, researchers can screen the disease resistance of large-scale hybrid clones, in the second years of hybrid lesion, realizing earlier on the sliding selections in poplar hybrid grading of stem canker disease.The most advantageous aspects of the leaf inoculation method are as high efficiency and high throughput. First, the leaf method can sharply shorten the period of disease resistance selection from five to seven years after hybridization to two years after hybridization. Second, the method can assess the disease resistance of large-scale hybrid clones within several weeks.The leaf inoculation method will advance the hybrid breeding of poplar Botryosphaeria dothidea and Cytospora chrysosperma canker in China, and the poplar hybrid breeding of Septoria canker in North America. Moreover, the method can be used to mine disease resistance genes and improve the development of poplar molecular breeding. In addition to poplar hybrid breeding of stem canker disease, the leaf method can be used to screen the disease resistance of natural or mutant poplar populations, and the susceptibility of poplar stem canker pathogens.Moreover, the method can provide pathotypes for the mining of disease resistance genes, and the genetic analysis of disease resistance.

In this protocol, the schematic workflow was conducted in 48 poplar hybrid clones infected by the stem canker pathogen C. chrysosperma (Figure 1). The poplar hybrid clones are part of the hybridization progeny of P. deltoides, cultivated in the nursery at the Chinese Academy of Forestry (CAF), Beijing.
The stem canker pathogen C. chrysosperma isolate CZC is a typical fungal strain (with middle pathogenicity) used for the physiological re...

We provide a step-by-step protocol for assessing poplar resistance to stem canker pathogens using an in vivo leaf inoculation method. This method is especially suitable for large-scale assessment of Cytospora chrysosperma and Botryosphaeria dothidea canker disease resistance in poplar breeding progeny in China.

Stem canker diseases caused by the pathogen Cytospora chrysosperma (Pers.) Fr.) and Botryosphaeria dothidea (Moug. ex Fr.) Ces. &#38; de Not. are the two ...

in-vivo-leaf-inoculation-an-alternative-method-to-assess-disease