https://www.google.com.br/search?q="grafting greenhouse"&es_...

Explanation:
www.scielo.br/scielo.php?script=sci_arttext&pid=S0034...‎
de DL Siqueira - ‎2010 -
Budstick age had no influence on bud viability of cv. ... para estabelecimento e manutenção de matrizes, borbulheiras e viveiro de citros em ambiente protegido.

www.redalyc.org/pdf/3052/305226735017.pdf‎
de DL de Siqueira - ‎2010 -
levels of budsticks (r = 0,94; p <0,05) occurred for cv. 'Baianinha'.. Key words: ... registradas e/ou de borbulheiras protegidas, monitoradas para sua sanidade ...

www.oxforddictionaries.com/definition/.../budstick‎
Definition of budstick in British and World English in Oxford dictionary. Meaning, pronunciation and example sentences. English to English reference content.

www.courses.nres.uiuc.edu/hort100/tutorials/t.../11.htm‎Traduzir esta página
Budsticks. Budsticks are branches that developed in the last growing season. They should be healthy and about the diameter of a pencil or a little larger.

mother plant for propagation

mother plant

1. A mature plant from which cuttings are taken. 2. The female ancestor of a hybrid.

From the same glossary posted by Sarah. The definition is:

A foundation block is a planting of a group of mother plants from sources that have been tested for viruses or other diseases and are maintained in isolation under conditions that prevent (re)infection and tested regularly for pathogens identified on a regional basis. In most NCPN-crop certification standards, **these blocks are considered Generation 1 (G1) stock** within the meaning of the ‘Generation level’ concept recommended by the NAPPO Guidelines.

Definition of Generation level:

Generation, or, Generation level (G level)
‘Generation’ or ‘G-level’ signifies the degree to which plant stock is related to the original virus-tested plant material. Regulations developed by certification programs specify the conditions under which each Generation level must be maintained in order to qualify for the program.

Definition of Generation 1:
Generation 1 (G1) refers to the original mother plants (nuclear or foundation material) that are tested for viruses of concern and maintained in isolation in the foundation block in order to prevent (re)infection. G1 stock is the original source of virus-tested plant material that is distributed to nurseries, growers or other interested parties within certification programs, such as those for grapes, hops, berries, citrus, fruit and nut trees.

Now, the original definition doesn't refer to testing for viruses, but that is the reason why they are separated into a block, and the article in question is referring to the contamination of a block of plants.