ET-232 ^ i^T BOAR^ August 1%6 STATE P^^^ United otates department of ngriculture Hgricultural nesearch Adininistration Bureau of iaitomology and hlant quarantine GHOV'JTH STAGES OF CX)RN AS KKLATED TO EUROPEAN COiiN BOHEK INFESTATION By (J. H. aatc holder uivision of Cereal and Forage insect investigations The nomenclature of corn-plant growth stages described in this paper includes specific definitions useful in reporting field experi- ments in the control of the Jiiiropean corn borer ( Pyrausta nubilalis (Hbn.;)o This arbitrary classification has been developed and is presented in response to the need experienced by the author and many other entomologists for a uniform, specific, and readily conqjrehensible terminology for use in describing the com plant at the time when an observation is made or a treatment is applied. The use of plant height, number of days from planting or silking, and very loose terms, such as "tasseling" or -silking," does not provide an adequate concept of the growth stage of the plant to which reference is made. The accompany- ing illustrations, tables, and definitions have been developed from extensive field studiesi/ of the growth habits of various types and strains, and the nomenclature has been found applicable to most of the common commercial varieties of com. i-iirther designation of growth periods within the subdivisions itemised here is practicable for special purposes as required. The illustrations of corn-plant growth stages (figs. 1-3 j are semi-diagrammatic. An effort has been made to keep the drawings to scale and to show the normal growth and development of all parts of the com plant. This has required considerable generalization. The growth stages recognized, together with symbols useful in designating them, are listed as follows: Pre-whorl (.?„; Late green tassel ^13; li^rly whorl ^Wi; Early silk ^^iJ kid-whorl {yi2) Mid-silk ^a2; Late whorl ^^3^ late silk ^^3; uiarly green tassel . ^TiJ Koasting ear U.^.J Mid green tassel ... (T2; Mature {U) -'The studies were conducted near New Haven, Lionn., in commercial fields of sweet com. hybrids derived from ijolden £arly Market, (iolden uross Bantam, and various other northern strains of sweet and field corn were used in developing the classification. - 2 - The characters found most useful in distinguishing the several growth stages of corn are itemized in taoles 1 and 2. These tables include several terms, the definitions of which are reviewed here for the readers' convenience. The primary leaf blades are the first parts of the plant to make their appearance above ground, ihey are turgid and functional through- out the pre-whorl and the early whorl growth stages. Then they begin to shrivel, split, and become orown. uultivation usually buries the primary ieaf blades before the plant reaches the late-green-tasael stage. A rolled leaf blade is one that is rolled at its base and does not show a distinct line of juncture with its leaf sheath, the line of juncture being hidden by Leaf blades originating at points lower on the stalk. An unrolled leaf blade is one that, through expanding growth, has separated itself from the whorl and shows a distinct line of juncture with its leaf sheath. The leaf sheath is the part of the leaf that is attached to and wrapped around the stalk* The peduncle , or tassel stem, is the part of the stalk that extends from the topmost leaf sheath to the lowermost tassel branch, it is not seen until the late-green-tassel stage, when a small part is visible in some varieties. Husk laminae , or -streamers," are bladelike extensions from the husk leaves. An ear shoot is amy immature ear produced by the plant prior to and including the early silk stage of growth. With further development the topmost ear shoots become true ears and the remainder become rudi- mentary ears, A rudimentary ear is one that produces silk, but its kernels are UBually sterile and incapable of more than rudimentary growth, in this respect it differs from a nubbin, or small ear, some oi the kernels of which have been fertilized. -3 - Table 1. — Characteristics of the whorl (fig.l) and ^een- tassel (fig. 2) growth stages of the corn -olant (based on examination of 100 -nlants) Pri- Leaf Rolled Tassel Growth raaxy blades leaf scyryeax- Ear Length stage leaf visi- blades ance shoots Anthers of blades ble 2/ compoR- husk 1/ ing 11 laminae whorl Number Number Pre- v/horl Tur- gid & f\inc- tional 1-^ 2-3 Early whorl do b-7 3 Mid- Some S-9 li_5 whorl shrivel- ing & tipb\arn Late exten- 10 5-6 whorl sive brown- ing or lost Early 10 2-U green tassel LTumber Number Inches Shows in whorl cup 0-2 Tip shows 2-U above edge of whorl — 0-5 Mid green tassel 10-11 Clump of adhering branches 2-h 1-g Late green tassel 11 0-1 Unfurled 3-5 away from stem 0-a few 2-12 prominent on 2 shoots 1/ The first 2, or primary^ leaf blades are usually lost or buried prior to the mid-green-tassel stage. 2/ Does not include the 2 primary leaf blades. Many strains of field corn show I3 or more leaf blades. J/ Including all leaf blades that are rolled, at least at the base, and do not show a distinct line of juncture between the blade and the leaf sheath. - U ' Table 2. — characteristics of the silk, roasting-ear, and mature -growth stages ^fig. 3) of the com plant ^ based on examination of 100 plants ; . urowth stage Leaf blades vis- ible Tassel appear- Peduncle Anthers and pollen hAr shoots and ears iilk Husk laminae Kuraber iiarly silk 11-15 Branches cimall iinthers husk Yellowish i'roject droop portion dehisc- leaves green. forward shows ing or showing fresh and from above opening limited ear topmost in number tip leaf Uidsilk 11-15 branches xaay Maximum True Colored, Turned droop show pollen ears wilted outward 1 or shed- distinct at tips from more ding from and ear inches rudimen- taries numerous tip Late -silk 11-15 Branches Way Anthers Husk i;ried rolded straight show usually leaves and back and at 1-6 empty tight brown along an angle inches around at tips side to stem the ears only of ear itoasting- 11-15 Branches 3-6 ury One ear uried folded ears 1/ straight inches has and back and at reached bro>vn along an angle full most of side to stem size length of ear Mature 11-15 Branches 3-6 ury Less jjried rolded straight inches than 35 and brown back and at percent most of along an angle moisture Length side to stem in kernels of ear 1/ sometimes called prime-milk, or green -produce ear, The duration of any growth stage was found to vary with genetic strain, soil fertility, and weather. In early strains of sweet com the plants i^assed from the early whorl to the late- silk stage in 30 to U5 days, depending on temp era txires and rainfall in May and June. Mid-whorl and late-whorl staf^es lasted from U to 7 days each in early season sweet corn, but required a longer -oeriod in l?te-season sweet corn and field corn. The plants usually passed through the mid-green- tassel stage in 1 to 3 days. All growth stages were greatly prolonged, however, in strains of sweet corn or field com that required a longer period to reach maturity. Shelter areas on the plant are available to larvae of the European corn borer during each of the whorl, tassel, and silking growth stages of the corn plant. Such areas, cliaracteristic of each stage, occur successively as the plants change in form through grov/th. Com borer larvae are forced to migrate with the change or with the extinction of shelter areas; therefore larval establishment, migration, ajad reestablishment are affected by specific stages of plant growth. For these reasons the nomenclature of plant-growth stages described here is designed with reference to the sequence of biological events that take Dlace on a com plant. Figure 1, — The whorl stagefi of growth of the com plant (Wi , deBcribed in table 1: A, Early whorl; B, mid-whorl; and £, ^2* *J^» late-irtiorl stages- / /• .'if i ■ ( v^ «fc 1^, p xs 1) tc M a I >* • I— ( I. UNIVERSITY OF FLORIDA l|Tir!|iii|li<||!;llll|!|l!!11lllllllllllll 3 1262 09240 9035 A \ i' ( ( CQ V I V - U 9 t3 (0 "J CV I 0) •» 0) 1^ I' o 3 0) .* ^ 0) 01 (m (0 o ^ Is o ® M a> ^ -p .* CD H 0) rH a -p c 0) 0) ^& « M